I have noticed that there doesn't appear to be a clear explanation of what the this
keyword is and how it is correctly (and incorrectly) used in JavaScript on the Stack Overflow site.
I have witnessed some very strange behaviour with it and have failed to understand why it has occurred.
How does this
work and when should it be used?
This question is related to
javascript
this
Here is one good source of this
in JavaScript
.
Here is the summary:
global this
In a browser, at the global scope, this
is the window
object
<script type="text/javascript">
console.log(this === window); // true
var foo = "bar";
console.log(this.foo); // "bar"
console.log(window.foo); // "bar"
In node
using the repl, this
is the top namespace. You can refer to it as global
.
>this
{ ArrayBuffer: [Function: ArrayBuffer],
Int8Array: { [Function: Int8Array] BYTES_PER_ELEMENT: 1 },
Uint8Array: { [Function: Uint8Array] BYTES_PER_ELEMENT: 1 },
...
>global === this
true
In node
executing from a script, this
at the global scope starts as an empty object. It is not the same as global
\\test.js
console.log(this); \\ {}
console.log(this === global); \\ fasle
function this
Except in the case of DOM event handlers or when a thisArg
is provided (see further down), both in node and in a browser using this
in a function that is not called with new
references the global scope…
<script type="text/javascript">
foo = "bar";
function testThis() {
this.foo = "foo";
}
console.log(this.foo); //logs "bar"
testThis();
console.log(this.foo); //logs "foo"
</script>
If you use use strict;
, in which case this
will be undefined
<script type="text/javascript">
foo = "bar";
function testThis() {
"use strict";
this.foo = "foo";
}
console.log(this.foo); //logs "bar"
testThis(); //Uncaught TypeError: Cannot set property 'foo' of undefined
</script>
If you call a function with new
the this
will be a new context, it will not reference the global this
.
<script type="text/javascript">
foo = "bar";
function testThis() {
this.foo = "foo";
}
console.log(this.foo); //logs "bar"
new testThis();
console.log(this.foo); //logs "bar"
console.log(new testThis().foo); //logs "foo"
</script>
Functions you create become function objects. They automatically get a special prototype
property, which is something you can assign values to. When you create an instance by calling your function with new
you get access to the values you assigned to the prototype
property. You access those values using this
.
function Thing() {
console.log(this.foo);
}
Thing.prototype.foo = "bar";
var thing = new Thing(); //logs "bar"
console.log(thing.foo); //logs "bar"
It is usually a mistake to assign arrays or objects on the prototype
. If you want instances to each have their own arrays, create them in the function, not the prototype.
function Thing() {
this.things = [];
}
var thing1 = new Thing();
var thing2 = new Thing();
thing1.things.push("foo");
console.log(thing1.things); //logs ["foo"]
console.log(thing2.things); //logs []
You can use this
in any function on an object to refer to other properties on that object. This is not the same as an instance created with new
.
var obj = {
foo: "bar",
logFoo: function () {
console.log(this.foo);
}
};
obj.logFoo(); //logs "bar"
In an HTML DOM event handler, this
is always a reference to the DOM element the event was attached to
function Listener() {
document.getElementById("foo").addEventListener("click",
this.handleClick);
}
Listener.prototype.handleClick = function (event) {
console.log(this); //logs "<div id="foo"></div>"
}
var listener = new Listener();
document.getElementById("foo").click();
Unless you bind
the context
function Listener() {
document.getElementById("foo").addEventListener("click",
this.handleClick.bind(this));
}
Listener.prototype.handleClick = function (event) {
console.log(this); //logs Listener {handleClick: function}
}
var listener = new Listener();
document.getElementById("foo").click();
Inside HTML attributes in which you can put JavaScript, this
is a reference to the element.
<div id="foo" onclick="console.log(this);"></div>
<script type="text/javascript">
document.getElementById("foo").click(); //logs <div id="foo"...
</script>
You can use eval
to access this
.
function Thing () {
}
Thing.prototype.foo = "bar";
Thing.prototype.logFoo = function () {
eval("console.log(this.foo)"); //logs "bar"
}
var thing = new Thing();
thing.logFoo();
You can use with
to add this
to the current scope to read and write to values on this
without referring to this
explicitly.
function Thing () {
}
Thing.prototype.foo = "bar";
Thing.prototype.logFoo = function () {
with (this) {
console.log(foo);
foo = "foo";
}
}
var thing = new Thing();
thing.logFoo(); // logs "bar"
console.log(thing.foo); // logs "foo"
the jQuery will in many places have this
refer to a DOM element.
<div class="foo bar1"></div>
<div class="foo bar2"></div>
<script type="text/javascript">
$(".foo").each(function () {
console.log(this); //logs <div class="foo...
});
$(".foo").on("click", function () {
console.log(this); //logs <div class="foo...
});
$(".foo").each(function () {
this.click();
});
</script>
Since this thread has bumped up, I have compiled few points for readers new to this
topic.
this
determined?We use this similar to the way we use pronouns in natural languages like English: “John is running fast because he is trying to catch the train.” Instead we could have written “… John is trying to catch the train”.
var person = {
firstName: "Penelope",
lastName: "Barrymore",
fullName: function () {
// We use "this" just as in the sentence above:
console.log(this.firstName + " " + this.lastName);
// We could have also written:
console.log(person.firstName + " " + person.lastName);
}
}
this
is not assigned a value until an object invokes the function where it is defined. In the global scope, all global variables and functions are defined on the window
object. Therefore, this
in a global function refers to (and has the value of) the global window
object.
When use strict
, this
in global and in anonymous functions that are not bound to any object holds a value of undefined
.
The this
keyword is most misunderstood when: 1) we borrow a method that uses this
, 2) we assign a method that uses this
to a variable, 3) a function that uses this
is passed as a callback function, and 4) this
is used inside a closure — an inner function. (2)
Defined in ECMA Script 6, arrow-functions adopt the this
binding from the
enclosing (function or global) scope.
function foo() {
// return an arrow function
return (a) => {
// `this` here is lexically inherited from `foo()`
console.log(this.a);
};
}
var obj1 = { a: 2 };
var obj2 = { a: 3 };
var bar = foo.call(obj1);
bar.call( obj2 ); // 2, not 3!
While arrow-functions provide an alternative to using bind()
, it’s important to note that they essentially are disabling the traditional this
mechanism in favor of more widely understood lexical scoping. (1)
References:
I have a different take on this
from the other answers that I hope is helpful.
One way to look at JavaScript is to see that there are only 1 way to call a function1. It is
functionObject.call(objectForThis, arg0, arg1, arg2, ...);
There is always some value supplied for objectForThis
.
Everything else is syntactic sugar for functionObject.call
So, everything else can be described by how it translates into functionObject.call
.
If you just call a function then this
is the "global object" which in the browser is the window
function foo() {_x000D_
console.log(this);_x000D_
}_x000D_
_x000D_
foo(); // this is the window object
_x000D_
In other words,
foo();
was effectively translated into
foo.call(window);
Note that if you use strict mode then this
will be undefined
'use strict';_x000D_
_x000D_
function foo() {_x000D_
console.log(this);_x000D_
}_x000D_
_x000D_
foo(); // this is the window object
_x000D_
which means
In other words,
foo();
was effectively translated into
foo.call(undefined);
In JavaScript there are operators like +
and -
and *
. There is also the dot operator which is .
The .
operator when used with a function on the right and an object on the left effectively means "pass object as this
to function.
Example
const bar = {_x000D_
name: 'bar',_x000D_
foo() { _x000D_
console.log(this); _x000D_
},_x000D_
};_x000D_
_x000D_
bar.foo(); // this is bar
_x000D_
In other words bar.foo()
translates into const temp = bar.foo; temp.call(bar);
Note that it doesn't matter how the function was created (mostly...). All of these will produce the same results
const bar = {_x000D_
name: 'bar',_x000D_
fn1() { console.log(this); },_x000D_
fn2: function() { console.log(this); },_x000D_
fn3: otherFunction,_x000D_
};_x000D_
_x000D_
function otherFunction() { console.log(this) };_x000D_
_x000D_
bar.fn1(); // this is bar_x000D_
bar.fn2(); // this is bar_x000D_
bar.fn3(); // this is bar
_x000D_
Again these all are just syntactic sugar for
{ const temp = bar.fn1; temp.call(bar); }
{ const temp = bar.fn2; temp.call(bar); }
{ const temp = bar.fn3; temp.call(bar); }
One other wrinkle is the prototype chain. When you use a.b
JavaScript first looks on the object referenced directly by a
for the property b
. If b
is not found on the object then JavaScript will look in the object's prototype to find b
.
There are various ways to define an object's prototype, the most common in 2019 is the class
keyword. For the purposes of this
though it doesn't matter. What matters is that as it looks in object a
for property b
if it finds property b
on the object or in it's prototype chain if b
ends up being a function then the same rules as above apply. The function b
references will be called using the call
method and passing a
as objectForThis as shown a the top of this answer.
Now. Let's imagine we make a function that explicitly sets this
before calling another function and then call it with the .
(dot) operator
function foo() {_x000D_
console.log(this);_x000D_
}_x000D_
_x000D_
function bar() {_x000D_
const objectForThis = {name: 'moo'}_x000D_
foo.call(objectForThis); // explicitly passing objectForThis_x000D_
}_x000D_
_x000D_
const obj = {_x000D_
bar,_x000D_
};_x000D_
_x000D_
obj.bar();
_x000D_
Following the translation to use call
, obj.bar()
becomes const temp = obj.bar; temp.call(obj);
. When we enter the bar
function we call foo
but we explicitly passed in another object for objectForThis so when we arrive at foo this
is that inner object.
This is what both bind
and =>
functions effectively do. They are more syntactic sugar. They effectively build a new invisible function exactly like bar
above that explicitly sets this
before it calls whatever function is specified. In the case of bind this
is set to whatever you pass to bind
.
function foo() {_x000D_
console.log(this);_x000D_
}_x000D_
_x000D_
const bar = foo.bind({name: 'moo'});_x000D_
_x000D_
// bind created a new invisible function that calls foo with the bound object._x000D_
_x000D_
bar(); _x000D_
_x000D_
// the objectForThis we are passing to bar here is ignored because_x000D_
// the invisible function that bind created will call foo with with_x000D_
// the object we bound above_x000D_
_x000D_
bar.call({name: 'other'});
_x000D_
Note that if functionObject.bind
did not exist we could make our own like this
function bind(fn, objectForThis) {
return function(...args) {
return fn.call(objectForthis, ...args);
};
}
and then we could call it like this
function foo() {
console.log(this);
}
const bar = bind(foo, {name:'abc'});
Arrow functions, the =>
operator are syntactic sugar for bind
const a = () => {console.log(this)};
is the same as
const tempFn = function() {console.log(this)};
const a = tempFn.bind(this);
Just like bind
, a new invisible function is created that calls the given function with a bound value for objectForThis
but unlike bind
the object to be bound is implicit. It's whatever this
happens to be when the =>
operator is used.
So, just like the rules above
const a = () => { console.log(this); } // this is the global object
'use strict';
const a = () => { console.log(this); } // this is undefined
function foo() {
return () => { console.log(this); }
}
const obj = {
foo,
};
const b = obj.foo();
b();
obj.foo()
translates to const temp = obj.foo; temp.call(obj);
which means the arrow operator inside foo
will bind obj
to a new invisible function and return that new invisible function which is assigned to b
. b()
will work as it always has as b.call(window)
or b.call(undefined)
calling the new invisible function that foo
created. That invisible function ignores the this
passed into it and passes obj
as objectForThis` to the arrow function.
The code above translates to
function foo() {
function tempFn() {
console.log(this);
}
return tempFn.bind(this);
}
const obj = {
foo,
};
const b = obj.foo();
b.call(window or undefined if strict mode);
1apply
is another function similar to call
functionName.apply(objectForThis, arrayOfArgs);
But as of ES6 conceptually you can even translate that into
functionName.call(objectForThis, ...arrayOfArgs);
this use for Scope just like this
<script type="text/javascript" language="javascript">
$('#tbleName tbody tr').each(function{
var txt='';
txt += $(this).find("td").eq(0).text();
\\same as above but synatx different
var txt1='';
txt1+=$('#tbleName tbody tr').eq(0).text();
alert(txt1)
});
</script>
value of txt1 and txt is same in Above example $(this)=$('#tbleName tbody tr') is Same
"this" is all about scope. Every function has its own scope, and since everything in JS is an object, even a function can store some values into itself using "this". OOP 101 teaches that "this" is only applicable to instances of an object. Therefore, every-time a function executes, a new "instance" of that function has a new meaning of "this".
Most people get confused when they try to use "this" inside of anonymous closure functions like:
(function(value) { this.value = value; $('.some-elements').each(function(elt){ elt.innerHTML = this.value; // uh oh!! possibly undefined }); })(2);
So here, inside each(), "this" doesn't hold the "value" that you expect it to (from
this.value = value;above it). So, to get over this (no pun intended) problem, a developer could:
(function(value) { var self = this; // small change self.value = value; $('.some-elements').each(function(elt){ elt.innerHTML = self.value; // phew!! == 2 }); })(2);
Try it out; you'll begin to like this pattern of programming
this
in JavaScript always refers to the 'owner' of the function that is being executed.
If no explicit owner is defined, then the top most owner, the window object, is referenced.
So if I did
function someKindOfFunction() {
this.style = 'foo';
}
element.onclick = someKindOfFunction;
this
would refer to the element object. But be careful, a lot of people make this mistake.
<element onclick="someKindOfFunction()">
In the latter case, you merely reference the function, not hand it over to the element. Therefore, this
will refer to the window object.
The value of "this" depends on the "context" in which the function is executed. The context can be any object or the global object, i.e., window.
So the Semantic of "this" is different from the traditional OOP languages. And it causes problems: 1. when a function is passed to another variable (most likely, a callback); and 2. when a closure is invoked from a member method of a class.
In both cases, this is set to window.
this
Javascript:this
is determined by how the function is invoked not, where it was created!this
is determined by the Object which is left of the dot. (window
in global space)this
refers to the DOM element on which the event was called.new
keyword the value of this
refers to the newly created objectthis
with the functions: call
, apply
, bind
let object = {_x000D_
prop1: function () {console.log(this);}_x000D_
}_x000D_
_x000D_
object.prop1(); // object is left of the dot, thus this is object_x000D_
_x000D_
const myFunction = object.prop1 // We store the function in the variable myFunction_x000D_
_x000D_
myFunction(); // Here we are in the global space_x000D_
// myFunction is a property on the global object_x000D_
// Therefore it logs the window object_x000D_
_x000D_
_x000D_
document.querySelector('.foo').addEventListener('click', function () {_x000D_
console.log(this); // This refers to the DOM element the eventListener was invoked from_x000D_
})_x000D_
_x000D_
_x000D_
document.querySelector('.foo').addEventListener('click', () => {_x000D_
console.log(this); // Tip, es6 arrow function don't have their own binding to the this v_x000D_
}) // Therefore this will log the global object
_x000D_
.foo:hover {_x000D_
color: red;_x000D_
cursor: pointer;_x000D_
}
_x000D_
<div class="foo">click me</div>
_x000D_
function Person (name) {_x000D_
this.name = name;_x000D_
}_x000D_
_x000D_
const me = new Person('Willem');_x000D_
// When using the new keyword the this in the constructor function will refer to the newly created object_x000D_
_x000D_
console.log(me.name); _x000D_
// Therefore, the name property was placed on the object created with new keyword.
_x000D_
Daniel, awesome explanation! A couple of words on this and good list of this
execution context pointer in case of event handlers.
In two words, this
in JavaScript points the object from whom (or from whose execution context) the current function was run and it's always read-only, you can't set it anyway (such an attempt will end up with 'Invalid left-hand side in assignment' message.
For event handlers: inline event handlers, such as <element onclick="foo">
, override any other handlers attached earlier and before, so be careful and it's better to stay off of inline event delegation at all.
And thanks to Zara Alaverdyan who inspired me to this list of examples through a dissenting debate :)
el.onclick = foo; // in the foo - obj
el.onclick = function () {this.style.color = '#fff';} // obj
el.onclick = function() {doSomething();} // In the doSomething -
Window
el.addEventListener('click',foo,false) // in the foo - obj
el.attachEvent('onclick, function () { // this }') // window, all the
compliance to IE :)
<button onclick="this.style.color = '#fff';"> // obj
<button onclick="foo"> // In the foo - window, but you can <button
onclick="foo(this)">
This is the best explanation I've seen: Understand JavaScripts this with Clarity
The this reference ALWAYS refers to (and holds the value of) an object—a singular object—and it is usually used inside a function or a method, although it can be used outside a function in the global scope. Note that when we use strict mode, this holds the value of undefined in global functions and in anonymous functions that are not bound to any object.
There are Four Scenarios where this can be confusing:
He gives code examples, explanations, and solutions, which I thought was very helpful.
The this
keyword behaves differently in JavaScript compared to other languages. In Object Oriented languages, the this
keyword refers to the current instance of the class. In JavaScript the value of this
is determined by the invocation context of function (context.function()
) and where it is called.
1. When used in global context
When you use this
in global context, it is bound to global object (window
in browser)
document.write(this); //[object Window]
When you use this
inside a function defined in the global context, this
is still bound to global object since the function is actually made a method of global context.
function f1()
{
return this;
}
document.write(f1()); //[object Window]
Above f1
is made a method of global object. Thus we can also call it on window
object as follows:
function f()
{
return this;
}
document.write(window.f()); //[object Window]
2. When used inside object method
When you use this
keyword inside an object method, this
is bound to the "immediate" enclosing object.
var obj = {
name: "obj",
f: function () {
return this + ":" + this.name;
}
};
document.write(obj.f()); //[object Object]:obj
Above I have put the word immediate in double quotes. It is to make the point that if you nest the object inside another object, then this
is bound to the immediate parent.
var obj = {
name: "obj1",
nestedobj: {
name:"nestedobj",
f: function () {
return this + ":" + this.name;
}
}
}
document.write(obj.nestedobj.f()); //[object Object]:nestedobj
Even if you add function explicitly to the object as a method, it still follows above rules, that is this
still points to the immediate parent object.
var obj1 = {
name: "obj1",
}
function returnName() {
return this + ":" + this.name;
}
obj1.f = returnName; //add method to object
document.write(obj1.f()); //[object Object]:obj1
3. When invoking context-less function
When you use this
inside function that is invoked without any context (i.e. not on any object), it is bound to the global object (window
in browser)(even if the function is defined inside the object) .
var context = "global";
var obj = {
context: "object",
method: function () {
function f() {
var context = "function";
return this + ":" +this.context;
};
return f(); //invoked without context
}
};
document.write(obj.method()); //[object Window]:global
Trying it all with functions
We can try above points with functions too. However there are some differences.
this
. to specify them.new
operator.Below I tried out all the things that we did with Object and this
above, but by first creating function instead of directly writing an object.
/*********************************************************************
1. When you add variable to the function using this keyword, it
gets added to the function prototype, thus allowing all function
instances to have their own copy of the variables added.
*********************************************************************/
function functionDef()
{
this.name = "ObjDefinition";
this.getName = function(){
return this+":"+this.name;
}
}
obj1 = new functionDef();
document.write(obj1.getName() + "<br />"); //[object Object]:ObjDefinition
/*********************************************************************
2. Members explicitly added to the function protorype also behave
as above: all function instances have their own copy of the
variable added.
*********************************************************************/
functionDef.prototype.version = 1;
functionDef.prototype.getVersion = function(){
return "v"+this.version; //see how this.version refers to the
//version variable added through
//prototype
}
document.write(obj1.getVersion() + "<br />"); //v1
/*********************************************************************
3. Illustrating that the function variables added by both above
ways have their own copies across function instances
*********************************************************************/
functionDef.prototype.incrementVersion = function(){
this.version = this.version + 1;
}
var obj2 = new functionDef();
document.write(obj2.getVersion() + "<br />"); //v1
obj2.incrementVersion(); //incrementing version in obj2
//does not affect obj1 version
document.write(obj2.getVersion() + "<br />"); //v2
document.write(obj1.getVersion() + "<br />"); //v1
/*********************************************************************
4. `this` keyword refers to the immediate parent object. If you
nest the object through function prototype, then `this` inside
object refers to the nested object not the function instance
*********************************************************************/
functionDef.prototype.nestedObj = { name: 'nestedObj',
getName1 : function(){
return this+":"+this.name;
}
};
document.write(obj2.nestedObj.getName1() + "<br />"); //[object Object]:nestedObj
/*********************************************************************
5. If the method is on an object's prototype chain, `this` refers
to the object the method was called on, as if the method was on
the object.
*********************************************************************/
var ProtoObj = { fun: function () { return this.a } };
var obj3 = Object.create(ProtoObj); //creating an object setting ProtoObj
//as its prototype
obj3.a = 999; //adding instance member to obj3
document.write(obj3.fun()+"<br />");//999
//calling obj3.fun() makes
//ProtoObj.fun() to access obj3.a as
//if fun() is defined on obj3
4. When used inside constructor function.
When the function is used as a constructor (that is when it is called with new
keyword), this
inside function body points to the new object being constructed.
var myname = "global context";
function SimpleFun()
{
this.myname = "simple function";
}
var obj1 = new SimpleFun(); //adds myname to obj1
//1. `new` causes `this` inside the SimpleFun() to point to the
// object being constructed thus adding any member
// created inside SimipleFun() using this.membername to the
// object being constructed
//2. And by default `new` makes function to return newly
// constructed object if no explicit return value is specified
document.write(obj1.myname); //simple function
5. When used inside function defined on prototype chain
If the method is on an object's prototype chain, this
inside such method refers to the object the method was called on, as if the method is defined on the object.
var ProtoObj = {
fun: function () {
return this.a;
}
};
//Object.create() creates object with ProtoObj as its
//prototype and assigns it to obj3, thus making fun()
//to be the method on its prototype chain
var obj3 = Object.create(ProtoObj);
obj3.a = 999;
document.write(obj3.fun()); //999
//Notice that fun() is defined on obj3's prototype but
//`this.a` inside fun() retrieves obj3.a
6. Inside call(), apply() and bind() functions
Function.prototype
.this
which will be used while the function is being executed. They also take any parameters to be passed to the original function when it is invoked.fun.apply(obj1 [, argsArray])
Sets obj1
as the value of this
inside fun()
and calls fun()
passing elements of argsArray
as its arguments.fun.call(obj1 [, arg1 [, arg2 [,arg3 [, ...]]]])
- Sets obj1
as the value of this
inside fun()
and calls fun()
passing arg1, arg2, arg3, ...
as its arguments.fun.bind(obj1 [, arg1 [, arg2 [,arg3 [, ...]]]])
- Returns the reference to the function fun
with this
inside fun bound to obj1
and parameters of fun
bound to the parameters specified arg1, arg2, arg3,...
.apply
, call
and bind
must have become apparent. apply
allows to specify the arguments to function as array-like object i.e. an object with a numeric length
property and corresponding non-negative integer properties. Whereas call
allows to specify the arguments to the function directly. Both apply
and call
immediately invokes the function in the specified context and with the specified arguments. On the other hand, bind
simply returns the function bound to the specified this
value and the arguments. We can capture the reference to this returned function by assigning it to a variable and later we can call it any time.function add(inc1, inc2)
{
return this.a + inc1 + inc2;
}
var o = { a : 4 };
document.write(add.call(o, 5, 6)+"<br />"); //15
//above add.call(o,5,6) sets `this` inside
//add() to `o` and calls add() resulting:
// this.a + inc1 + inc2 =
// `o.a` i.e. 4 + 5 + 6 = 15
document.write(add.apply(o, [5, 6]) + "<br />"); //15
// `o.a` i.e. 4 + 5 + 6 = 15
var g = add.bind(o, 5, 6); //g: `o.a` i.e. 4 + 5 + 6
document.write(g()+"<br />"); //15
var h = add.bind(o, 5); //h: `o.a` i.e. 4 + 5 + ?
document.write(h(6) + "<br />"); //15
// 4 + 5 + 6 = 15
document.write(h() + "<br />"); //NaN
//no parameter is passed to h()
//thus inc2 inside add() is `undefined`
//4 + 5 + undefined = NaN</code>
7. this
inside event handlers
this
directly inside event handling function refers to the corresponding element. Such direct function assignment can be done using addeventListener
method or through the traditional event registration methods like onclick
.this
directly inside the event property (like <button onclick="...this..." >
) of the element, it refers to the element.this
indirectly through the other function called inside the event handling function or event property resolves to the global object window
.attachEvent
. Instead of assigning the function to the event handler (and the thus making the function method of the element), it calls the function on the event (effectively calling it in global context).I recommend to better try this in JSFiddle.
<script>
function clickedMe() {
alert(this + " : " + this.tagName + " : " + this.id);
}
document.getElementById("button1").addEventListener("click", clickedMe, false);
document.getElementById("button2").onclick = clickedMe;
document.getElementById("button5").attachEvent('onclick', clickedMe);
</script>
<h3>Using `this` "directly" inside event handler or event property</h3>
<button id="button1">click() "assigned" using addEventListner() </button><br />
<button id="button2">click() "assigned" using click() </button><br />
<button id="button3" onclick="alert(this+ ' : ' + this.tagName + ' : ' + this.id);">used `this` directly in click event property</button>
<h3>Using `this` "indirectly" inside event handler or event property</h3>
<button onclick="alert((function(){return this + ' : ' + this.tagName + ' : ' + this.id;})());">`this` used indirectly, inside function <br /> defined & called inside event property</button><br />
<button id="button4" onclick="clickedMe()">`this` used indirectly, inside function <br /> called inside event property</button> <br />
IE only: <button id="button5">click() "attached" using attachEvent() </button>
8. this
in ES6 arrow function
In an arrow function, this
will behave like common variables: it will be inherited from its lexical scope. The function's this
, where the arrow function is defined, will be the arrow function's this
.
So, that's the same behavior as:
(function(){}).bind(this)
See the following code:
const globalArrowFunction = () => {
return this;
};
console.log(globalArrowFunction()); //window
const contextObject = {
method1: () => {return this},
method2: function(){
return () => {return this};
}
};
console.log(contextObject.method1()); //window
const contextLessFunction = contextObject.method1;
console.log(contextLessFunction()); //window
console.log(contextObject.method2()()) //contextObject
const innerArrowFunction = contextObject.method2();
console.log(innerArrowFunction()); //contextObject
In pseudoclassical terms, the way many lectures teach the 'this' keyword is as an object instantiated by a class or object constructor. Each time a new object is constructed from a class, imagine that under the hood a local instance of a 'this' object is created and returned. I remember it taught like this:
function Car(make, model, year) {
var this = {}; // under the hood, so to speak
this.make = make;
this.model = model;
this.year = year;
return this; // under the hood
}
var mycar = new Car('Eagle', 'Talon TSi', 1993);
// ========= under the hood
var this = {};
this.make = 'Eagle';
this.model = 'Talon TSi';
this.year = 1993;
return this;
There is a lot of confusion regarding how "this" keyword is interpreted in JavaScript. Hopefully this article will lay all those to rest once and for all. And a lot more. Please read the entire article carefully. Be forewarned that this article is long.
Irrespective of the context in which it is used, "this" always references the "current object" in Javascript. However, what the "current object" is differs according to context. The context may be exactly 1 of the 6 following:
The following describes each of this contexts one by one:
Global Context (i.e. Outside all functions):
Outside all functions (i.e. in global context) the "current object" (and hence the value of "this") is always the "window" object for browsers.
Inside Direct "Non Bound Function" Call:
Inside a Direct "Non Bound Function" Call, the object that invoked the function call becomes the "current object" (and hence the value of "this"). If a function is called without a explicit current object, the current object is either the "window" object (For Non Strict Mode) or undefined (For Strict Mode) . Any function (or variable) defined in Global Context automatically becomes a property of the "window" object.For e.g Suppose function is defined in Global Context as
function UserDefinedFunction(){
alert(this)
}
it becomes the property of the window object, as if you have defined it as
window.UserDefinedFunction=function(){
alert(this)
}
In "Non Strict Mode", Calling/Invoking this function directly through "UserDefinedFunction()" will automatically call/invoke it as "window.UserDefinedFunction()" making "window" as the "current object" (and hence the value of "this") within "UserDefinedFunction".Invoking this function in "Non Strict Mode" will result in the following
UserDefinedFunction() // displays [object Window] as it automatically gets invoked as window.UserDefinedFunction()
In "Strict Mode", Calling/Invoking the function directly through "UserDefinedFunction()" will "NOT" automatically call/invoke it as "window.UserDefinedFunction()".Hence the "current object" (and the value of "this") within "UserDefinedFunction" shall be undefined. Invoking this function in "Strict Mode" will result in the following
UserDefinedFunction() // displays undefined
However, invoking it explicitly using window object shall result in the following
window.UserDefinedFunction() // "always displays [object Window] irrespective of mode."
Let us look at another example. Please look at the following code
function UserDefinedFunction()
{
alert(this.a + "," + this.b + "," + this.c + "," + this.d)
}
var o1={
a:1,
b:2,
f:UserDefinedFunction
}
var o2={
c:3,
d:4,
f:UserDefinedFunction
}
o1.f() // Shall display 1,2,undefined,undefined
o2.f() // Shall display undefined,undefined,3,4
In the above example we see that when "UserDefinedFunction" was invoked through o1, "this" takes value of o1 and the value of its properties "a" and "b" get displayed. The value of "c" and "d" were shown as undefined as o1 does not define these properties
Similarly when "UserDefinedFunction" was invoked through o2, "this" takes value of o2 and the value of its properties "c" and "d" get displayed.The value of "a" and "b" were shown as undefined as o2 does not define these properties.
Inside Indirect "Non Bound Function" Call through functionName.call and functionName.apply:
When a "Non Bound Function" is called through functionName.call or functionName.apply, the "current object" (and hence the value of "this") is set to the value of "this" parameter (first parameter) passed to call/apply. The following code demonstrates the same.
function UserDefinedFunction()
{
alert(this.a + "," + this.b + "," + this.c + "," + this.d)
}
var o1={
a:1,
b:2,
f:UserDefinedFunction
}
var o2={
c:3,
d:4,
f:UserDefinedFunction
}
UserDefinedFunction.call(o1) // Shall display 1,2,undefined,undefined
UserDefinedFunction.apply(o1) // Shall display 1,2,undefined,undefined
UserDefinedFunction.call(o2) // Shall display undefined,undefined,3,4
UserDefinedFunction.apply(o2) // Shall display undefined,undefined,3,4
o1.f.call(o2) // Shall display undefined,undefined,3,4
o1.f.apply(o2) // Shall display undefined,undefined,3,4
o2.f.call(o1) // Shall display 1,2,undefined,undefined
o2.f.apply(o1) // Shall display 1,2,undefined,undefined
The above code clearly shows that the "this" value for any "NON Bound Function" can be altered through call/apply. Also,if the "this" parameter is not explicitly passed to call/apply, "current object" (and hence the value of "this") is set to "window" in Non strict mode and "undefined" in strict mode.
Inside "Bound Function" Call (i.e. a function that has been bound by calling functionName.bind):
A bound function is a function whose "this" value has been fixed. The following code demonstrated how "this" works in case of bound function
function UserDefinedFunction()
{
alert(this.a + "," + this.b + "," + this.c + "," + this.d)
}
var o1={
a:1,
b:2,
f:UserDefinedFunction,
bf:null
}
var o2={
c:3,
d:4,
f:UserDefinedFunction,
bf:null
}
var bound1=UserDefinedFunction.bind(o1); // permanantly fixes "this" value of function "bound1" to Object o1
bound1() // Shall display 1,2,undefined,undefined
var bound2=UserDefinedFunction.bind(o2); // permanantly fixes "this" value of function "bound2" to Object o2
bound2() // Shall display undefined,undefined,3,4
var bound3=o1.f.bind(o2); // permanantly fixes "this" value of function "bound3" to Object o2
bound3() // Shall display undefined,undefined,3,4
var bound4=o2.f.bind(o1); // permanantly fixes "this" value of function "bound4" to Object o1
bound4() // Shall display 1,2,undefined,undefined
o1.bf=UserDefinedFunction.bind(o2) // permanantly fixes "this" value of function "o1.bf" to Object o2
o1.bf() // Shall display undefined,undefined,3,4
o2.bf=UserDefinedFunction.bind(o1) // permanantly fixes "this" value of function "o2.bf" to Object o1
o2.bf() // Shall display 1,2,undefined,undefined
bound1.call(o2) // Shall still display 1,2,undefined,undefined. "call" cannot alter the value of "this" for bound function
bound1.apply(o2) // Shall still display 1,2,undefined,undefined. "apply" cannot alter the value of "this" for bound function
o2.bf.call(o2) // Shall still display 1,2,undefined,undefined. "call" cannot alter the value of "this" for bound function
o2.bf.apply(o2) // Shall still display 1,2,undefined,undefined."apply" cannot alter the value of "this" for bound function
As given in the code above, "this" value for any "Bound Function" CANNOT be altered through call/apply. Also, if the "this" parameter is not explicitly passed to bind, "current object" (and hence the value of "this" ) is set to "window" in Non strict mode and "undefined" in strict mode. One more thing. Binding an already bound function does not change the value of "this". It remains set as the value set by first bind function.
While Object Creation through "new":
Inside a constructor function, the "current object" (and hence the value of "this") references the object that is currently being created through "new" irrespective of the bind status of the function. However if the constructor is a bound function it shall get called with predefined set of arguments as set for the bound function.
Inside Inline DOM event handler:
Please look at the following HTML Snippet
<button onclick='this.style.color=white'>Hello World</button>
<div style='width:100px;height:100px;' onclick='OnDivClick(event,this)'>Hello World</div>
The "this" in above examples refer to "button" element and the "div" element respectively.
In the first example, the font color of the button shall be set to white when it is clicked.
In the second example when the "div" element is clicked it shall call the OnDivClick function with its second parameter referencing the clicked div element. However the value of "this" within OnDivClick SHALL NOT reference the clicked div element. It shall be set as the "window object" or "undefined" in Non strict and Strict Modes respectively (if OnDivClick is an unbound function) or set to a predefined Bound value (if OnDivClick is a bound function)
The following summarizes the entire article
In Global Context "this" always refers to the "window" object
Whenever a function is invoked, it is invoked in context of an object ("current object"). If the current object is not explicitly provided, the current object is the "window object" in NON Strict Mode and "undefined" in Strict Mode by default.
The value of "this" within a Non Bound function is the reference to object in context of which the function is invoked ("current object")
The value of "this" within a Non Bound function can be overriden by call and apply methods of the function.
The value of "this" is fixed for a Bound function and cannot be overriden by call and apply methods of the function.
Binding and already bound function does not change the value of "this". It remains set as the value set by first bind function.
The value of "this" within a constructor is the object that is being created and initialized
The value of "this" within an inline DOM event handler is reference to the element for which the event handler is given.
this
Consider the following function:
function foo() {
console.log("bar");
console.log(this);
}
foo(); // calling the function
Note that we are running this in the normal mode, i.e. strict mode is not used.
When running in a browser, the value of this
would be logged as window
. This is because window
is the global variable in a web browser's scope.
If you run this same piece of code in an environment like node.js, this
would refer to the global variable in your app.
Now if we run this in strict mode by adding the statement "use strict";
to the beginning of the function declaration, this
would no longer refer to the global variable in either of the environments. This is done to avoid confusions in strict mode. this
would, in this case just log undefined
, because that is what it is, it is not defined.
In the following cases, we would see how to manipulate the value of this
.
There are different ways to do this. If you have called native methods in Javascript like forEach
and slice
, you should already know that the this
variable in that case refers to the Object
on which you called that function (Note that in javascript, just about everything is an Object
, including Array
s and Function
s). Take the following code for example.
var myObj = {key: "Obj"};
myObj.logThis = function () {
// I am a method
console.log(this);
}
myObj.logThis(); // myObj is logged
If an Object
contains a property which holds a Function
, the property is called a method. This method, when called, will always have it's this
variable set to the Object
it is associated with. This is true for both strict and non-strict modes.
Note that if a method is stored (or rather, copied) in another variable, the reference to this
is no longer preserved in the new variable. For example:
// continuing with the previous code snippet
var myVar = myObj.logThis;
myVar();
// logs either of window/global/undefined based on mode of operation
Considering a more commonly practical scenario:
var el = document.getElementById('idOfEl');
el.addEventListener('click', function() { console.log(this) });
// the function called by addEventListener contains this as the reference to the element
// so clicking on our element would log that element itself
new
keywordConsider a constructor function in Javascript:
function Person (name) {
this.name = name;
this.sayHello = function () {
console.log ("Hello", this);
}
}
var awal = new Person("Awal");
awal.sayHello();
// In `awal.sayHello`, `this` contains the reference to the variable `awal`
How does this work? Well, let's see what happens when we use the new
keyword.
new
keyword would immediately initialize an Object
of type Person
.Object
has its constructor set to Person
. Also, note that typeof awal
would return Object
only.Object
would be assigned the prototype of Person.prototype
. This means that any method or property in the Person
prototype would be available to all instances of Person
, including awal
.Person
itself is now invoked; this
being a reference to the newly constructed object awal
.Pretty straightforward, eh?
Note that the official ECMAScript spec nowhere states that such types of functions are actual constructor
functions. They are just normal functions, and new
can be used on any function. It's just that we use them as such, and so we call them as such only.
call
and apply
So yeah, since function
s are also Objects
(and in-fact first class variables in Javascript), even functions have methods which are... well, functions themselves.
All functions inherit from the global Function
, and two of its many methods are call
and apply
, and both can be used to manipulate the value of this
in the function on which they are called.
function foo () { console.log (this, arguments); }
var thisArg = {myObj: "is cool"};
foo.call(thisArg, 1, 2, 3);
This is a typical example of using call
. It basically takes the first parameter and sets this
in the function foo
as a reference to thisArg
. All other parameters passed to call
is passed to the function foo
as arguments.
So the above code will log {myObj: "is cool"}, [1, 2, 3]
in the console. Pretty nice way to change the value of this
in any function.
apply
is almost the same as call
accept that it takes only two parameters: thisArg
and an array which contains the arguments to be passed to the function. So the above call
call can be translated to apply
like this:
foo.apply(thisArg, [1,2,3])
Note that call
and apply
can override the value of this
set by dot method invocation we discussed in the second bullet.
Simple enough :)
bind
!bind
is a brother of call
and apply
. It is also a method inherited by all functions from the global Function
constructor in Javascript. The difference between bind
and call
/apply
is that both call
and apply
will actually invoke the function. bind
, on the other hand, returns a new function with the thisArg
and arguments
pre-set. Let's take an example to better understand this:
function foo (a, b) {
console.log (this, arguments);
}
var thisArg = {myObj: "even more cool now"};
var bound = foo.bind(thisArg, 1, 2);
console.log (typeof bound); // logs `function`
console.log (bound);
/* logs `function () { native code }` */
bound(); // calling the function returned by `.bind`
// logs `{myObj: "even more cool now"}, [1, 2]`
See the difference between the three? It is subtle, but they are used differently. Like call
and apply
, bind
will also over-ride the value of this
set by dot-method invocation.
Also note that neither of these three functions do any change to the original function. call
and apply
would return the value from freshly constructed functions while bind
will return the freshly constructed function itself, ready to be called.
Sometimes, you don't like the fact that this
changes with scope, especially nested scope. Take a look at the following example.
var myObj = {
hello: function () {
return "world"
},
myMethod: function () {
// copy this, variable names are case-sensitive
var that = this;
// callbacks ftw \o/
foo.bar("args", function () {
// I want to call `hello` here
this.hello(); // error
// but `this` references to `foo` damn!
// oh wait we have a backup \o/
that.hello(); // "world"
});
}
};
In the above code, we see that the value of this
changed with the nested scope, but we wanted the value of this
from the original scope. So we 'copied' this
to that
and used the copy instead of this
. Clever, eh?
Index:
this
by default?new
keyword?this
with call
and apply
?bind
.this
to solve nested-scope issues.Probably the most detailed and comprehensive article on this
is the following:
Gentle explanation of 'this' keyword in JavaScript
The idea behind this
is to understand that the function invocation types have the significant importance on setting this
value.
When having troubles identifying this
, do not ask yourself:
Where is
this
taken from?
but do ask yourself:
How is the function invoked?
For an arrow function (special case of context transparency) ask yourself:
What value has
this
where the arrow function is defined?
This mindset is correct when dealing with this
and will save you from headache.
Every execution context in javascript has a this parameter that is set by:
eval
You can set the value of this using func.call
, func.apply
or func.bind
.
By default, and what confuses most beginners, when a listener is called after an event is raised on a DOM element, the this value of the function is the DOM element.
jQuery makes this trivial to change with jQuery.proxy.
A little bit info about this keyword
Let's log this
keyword to the console in global scope without any more code but
console.log(this)
In Client/Browser this
keyword is a global object which is window
console.log(this === window) // true
and
In Server/Node/Javascript runtime this
keyword is also a global object which is module.exports
console.log(this === module.exports) // true
console.log(this === exports) // true
Keep in mind exports
is just a reference to module.exports
Whould this help? (Most confusion of 'this' in javascript is coming from the fact that it generally is not linked to your object, but to the current executing scope -- that might not be exactly how it works but is always feels like that to me -- see the article for a complete explanation)
To understand "this" properly one must understand the context and scope and difference between them.
Scope: In javascript scope is related to the visibility of the variables, scope achieves through the use of the function. (Read more about scope)
Context: Context is related to objects. It refers to the object to which a function belongs. When you use the JavaScript “this” keyword, it refers to the object to which function belongs. For example, inside of a function, when you say: “this.accoutNumber”, you are referring to the property “accoutNumber”, that belongs to the object to which that function belongs.
If the object “myObj” has a method called “getMyName”, when the JavaScript keyword “this” is used inside of “getMyName”, it refers to “myObj”. If the function “getMyName” were executed in the global scope, then “this” refers to the window object (except in strict mode).
Now let's see some example:
<script>
console.log('What is this: '+this);
console.log(this);
</script>
Runnig abobve code in browser output will:
According to the output you are inside of the context of the window object, it is also visible that window prototype refers to the Object.
Now let's try inside of a function:
<script>
function myFunc(){
console.log('What is this: '+this);
console.log(this);
}
myFunc();
</script>
Output:
The output is the same because we logged 'this' variable in the global scope and we logged it in functional scope, we didn't change the context. In both case context was same, related to widow object.
Now let's create our own object. In javascript, you can create an object in many ways.
<script>
var firstName = "Nora";
var lastName = "Zaman";
var myObj = {
firstName:"Lord",
lastName:'Baron',
printNameGetContext:function(){
console.log(firstName + " "+lastName);
console.log(this.firstName +" "+this.lastName);
return this;
}
}
var context = myObj.printNameGetContext();
console.log(context);
</script>
So from the above example, we found that 'this' keyword is referring to a new context that is related to myObj, and myObject also has prototype chain to Object.
Let's go throw another example:
<body>
<button class="btn">Click Me</button>
<script>
function printMe(){
//Terminal2: this function declared inside window context so this function belongs to the window object.
console.log(this);
}
document.querySelector('.btn').addEventListener('click', function(){
//Terminal1: button context, this callback function belongs to DOM element
console.log(this);
printMe();
})
</script>
</body>
output: Make sense right? (read comments)
If you having trouble to understand the above example let's try with our own callback;
<script>
var myObj = {
firstName:"Lord",
lastName:'Baron',
printName:function(callback1, callback2){
//Attaching callback1 with this myObj context
this.callback1 = callback1;
this.callback1(this.firstName +" "+this.lastName)
//We did not attached callback2 with myObj so, it's reamin with window context by default
callback2();
/*
//test bellow codes
this.callback2 = callback2;
this.callback2();
*/
}
}
var callback2 = function (){
console.log(this);
}
myObj.printName(function(data){
console.log(data);
console.log(this);
}, callback2);
</script>
Now let's Understand Scope, Self, IIFE and THIS how behaves
var color = 'red'; // property of window
var obj = {
color:'blue', // property of window
printColor: function(){ // property of obj, attached with obj
var self = this;
console.log('In printColor -- this.color: '+this.color);
console.log('In printColor -- self.color: '+self.color);
(function(){ // decleard inside of printColor but not property of object, it will executed on window context.
console.log(this)
console.log('In IIFE -- this.color: '+this.color);
console.log('In IIFE -- self.color: '+self.color);
})();
function nestedFunc(){// decleard inside of printColor but not property of object, it will executed on window context.
console.log('nested fun -- this.color: '+this.color);
console.log('nested fun -- self.color: '+self.color);
}
nestedFunc(); // executed on window context
return nestedFunc;
}
};
obj.printColor()(); // returned function executed on window context
</script>
this
is one of the misunderstood concept in JavaScript because it behaves little differently from place to place. Simply, this
refers to the "owner" of the function we are currently executing.
this
helps to get the current object (a.k.a. execution context) we work with. If you understand in which object the current function is getting executed, you can understand easily what current this
is
var val = "window.val"
var obj = {
val: "obj.val",
innerMethod: function () {
var val = "obj.val.inner",
func = function () {
var self = this;
return self.val;
};
return func;
},
outerMethod: function(){
return this.val;
}
};
//This actually gets executed inside window object
console.log(obj.innerMethod()()); //returns window.val
//Breakdown in to 2 lines explains this in detail
var _inn = obj.innerMethod();
console.log(_inn()); //returns window.val
console.log(obj.outerMethod()); //returns obj.val
Above we create 3 variables with same name 'val'. One in global context, one inside obj and the other inside innerMethod of obj. JavaScript resolves identifiers within a particular context by going up the scope chain from local go global.
Few places where this
can be differentiated
var status = 1;
var helper = {
status : 2,
getStatus: function () {
return this.status;
}
};
var theStatus1 = helper.getStatus(); //line1
console.log(theStatus1); //2
var theStatus2 = helper.getStatus;
console.log(theStatus2()); //1
When line1 is executed, JavaScript establishes an execution context (EC) for the function call, setting this
to the object referenced by whatever came before the last ".". so in the last line you can understand that a()
was executed in the global context which is the window
.
this
can be used to refer to the object being created
function Person(name){
this.personName = name;
this.sayHello = function(){
return "Hello " + this.personName;
}
}
var person1 = new Person('Scott');
console.log(person1.sayHello()); //Hello Scott
var person2 = new Person('Hugh');
var sayHelloP2 = person2.sayHello;
console.log(sayHelloP2()); //Hello undefined
When new Person()
is executed, a completely new object is created. Person
is called and its this
is set to reference that new object.
function testFunc() {
this.name = "Name";
this.myCustomAttribute = "Custom Attribute";
return this;
}
var whatIsThis = testFunc();
console.log(whatIsThis); //window
var whatIsThis2 = new testFunc();
console.log(whatIsThis2); //testFunc() / object
console.log(window.myCustomAttribute); //Custom Attribute
If we miss new
keyword, whatIsThis
referes to the most global context it can find(window
)
If the event handler is inline, this
refers to global object
<script type="application/javascript">
function click_handler() {
alert(this); // alerts the window object
}
</script>
<button id='thebutton' onclick='click_handler()'>Click me!</button>
When adding event handler through JavaScript, this
refers to DOM element that generated the event.
.apply()
.call()
and .bind()
var that = this
means in JavaScriptSource: Stackoverflow.com