What is the difference between using call
and apply
to invoke a function?
var func = function() {
alert('hello!');
};
func.apply();
vs func.call();
Are there performance differences between the two aforementioned methods? When is it best to use call
over apply
and vice versa?
This question is related to
javascript
performance
function
dynamic
The call()
method calls a function with a given this
value and arguments provided individually.
apply()
-
Similar to the call()
method, the first parameter in the apply()
method sets the this
value which is the object upon which the function is invoked. In this case, it's the obj
object above. The only difference between the apply()
and call()
method is that the second parameter of the apply()
method accepts the arguments to the actual function as an array.
A well explained by flatline. I just want to add a simple example. which makes it easy to understand for beginners.
func.call(context, args1 , args2 ); // pass arguments as "," saprated value
func.apply(context, [args1 , args2 ]); // pass arguments as "Array"
we also use "Call" and "Apply" method for changing reference as defined in code below
let Emp1 = {_x000D_
name: 'X',_x000D_
getEmpDetail: function (age, department) {_x000D_
console.log('Name :', this.name, ' Age :', age, ' Department :', department)_x000D_
}_x000D_
}_x000D_
Emp1.getEmpDetail(23, 'Delivery')_x000D_
_x000D_
// 1st approch of chenging "this"_x000D_
let Emp2 = {_x000D_
name: 'Y',_x000D_
getEmpDetail: Emp1.getEmpDetail_x000D_
}_x000D_
Emp2.getEmpDetail(55, 'Finance')_x000D_
_x000D_
// 2nd approch of changing "this" using "Call" and "Apply"_x000D_
let Emp3 = {_x000D_
name: 'Z',_x000D_
}_x000D_
_x000D_
Emp1.getEmpDetail.call(Emp3, 30, 'Admin') _x000D_
// here we have change the ref from **Emp1 to Emp3** object_x000D_
// now this will print "Name = X" because it is pointing to Emp3 object_x000D_
Emp1.getEmpDetail.apply(Emp3, [30, 'Admin']) //
_x000D_
The difference is that call()
takes the function arguments separately, and apply()
takes the function arguments in an array.
Here's a small-ish post, I wrote on this:
http://sizeableidea.com/call-versus-apply-javascript/
var obj1 = { which : "obj1" },
obj2 = { which : "obj2" };
function execute(arg1, arg2){
console.log(this.which, arg1, arg2);
}
//using call
execute.call(obj1, "dan", "stanhope");
//output: obj1 dan stanhope
//using apply
execute.apply(obj2, ["dan", "stanhope"]);
//output: obj2 dan stanhope
//using old school
execute("dan", "stanhope");
//output: undefined "dan" "stanhope"
The main difference is, using call, we can change the scope and pass arguments as normal, but apply lets you call it using arguments as an Array (pass them as an array). But in terms of what they to do in your code, they are pretty similar.
While the syntax of this function is almost identical to that of apply(), the fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.
So as you see, there is not a big difference, but still, there are cases we prefer using call() or apply(). For example, look at the code below, which finding the smallest and largest number in an array from MDN, using the apply method:
// min/max number in an array
var numbers = [5, 6, 2, 3, 7];
// using Math.min/Math.max apply
var max = Math.max.apply(null, numbers);
// This about equal to Math.max(numbers[0], ...)
// or Math.max(5, 6, ...)
var min = Math.min.apply(null, numbers)
So the main difference is just the way we passing the arguments:
Call:
function.call(thisArg, arg1, arg2, ...);
Apply:
function.apply(thisArg, [argsArray]);
call() It’s a predefined method in javascript. This method invokes a method (function) by specifying the owner object.
function sayHello(){
return "Hello " + this.name;
}
var obj = {name: "Sandy"};
sayHello.call(obj);
// Returns "Hello Sandy"
Call accepts argument
function saySomething(message){
return this.name + " is " + message;
}
var person4 = {name: "John"};
saySomething.call(person4, "awesome");
// Returns "John is awesome"
apply() The apply method is similar to the call() method. The only difference is that, call() method takes arguments separately whereas, apply() method takes arguments as an array.
example
function saySomething(message){
return this.name + " is " + message;
}
var person4 = {name: "John"};
saySomething.apply(person4, ["awesome"]);
Call() takes comma-separated arguments, ex:
.call(scope, arg1, arg2, arg3)
and apply() takes an array of arguments, ex:
.apply(scope, [arg1, arg2, arg3])
here are few more usage examples: http://blog.i-evaluation.com/2012/08/15/javascript-call-and-apply/
To answer the part about when to use each function, use apply
if you don't know the number of arguments you will be passing, or if they are already in an array or array-like object (like the arguments
object to forward your own arguments. Use call
otherwise, since there's no need to wrap the arguments in an array.
f.call(thisObject, a, b, c); // Fixed number of arguments
f.apply(thisObject, arguments); // Forward this function's arguments
var args = [];
while (...) {
args.push(some_value());
}
f.apply(thisObject, args); // Unknown number of arguments
When I'm not passing any arguments (like your example), I prefer call
since I'm calling the function. apply
would imply you are applying the function to the (non-existent) arguments.
There shouldn't be any performance differences, except maybe if you use apply
and wrap the arguments in an array (e.g. f.apply(thisObject, [a, b, c])
instead of f.call(thisObject, a, b, c)
). I haven't tested it, so there could be differences, but it would be very browser specific. It's likely that call
is faster if you don't already have the arguments in an array and apply
is faster if you do.
Both call and apply the same way. It calls immediately when we use call and apply.
Both call and apply takes "this" parameter as the first argument and the second argument only differs.
the call takes the arguments of the functions as a list (comma ) Apply takes the arguments of the functions as an array.
You can find the complete difference between bind, call, and apply in the bellow youtube video.
Both call()
and apply()
are methods which are located on Function.prototype
. Therefore they are available on every function object via the prototype chain. Both call()
and apply()
can execute a function with a specified value of the this
.
The main difference between call()
and apply()
is the way you have to pass in arguments into it. In both call()
and apply()
you pass as a first argument the object you want to be the value as this
. The other arguments differ in the following way:
call()
you have to put in the arguments normally (starting from the second argument)apply()
you have to pass in array of arguments.let obj = {_x000D_
val1: 5,_x000D_
val2: 10_x000D_
}_x000D_
_x000D_
const summation = function (val3, val4) {_x000D_
return this.val1 + this.val2 + val3 + val4;_x000D_
}_x000D_
_x000D_
console.log(summation.apply(obj, [2 ,3]));_x000D_
// first we assign we value of this in the first arg_x000D_
// with apply we have to pass in an array_x000D_
_x000D_
_x000D_
console.log(summation.call(obj, 2, 3));_x000D_
// with call we can pass in each arg individually
_x000D_
The this
value can be tricky sometimes in javascript. The value of this
determined when a function is executed not when a function is defined. If our function is dependend on a right this
binding we can use call()
and apply()
to enforce this behaviour. For example:
var name = 'unwantedGlobalName';_x000D_
_x000D_
const obj = {_x000D_
name: 'Willem',_x000D_
sayName () { console.log(this.name);}_x000D_
}_x000D_
_x000D_
_x000D_
let copiedMethod = obj.sayName;_x000D_
// we store the function in the copiedmethod variable_x000D_
_x000D_
_x000D_
_x000D_
copiedMethod();_x000D_
// this is now window, unwantedGlobalName gets logged_x000D_
_x000D_
copiedMethod.call(obj);_x000D_
// we enforce this to be obj, Willem gets logged
_x000D_
Another example with Call, Apply and Bind. The difference between Call and Apply is evident, but Bind works like this:
}
function Person(name) {
this.name = name;
}
Person.prototype.getName = function(a,b) {
return this.name + " " + a + " " + b;
}
var reader = new Person('John Smith');
reader.getName = function() {
// Apply and Call executes the function and returns value
// Also notice the different ways of extracting 'getName' prototype
var baseName = Object.getPrototypeOf(this).getName.apply(this,["is a", "boy"]);
console.log("Apply: " + baseName);
var baseName = Object.getPrototypeOf(reader).getName.call(this, "is a", "boy");
console.log("Call: " + baseName);
// Bind returns function which can be invoked
var baseName = Person.prototype.getName.bind(this, "is a", "boy");
console.log("Bind: " + baseName());
}
reader.getName();
/* Output
Apply: John Smith is a boy
Call: John Smith is a boy
Bind: John Smith is a boy
*/
Even though call
and apply
achive the same thing, I think there is atleast one place where you cannot use call
but can only use apply
. That is when you want to support inheritance and want to call the constructor.
Here is a function allows you to create classes which also supports creating classes by extending other classes.
function makeClass( properties ) {
var ctor = properties['constructor'] || function(){}
var Super = properties['extends'];
var Class = function () {
// Here 'call' cannot work, only 'apply' can!!!
if(Super)
Super.apply(this,arguments);
ctor.apply(this,arguments);
}
if(Super){
Class.prototype = Object.create( Super.prototype );
Class.prototype.constructor = Class;
}
Object.keys(properties).forEach( function(prop) {
if(prop!=='constructor' && prop!=='extends')
Class.prototype[prop] = properties[prop];
});
return Class;
}
//Usage
var Car = makeClass({
constructor: function(name){
this.name=name;
},
yourName: function() {
return this.name;
}
});
//We have a Car class now
var carInstance=new Car('Fiat');
carInstance.youName();// ReturnsFiat
var SuperCar = makeClass({
constructor: function(ignore,power){
this.power=power;
},
extends:Car,
yourPower: function() {
return this.power;
}
});
//We have a SuperCar class now, which is subclass of Car
var superCar=new SuperCar('BMW xy',2.6);
superCar.yourName();//Returns BMW xy
superCar.yourPower();// Returns 2.6
From the MDN docs on Function.prototype.apply() :
The apply() method calls a function with a given
this
value and arguments provided as an array (or an array-like object).Syntax
fun.apply(thisArg, [argsArray])
From the MDN docs on Function.prototype.call() :
The call() method calls a function with a given
this
value and arguments provided individually.Syntax
fun.call(thisArg[, arg1[, arg2[, ...]]])
From Function.apply and Function.call in JavaScript :
The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method.
var doSomething = function() {_x000D_
var arr = [];_x000D_
for(i in arguments) {_x000D_
if(typeof this[arguments[i]] !== 'undefined') {_x000D_
arr.push(this[arguments[i]]);_x000D_
}_x000D_
}_x000D_
return arr;_x000D_
}_x000D_
_x000D_
var output = function(position, obj) {_x000D_
document.body.innerHTML += '<h3>output ' + position + '</h3>' + JSON.stringify(obj) + '\n<br>\n<br><hr>';_x000D_
}_x000D_
_x000D_
output(1, doSomething(_x000D_
'one',_x000D_
'two',_x000D_
'two',_x000D_
'one'_x000D_
));_x000D_
_x000D_
output(2, doSomething.apply({one : 'Steven', two : 'Jane'}, [_x000D_
'one',_x000D_
'two',_x000D_
'two',_x000D_
'one'_x000D_
]));_x000D_
_x000D_
output(3, doSomething.call({one : 'Steven', two : 'Jane'},_x000D_
'one',_x000D_
'two',_x000D_
'two',_x000D_
'one'_x000D_
));
_x000D_
See also this Fiddle.
We can differentiate call and apply methods as below
CALL : A function with argument provide individually. If you know the arguments to be passed or there are no argument to pass you can use call.
APPLY : Call a function with argument provided as an array. You can use apply if you don't know how many argument are going to pass to the function.
There is a advantage of using apply over call, we don't need to change the number of argument only we can change a array that is passed.
There is not big difference in performance. But we can say call is bit faster as compare to apply because an array need to evaluate in apply method.
Follows an extract from Closure: The Definitive Guide by Michael Bolin. It might look a bit lengthy, but it's saturated with a lot of insight. From "Appendix B. Frequently Misunderstood JavaScript Concepts":
this
Refers to When a Function is CalledWhen calling a function of the form foo.bar.baz()
, the object foo.bar
is referred to as the receiver. When the function is called, it is the receiver that is used as the value for this
:
var obj = {};
obj.value = 10;
/** @param {...number} additionalValues */
obj.addValues = function(additionalValues) {
for (var i = 0; i < arguments.length; i++) {
this.value += arguments[i];
}
return this.value;
};
// Evaluates to 30 because obj is used as the value for 'this' when
// obj.addValues() is called, so obj.value becomes 10 + 20.
obj.addValues(20);
If there is no explicit receiver when a function is called, then the global object becomes the receiver. As explained in "goog.global" on page 47, window is the global object when JavaScript is executed in a web browser. This leads to some surprising behavior:
var f = obj.addValues;
// Evaluates to NaN because window is used as the value for 'this' when
// f() is called. Because and window.value is undefined, adding a number to
// it results in NaN.
f(20);
// This also has the unintentional side effect of adding a value to window:
alert(window.value); // Alerts NaN
Even though obj.addValues
and f
refer to the same function, they behave differently when called because the value of the receiver is different in each call. For this reason, when calling a function that refers to this
, it is important to ensure that this
will have the correct value when it is called. To be clear, if this
were not referenced in the function body, then the behavior of f(20)
and obj.addValues(20)
would be the same.
Because functions are first-class objects in JavaScript, they can have their own methods. All functions have the methods call()
and apply()
which make it possible to redefine the receiver (i.e., the object that this
refers to) when calling the function. The method signatures are as follows:
/**
* @param {*=} receiver to substitute for 'this'
* @param {...} parameters to use as arguments to the function
*/
Function.prototype.call;
/**
* @param {*=} receiver to substitute for 'this'
* @param {Array} parameters to use as arguments to the function
*/
Function.prototype.apply;
Note that the only difference between call()
and apply()
is that call()
receives the function parameters as individual arguments, whereas apply()
receives them as a single array:
// When f is called with obj as its receiver, it behaves the same as calling
// obj.addValues(). Both of the following increase obj.value by 60:
f.call(obj, 10, 20, 30);
f.apply(obj, [10, 20, 30]);
The following calls are equivalent, as f
and obj.addValues
refer to the same function:
obj.addValues.call(obj, 10, 20, 30);
obj.addValues.apply(obj, [10, 20, 30]);
However, since neither call()
nor apply()
uses the value of its own receiver to substitute for the receiver argument when it is unspecified, the following will not work:
// Both statements evaluate to NaN
obj.addValues.call(undefined, 10, 20, 30);
obj.addValues.apply(undefined, [10, 20, 30]);
The value of this
can never be null
or undefined
when a function is called. When null
or undefined
is supplied as the receiver to call()
or apply()
, the global object is used as the value for receiver instead. Therefore, the previous code has the same undesirable side effect of adding a property named value
to the global object.
It may be helpful to think of a function as having no knowledge of the variable to which it is assigned. This helps reinforce the idea that the value of this will be bound when the function is called rather than when it is defined.
End of extract.
Fundamental difference is that call()
accepts an argument list, while apply()
accepts a single array of arguments.
Difference between these to methods are, how you want to pass the parameters.
“A for array and C for comma” is a handy mnemonic.
I'd like to show an example, where the 'valueForThis' argument is used:
Array.prototype.push = function(element) {
/*
Native code*, that uses 'this'
this.put(element);
*/
}
var array = [];
array.push(1);
array.push.apply(array,[2,3]);
Array.prototype.push.apply(array,[4,5]);
array.push.call(array,6,7);
Array.prototype.push.call(array,8,9);
//[1, 2, 3, 4, 5, 6, 7, 8, 9]
**details: http://es5.github.io/#x15.4.4.7*
K. Scott Allen has a nice writeup on the matter.
Basically, they differ on how they handle function arguments.
The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method."
So:
// assuming you have f
function f(message) { ... }
f.call(receiver, "test");
f.apply(receiver, ["test"]);
Let me add a little detail to this.
these two calls are almost equivalent:
func.call(context, ...args); // pass an array as list with spread operator
func.apply(context, args); // is same as using apply
There’s only a minor difference:
- The
spread
operator ... allows passing iterableargs
as the list to call.- The
apply
accepts only array-like args.
So, these calls complement each other. Where we expect an iterable, call
works, where we expect an array-like, apply
works.
And for objects that are both iterable and array-like, like a real array, we technically could use any of them, but apply will probably be faster because most JavaScript engines internally optimize it better.
Here's a good mnemonic. Apply uses Arrays and Always takes one or two Arguments. When you use Call you have to Count the number of arguments.
It is useful at times for one object to borrow the function of another object, meaning that the borrowing object simply executes the lent function as if it were its own.
A small code example:
var friend = {
car: false,
lendCar: function ( canLend ){
this.car = canLend;
}
};
var me = {
car: false,
gotCar: function(){
return this.car === true;
}
};
console.log(me.gotCar()); // false
friend.lendCar.call(me, true);
console.log(me.gotCar()); // true
friend.lendCar.apply(me, [false]);
console.log(me.gotCar()); // false
These methods are very useful for giving objects temporary functionality.
Call and apply both are used to force the this
value when a function is executed. The only difference is that call
takes n+1
arguments where 1 is this
and 'n' arguments
. apply
takes only two arguments, one is this
the other is argument array.
The advantage I see in apply
over call
is that we can easily delegate a function call to other function without much effort;
function sayHello() {
console.log(this, arguments);
}
function hello() {
sayHello.apply(this, arguments);
}
var obj = {name: 'my name'}
hello.call(obj, 'some', 'arguments');
Observe how easily we delegated hello
to sayHello
using apply
, but with call
this is very difficult to achieve.
While this is an old topic, I just wanted to point out that .call is slightly faster than .apply. I can't tell you exactly why.
See jsPerf, http://jsperf.com/test-call-vs-apply/3
[UPDATE!
]
Douglas Crockford mentions briefly the difference between the two, which may help explain the performance difference... http://youtu.be/ya4UHuXNygM?t=15m52s
Apply takes an array of arguments, while Call takes zero or more individual parameters! Ah hah!
.apply(this, [...])
.call(this, param1, param2, param3, param4...)
Source: Stackoverflow.com