What's the best way of checking if an object property in JavaScript is undefined?
This question is related to
javascript
object
undefined
object-property
The solution is incorrect. In JavaScript,
null == undefined
will return true, because they both are "casted" to a boolean and are false. The correct way would be to check
if (something === undefined)
which is the identity operator...
I believe there are a number of incorrect answers to this topic. Contrary to common belief, "undefined" is not a keyword in JavaScript and can in fact have a value assigned to it.
The most robust way to perform this test is:
if (typeof myVar === "undefined")
This will always return the correct result, and even handles the situation where myVar
is not declared.
var undefined = false; // Shockingly, this is completely legal!
if (myVar === undefined) {
alert("You have been misled. Run away!");
}
Additionally, myVar === undefined
will raise an error in the situation where myVar is undeclared.
I use if (this.variable)
to test if it is defined. A simple if (variable)
, recommended in a previous answer, fails for me.
It turns out that it works only when a variable is a field of some object, obj.someField
to check if it is defined in the dictionary. But we can use this
or window
as the dictionary object since any variable is a field in the current window, as I understand it. Therefore here is a test:
if (this.abc)
alert("defined");
else
alert("undefined");
abc = "abc";
if (this.abc)
alert("defined");
else
alert("undefined");
_x000D_
It first detects that variable abc
is undefined and it is defined after initialization.
I didn't see (hope I didn't miss it) anyone checking the object before the property. So, this is the shortest and most effective (though not necessarily the most clear):
if (obj && obj.prop) {
// Do something;
}
If the obj or obj.prop is undefined, null, or "falsy", the if statement will not execute the code block. This is usually the desired behavior in most code block statements (in JavaScript).
All the answers are incomplete. This is the right way of knowing that there is a property 'defined as undefined':
var hasUndefinedProperty = function hasUndefinedProperty(obj, prop){
return ((prop in obj) && (typeof obj[prop] == 'undefined'));
};
Example:
var a = { b : 1, e : null };
a.c = a.d;
hasUndefinedProperty(a, 'b'); // false: b is defined as 1
hasUndefinedProperty(a, 'c'); // true: c is defined as undefined
hasUndefinedProperty(a, 'd'); // false: d is undefined
hasUndefinedProperty(a, 'e'); // false: e is defined as null
// And now...
delete a.c ;
hasUndefinedProperty(a, 'c'); // false: c is undefined
Too bad that this been the right answer and is buried in wrong answers >_<
So, for anyone who pass by, I will give you undefined's for free!!
var undefined ; undefined ; // undefined
({}).a ; // undefined
[].a ; // undefined
''.a ; // undefined
(function(){}()) ; // undefined
void(0) ; // undefined
eval() ; // undefined
1..a ; // undefined
/a/.a ; // undefined
(true).a ; // undefined
In the article Exploring the Abyss of Null and Undefined in JavaScript I read that frameworks like Underscore.js use this function:
function isUndefined(obj){
return obj === void 0;
}
There is a very easy and simple way.
You can use optional chaining:
x = {prop:{name:"sajad"}}
console.log(x.prop?.name) // Output is: "sajad"
console.log(x.prop?.lastName) // Output is: undefined
or
if(x.prop?.lastName) // The result of this 'if' statement is false and is not throwing an error
You can use optional chaining even for functions or arrays.
As of mid-2020 this is not universally implemented. Check the documentation at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining
Introduced in ECMAScript 6, we can now deal with undefined
in a new way using Proxies. It can be used to set a default value to any properties which doesn't exist so that we don't have to check each time whether it actually exists.
var handler = {
get: function(target, name) {
return name in target ? target[name] : 'N/A';
}
};
var p = new Proxy({}, handler);
p.name = 'Kevin';
console.log('Name: ' +p.name, ', Age: '+p.age, ', Gender: '+p.gender)
Will output the below text without getting any undefined.
Name: Kevin , Age: N/A , Gender: N/A
Going through the comments, for those who want to check both is it undefined or its value is null:
//Just in JavaScript
var s; // Undefined
if (typeof s == "undefined" || s === null){
alert('either it is undefined or value is null')
}
If you are using jQuery Library then jQuery.isEmptyObject()
will suffice for both cases,
var s; // Undefined
jQuery.isEmptyObject(s); // Will return true;
s = null; // Defined as null
jQuery.isEmptyObject(s); // Will return true;
//Usage
if (jQuery.isEmptyObject(s)) {
alert('Either variable:s is undefined or its value is null');
} else {
alert('variable:s has value ' + s);
}
s = 'something'; // Defined with some value
jQuery.isEmptyObject(s); // Will return false;
A simple way to check if a key exists is to use in
:
if (key in obj) {
// Do something
} else {
// Create key
}
const obj = {
0: 'abc',
1: 'def'
}
const hasZero = 0 in obj
console.log(hasZero) // true
You can use the JavaScript object function like this:
var ojb ={
age: 12
}
if(ojb.hasOwnProperty('name')){
console.log('property exists and is not undefined');
}
The above method returns true
if it got that property or the property is not undefined.
Compare with void 0
, for terseness.
if (foo !== void 0)
It's not as verbose as if (typeof foo !== 'undefined')
The issue boils down to three cases:
undefined
.undefined
.This tells us something I consider important:
There is a difference between an undefined member and a defined member with an undefined value.
But unhappily typeof obj.foo
does not tell us which of the three cases we have. However we can combine this with "foo" in obj
to distinguish the cases.
| typeof obj.x === 'undefined' | !("x" in obj)
1. { x:1 } | false | false
2. { x : (function(){})() } | true | false
3. {} | true | true
Its worth noting that these tests are the same for null
entries too
| typeof obj.x === 'undefined' | !("x" in obj)
{ x:null } | false | false
I'd argue that in some cases it makes more sense (and is clearer) to check whether the property is there, than checking whether it is undefined, and the only case where this check will be different is case 2, the rare case of an actual entry in the object with an undefined value.
For example: I've just been refactoring a bunch of code that had a bunch of checks whether an object had a given property.
if( typeof blob.x != 'undefined' ) { fn(blob.x); }
Which was clearer when written without a check for undefined.
if( "x" in blob ) { fn(blob.x); }
But as has been mentioned these are not exactly the same (but are more than good enough for my needs).
I'm not sure where the origin of using ===
with typeof
came from, and as a convention I see it used in many libraries, but the typeof operator returns a string literal, and we know that up front, so why would you also want to type check it too?
typeof x; // some string literal "string", "object", "undefined"
if (typeof x === "string") { // === is redundant because we already know typeof returns a string literal
if (typeof x == "string") { // sufficient
I'm surprised I haven't seen this suggestion yet, but it gets even more specificity than testing with typeof
. Use Object.getOwnPropertyDescriptor()
if you need to know whether an object property was initialized with undefined
or if it was never initialized:
// to test someObject.someProperty
var descriptor = Object.getOwnPropertyDescriptor(someObject, 'someProperty');
if (typeof descriptor === 'undefined') {
// was never initialized
} else if (typeof descriptor.value === 'undefined') {
if (descriptor.get || descriptor.set) {
// is an accessor property, defined via getter and setter
} else {
// is initialized with `undefined`
}
} else {
// is initialized with some other value
}
if (somevariable == undefined) {
alert('the variable is not defined!');
}
You can also make it into a function, as shown here:
function isset(varname){
return(typeof(window[varname]) != 'undefined');
}
You can get an array all undefined with path using the following code.
function getAllUndefined(object) {
function convertPath(arr, key) {
var path = "";
for (var i = 1; i < arr.length; i++) {
path += arr[i] + "->";
}
path += key;
return path;
}
var stack = [];
var saveUndefined= [];
function getUndefiend(obj, key) {
var t = typeof obj;
switch (t) {
case "object":
if (t === null) {
return false;
}
break;
case "string":
case "number":
case "boolean":
case "null":
return false;
default:
return true;
}
stack.push(key);
for (k in obj) {
if (obj.hasOwnProperty(k)) {
v = getUndefiend(obj[k], k);
if (v) {
saveUndefined.push(convertPath(stack, k));
}
}
}
stack.pop();
}
getUndefiend({
"": object
}, "");
return saveUndefined;
}
jsFiddle link
You can also use a Proxy. It will work with nested calls, but it will require one extra check:
function resolveUnknownProps(obj, resolveKey) {
const handler = {
get(target, key) {
if (
target[key] !== null &&
typeof target[key] === 'object'
) {
return resolveUnknownProps(target[key], resolveKey);
} else if (!target[key]) {
return resolveUnknownProps({ [resolveKey]: true }, resolveKey);
}
return target[key];
},
};
return new Proxy(obj, handler);
}
const user = {}
console.log(resolveUnknownProps(user, 'isUndefined').personalInfo.name.something.else); // { isUndefined: true }
So you will use it like:
const { isUndefined } = resolveUnknownProps(user, 'isUndefined').personalInfo.name.something.else;
if (!isUndefined) {
// Do something
}
In JavaScript there is null and there is undefined. They have different meanings.
Marijn Haverbeke states, in his free, online book "Eloquent JavaScript" (emphasis mine):
There is also a similar value, null, whose meaning is 'this value is defined, but it does not have a value'. The difference in meaning between undefined and null is mostly academic, and usually not very interesting. In practical programs, it is often necessary to check whether something 'has a value'. In these cases, the expression something == undefined may be used, because, even though they are not exactly the same value, null == undefined will produce true.
So, I guess the best way to check if something was undefined would be:
if (something == undefined)
Object properties should work the same way.
var person = {
name: "John",
age: 28,
sex: "male"
};
alert(person.name); // "John"
alert(person.fakeVariable); // undefined
This is probably the only explicit form of determining if the existing property-name has an explicit and intended value of undefined
; which is, nonetheless, a JavaScript type.
"propertyName" in containerObject && ""+containerObject["propertyName"] == "undefined";
>> true \ false
This expression will only return true
if the property name of the given context exists (truly) and only if its intended value is explicitly undefined
.
There will be no false positives like with empty or blank strings zeros nulls or empty arrays and alike. This does exactly that. Checks i.e., makes sure the property name exists (otherwise it would be a false positive), than it explicitly checks if its value is undefined
e.g. of an undefined JavaScript type in it's string representation form (literally "undefined") therefore ==
instead of ===
because no further conversion is possible. And this expression will only return true if both, that is all conditions are met. E.g. if the property-name doesn't exist, - it will return false. Which is the only correct return since nonexistent properties can't have values, not even an undefined one.
Example:
containerObject = { propertyName: void "anything" }
>> Object { propertyName: undefined }
// Now the testing
"propertyName" in containerObject && ""+containerObject["propertyName"] == "undefined";
>> true
/* Which makes sure that nonexistent property will not return a false positive
* unless it is previously defined */
"foo" in containerObject && ""+containerObject["foo"] == "undefined";
>> false
Here is my situation:
I am using the result of a REST call. The result should be parsed from JSON to a JavaScript object.
There is one error I need to defend. If the arguments to the REST call were incorrect as far as the user specifying the arguments wrong, the REST call comes back basically empty.
While using this post to help me defend against this, I tried this:
if( typeof restResult.data[0] === "undefined" ) { throw "Some error"; }
For my situation, if restResult.data[0] === "object", then I can safely start inspecting the rest of the members. If undefined then throw the error as above.
What I am saying is that for my situation, all the previous suggestions in this post did not work. I'm not saying I'm right and everyone is wrong. I am not a JavaScript master at all, but hopefully this will help someone.
"propertyName" in obj //-> true | false
What does this mean: "undefined object property"?
Actually it can mean two quite different things! First, it can mean the property that has never been defined in the object and, second, it can mean the property that has an undefined value. Let's look at this code:
var o = { a: undefined }
Is o.a
undefined? Yes! Its value is undefined. Is o.b
undefined? Sure! There is no property 'b' at all! OK, see now how different approaches behave in both situations:
typeof o.a == 'undefined' // true
typeof o.b == 'undefined' // true
o.a === undefined // true
o.b === undefined // true
'a' in o // true
'b' in o // false
We can clearly see that typeof obj.prop == 'undefined'
and obj.prop === undefined
are equivalent, and they do not distinguish those different situations. And 'prop' in obj
can detect the situation when a property hasn't been defined at all and doesn't pay attention to the property value which may be undefined.
1) You want to know if a property is undefined by either the first or second meaning (the most typical situation).
obj.prop === undefined // IMHO, see "final fight" below
2) You want to just know if object has some property and don't care about its value.
'prop' in obj
x.a === undefined
or this typeof x.a == 'undefined'
raises ReferenceError: x is not defined
if x is not defined.undefined
is a global variable (so actually it is window.undefined
in browsers). It has been supported since ECMAScript 1st Edition and since ECMAScript 5 it is read only. So in modern browsers it can't be redefined to true as many authors love to frighten us with, but this is still a true for older browsers.obj.prop === undefined
vs typeof obj.prop == 'undefined'
Pluses of obj.prop === undefined
:
undefined
Minuses of obj.prop === undefined
:
undefined
can be overridden in old browsersPluses of typeof obj.prop == 'undefined'
:
Minuses of typeof obj.prop == 'undefined'
:
'undefned'
(misspelled) here is just a string constant, so the JavaScript engine can't help you if you have misspelled it like I just did.Node.js supports the global variable undefined
as global.undefined
(it can also be used without the 'global' prefix). I don't know about other implementations of server-side JavaScript.
From lodash.js.
var undefined;
function isUndefined(value) {
return value === undefined;
}
It creates a local variable named undefined
which is initialized with the default value -- the real undefined
, then compares value
with the variable undefined
.
Update 9/9/2019
I found Lodash updated its implementation. See my issue and the code.
To be bullet-proof, simply use:
function isUndefined(value) {
return value === void 0;
}
Simply anything is not defined in JavaScript, is undefined, doesn't matter if it's a property inside an Object/Array or as just a simple variable...
JavaScript has typeof
which make it very easy to detect an undefined variable.
Simply check if typeof whatever === 'undefined'
and it will return a boolean.
That's how the famous function isUndefined()
in AngularJs v.1x is written:
function isUndefined(value) {return typeof value === 'undefined';}
So as you see the function receive a value, if that value is defined, it will return false
, otherwise for undefined values, return true
.
So let's have a look what gonna be the results when we passing values, including object properties like below, this is the list of variables we have:
var stackoverflow = {};
stackoverflow.javascipt = 'javascript';
var today;
var self = this;
var num = 8;
var list = [1, 2, 3, 4, 5];
var y = null;
and we check them as below, you can see the results in front of them as a comment:
isUndefined(stackoverflow); //false
isUndefined(stackoverflow.javascipt); //false
isUndefined(today); //true
isUndefined(self); //false
isUndefined(num); //false
isUndefined(list); //false
isUndefined(y); //false
isUndefined(stackoverflow.java); //true
isUndefined(stackoverflow.php); //true
isUndefined(stackoverflow && stackoverflow.css); //true
As you see we can check anything with using something like this in our code, as mentioned you can simply use typeof
in your code, but if you are using it over and over, create a function like the angular sample which I share and keep reusing as following DRY code pattern.
Also one more thing, for checking property on an object in a real application which you not sure even the object exists or not, check if the object exists first.
If you check a property on an object and the object doesn't exist, will throw an error and stop the whole application running.
isUndefined(x.css);
VM808:2 Uncaught ReferenceError: x is not defined(…)
So simple you can wrap inside an if statement like below:
if(typeof x !== 'undefined') {
//do something
}
Which also equal to isDefined in Angular 1.x...
function isDefined(value) {return typeof value !== 'undefined';}
Also other javascript frameworks like underscore has similar defining check, but I recommend you use typeof
if you already not using any frameworks.
I also add this section from MDN which has got useful information about typeof, undefined and void(0).
Strict equality and undefined
You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the variable x is not defined, and the if statement evaluates to true.
var x;
if (x === undefined) {
// these statements execute
}
else {
// these statements do not execute
}
Note: The strict equality operator rather than the standard equality operator must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. null is not equivalent to undefined. See comparison operators for details.
Typeof operator and undefined
Alternatively, typeof can be used:
var x;
if (typeof x === 'undefined') {
// these statements execute
}
One reason to use typeof is that it does not throw an error if the variable has not been declared.
// x has not been declared before
if (typeof x === 'undefined') { // evaluates to true without errors
// these statements execute
}
if (x === undefined) { // throws a ReferenceError
}
However, this kind of technique should be avoided. JavaScript is a statically scoped language, so knowing if a variable is declared can be read by seeing whether it is declared in an enclosing context. The only exception is the global scope, but the global scope is bound to the global object, so checking the existence of a variable in the global context can be done by checking the existence of a property on the global object (using the in operator, for instance).
Void operator and undefined
The void operator is a third alternative.
var x;
if (x === void 0) {
// these statements execute
}
// y has not been declared before
if (y === void 0) {
// throws a ReferenceError (in contrast to `typeof`)
}
more > here
There are a few little helpers in the Lodash library:
isUndefined - to check if value is undefined
.
_.isUndefined(undefined) // => true
_.isUndefined(null) // => false
has - to check if object contains a property
const object = { 'a': { 'b': 2 } }
_.has(object, 'a.b') // => true
_.has(object, 'a.c') // => false
Crossposting my answer from related question How can I check for "undefined" in JavaScript?.
Specific to this question, see test cases with someObject.<whatever>
.
Some scenarios illustrating the results of the various answers: http://jsfiddle.net/drzaus/UVjM4/
(Note that the use of var
for in
tests make a difference when in a scoped wrapper)
Code for reference:
(function(undefined) {
var definedButNotInitialized;
definedAndInitialized = 3;
someObject = {
firstProp: "1"
, secondProp: false
// , undefinedProp not defined
}
// var notDefined;
var tests = [
'definedButNotInitialized in window',
'definedAndInitialized in window',
'someObject.firstProp in window',
'someObject.secondProp in window',
'someObject.undefinedProp in window',
'notDefined in window',
'"definedButNotInitialized" in window',
'"definedAndInitialized" in window',
'"someObject.firstProp" in window',
'"someObject.secondProp" in window',
'"someObject.undefinedProp" in window',
'"notDefined" in window',
'typeof definedButNotInitialized == "undefined"',
'typeof definedButNotInitialized === typeof undefined',
'definedButNotInitialized === undefined',
'! definedButNotInitialized',
'!! definedButNotInitialized',
'typeof definedAndInitialized == "undefined"',
'typeof definedAndInitialized === typeof undefined',
'definedAndInitialized === undefined',
'! definedAndInitialized',
'!! definedAndInitialized',
'typeof someObject.firstProp == "undefined"',
'typeof someObject.firstProp === typeof undefined',
'someObject.firstProp === undefined',
'! someObject.firstProp',
'!! someObject.firstProp',
'typeof someObject.secondProp == "undefined"',
'typeof someObject.secondProp === typeof undefined',
'someObject.secondProp === undefined',
'! someObject.secondProp',
'!! someObject.secondProp',
'typeof someObject.undefinedProp == "undefined"',
'typeof someObject.undefinedProp === typeof undefined',
'someObject.undefinedProp === undefined',
'! someObject.undefinedProp',
'!! someObject.undefinedProp',
'typeof notDefined == "undefined"',
'typeof notDefined === typeof undefined',
'notDefined === undefined',
'! notDefined',
'!! notDefined'
];
var output = document.getElementById('results');
var result = '';
for(var t in tests) {
if( !tests.hasOwnProperty(t) ) continue; // bleh
try {
result = eval(tests[t]);
} catch(ex) {
result = 'Exception--' + ex;
}
console.log(tests[t], result);
output.innerHTML += "\n" + tests[t] + ": " + result;
}
})();
And results:
definedButNotInitialized in window: true
definedAndInitialized in window: false
someObject.firstProp in window: false
someObject.secondProp in window: false
someObject.undefinedProp in window: true
notDefined in window: Exception--ReferenceError: notDefined is not defined
"definedButNotInitialized" in window: false
"definedAndInitialized" in window: true
"someObject.firstProp" in window: false
"someObject.secondProp" in window: false
"someObject.undefinedProp" in window: false
"notDefined" in window: false
typeof definedButNotInitialized == "undefined": true
typeof definedButNotInitialized === typeof undefined: true
definedButNotInitialized === undefined: true
! definedButNotInitialized: true
!! definedButNotInitialized: false
typeof definedAndInitialized == "undefined": false
typeof definedAndInitialized === typeof undefined: false
definedAndInitialized === undefined: false
! definedAndInitialized: false
!! definedAndInitialized: true
typeof someObject.firstProp == "undefined": false
typeof someObject.firstProp === typeof undefined: false
someObject.firstProp === undefined: false
! someObject.firstProp: false
!! someObject.firstProp: true
typeof someObject.secondProp == "undefined": false
typeof someObject.secondProp === typeof undefined: false
someObject.secondProp === undefined: false
! someObject.secondProp: true
!! someObject.secondProp: false
typeof someObject.undefinedProp == "undefined": true
typeof someObject.undefinedProp === typeof undefined: true
someObject.undefinedProp === undefined: true
! someObject.undefinedProp: true
!! someObject.undefinedProp: false
typeof notDefined == "undefined": true
typeof notDefined === typeof undefined: true
notDefined === undefined: Exception--ReferenceError: notDefined is not defined
! notDefined: Exception--ReferenceError: notDefined is not defined
!! notDefined: Exception--ReferenceError: notDefined is not defined
I provide three ways here for those who expect weird answers:
function isUndefined1(val) {
try {
val.a;
} catch (e) {
return /undefined/.test(e.message);
}
return false;
}
function isUndefined2(val) {
return !val && val+'' === 'undefined';
}
function isUndefined3(val) {
const defaultVal = {};
return ((input = defaultVal) => input === defaultVal)(val);
}
function test(func){
console.group(`test start :`+func.name);
console.log(func(undefined));
console.log(func(null));
console.log(func(1));
console.log(func("1"));
console.log(func(0));
console.log(func({}));
console.log(func(function () { }));
console.groupEnd();
}
test(isUndefined1);
test(isUndefined2);
test(isUndefined3);
_x000D_
Try to get a property of the input value, and check the error message if it exists. If the input value is undefined, the error message would be Uncaught TypeError: Cannot read property 'b' of undefined.
Convert the input value to a string to compare with "undefined"
and ensure it's a negative value.
In JavaScript, an optional parameter works when the input value is exactly undefined
.
ECMAScript 10 introduced a new feature - optional chaining which you can use to use a property of an object only when an object is defined like this:
const userPhone = user?.contactDetails?.phone;
It will reference to the phone property only when user and contactDetails are defined.
Ref. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining
function isUnset(inp) {
return (typeof inp === 'undefined')
}
Returns false if variable is set, and true if is undefined.
Then use:
if (isUnset(var)) {
// initialize variable here
}
A lot of the given answers give a wrong result because they do not distinguish between the case when an object property does not exist and the case when a property has value undefined
. Here is proof for most popular solutions:
let obj = {
a: 666,
u: undefined // The 'u' property has value 'undefined'
// The 'x' property does not exist
}
console.log('>>> good results:');
console.log('A', "u" in obj, "x" in obj);
console.log('B', obj.hasOwnProperty("u"), obj.hasOwnProperty("x"));
console.log('\n>>> bad results:');
console.log('C', obj.u === undefined, obj.x === undefined);
console.log('D', obj.u == undefined, obj.x == undefined);
console.log('E', obj["u"] === undefined, obj["x"] === undefined);
console.log('F', obj["u"] == undefined, obj["x"] == undefined);
console.log('G', !obj.u, !obj.x);
console.log('H', typeof obj.u === 'undefined', typeof obj.x === 'undefined');
_x000D_
Reading through this, I'm amazed I didn't see this. I have found multiple algorithms that would work for this.
If the value of an object was never defined, this will prevent from returning true
if it is defined as null
or undefined
. This is helpful if you want true to be returned for values set as undefined
if(obj.prop === void 0) console.log("The value has never been defined");
If you want it to result as true
for values defined with the value of undefined
, or never defined, you can simply use === undefined
if(obj.prop === undefined) console.log("The value is defined as undefined, or never defined");
Commonly, people have asked me for an algorithm to figure out if a value is either falsy, undefined
, or null
. The following works.
if(obj.prop == false || obj.prop === null || obj.prop === undefined) {
console.log("The value is falsy, null, or undefined");
}
If you do
if (myvar == undefined )
{
alert('var does not exists or is not initialized');
}
it will fail when the variable myvar
does not exists, because myvar is not defined, so the script is broken and the test has no effect.
Because the window object has a global scope (default object) outside a function, a declaration will be 'attached' to the window object.
For example:
var myvar = 'test';
The global variable myvar is the same as window.myvar or window['myvar']
To avoid errors to test when a global variable exists, you better use:
if(window.myvar == undefined )
{
alert('var does not exists or is not initialized');
}
The question if a variable really exists doesn't matter, its value is incorrect. Otherwise, it is silly to initialize variables with undefined, and it is better use the value false to initialize. When you know that all variables that you declare are initialized with false, you can simply check its type or rely on !window.myvar
to check if it has a proper/valid value. So even when the variable is not defined then !window.myvar
is the same for myvar = undefined
or myvar = false
or myvar = 0
.
When you expect a specific type, test the type of the variable. To speed up testing a condition you better do:
if( !window.myvar || typeof window.myvar != 'string' )
{
alert('var does not exists or is not type of string');
}
When the first and simple condition is true, the interpreter skips the next tests.
It is always better to use the instance/object of the variable to check if it got a valid value. It is more stable and is a better way of programming.
(y)
Most likely you want if (window.x)
. This check is safe even if x hasn't been declared (var x;
) - browser doesn't throw an error.
if (window.history) {
history.call_some_function();
}
window is an object which holds all global variables as its members, and it is legal to try to access a non-existing member. If x hasn't been declared or hasn't been set then window.x
returns undefined. undefined leads to false when if() evaluates it.
Use:
To check if property is undefined:
if (typeof something === "undefined") {
alert("undefined");
}
To check if property is not undefined:
if (typeof something !== "undefined") {
alert("not undefined");
}
I'm assuming you're going to also want to check for it being either undefined
or null
. If so, I suggest:
myVar == null
This is one of the only times a double equals is very helpful as it will evaluate to true
when myVar
is undefined
or null
, but it will evaluate to false
when it is other falsey values such as 0
, false
, ''
, and NaN
.
This the actual the source code for Lodash's isNil
method.
Also, the same things can be written shorter:
if (!variable){
// Do it if the variable is undefined
}
or
if (variable){
// Do it if the variable is defined
}
There is a nice and elegant way to assign a defined property to a new variable if it is defined or assign a default value to it as a fallback if it’s undefined.
var a = obj.prop || defaultValue;
It’s suitable if you have a function, which receives an additional configuration property:
var yourFunction = function(config){
this.config = config || {};
this.yourConfigValue = config.yourConfigValue || 1;
console.log(this.yourConfigValue);
}
Now executing
yourFunction({yourConfigValue:2});
//=> 2
yourFunction();
//=> 1
yourFunction({otherProperty:5});
//=> 1
I would like to show you something I'm using in order to protect the undefined
variable:
Object.defineProperty(window, 'undefined', {});
This forbids anyone to change the window.undefined
value therefore destroying the code based on that variable. If using "use strict"
, anything trying to change its value will end in error, otherwise it would be silently ignored.
In JavaScript, there are truthy and falsy expressions. If you want to check if the property is undefined or not, there is a straight way of using an if condition as given,
if(!ob.someProp){
console.log('someProp is falsy')
}
However, there are several more approaches to check the object has property or not, but it seems long to me. Here are those.
=== undefined
check in if
conditionif(ob.someProp === undefined){
console.log('someProp is undefined')
}
typeof
typeof
acts as a combined check for the value undefined and for whether a variable exists.
if(typeof ob.someProp === 'undefined'){
console.log('someProp is undefined')
}
hasOwnProperty
methodThe JavaScript object has built in the hasOwnProperty
function in the object prototype.
if(!ob.hasOwnProperty('someProp')){
console.log('someProp is undefined')
}
Not going in deep, but the 1st way looks shortened and good to me. Here are the details on truthy/falsy values in JavaScript and undefined
is the falsy value listed in there. So the if
condition behaves normally without any glitch. Apart from the undefined
, values NaN
, false
(Obviously), ''
(empty string) and number 0
are also the falsy values.
Warning: Make sure the property value does not contain any falsy value, otherwise the
if
condition will return false. For such a case, you can use thehasOwnProperty
method
if ( typeof( something ) == "undefined")
This worked for me while the others didn't.
Version for the use of dynamic variables Did you know?
var boo ='lala';
function check(){
if(this['foo']){
console.log('foo is here');}
else{
console.log('have no foo');
}
if(this['boo']){
console.log('boo is here');}
else{
console.log('have no boo');
}
}
check();
_x000D_
If you are using Angular:
angular.isUndefined(obj)
angular.isUndefined(obj.prop)
Underscore.js:
_.isUndefined(obj)
_.isUndefined(obj.prop)
Despite being vehemently recommended by many other answers here, typeof
is a bad choice. It should never be used for checking whether variables have the value undefined
, because it acts as a combined check for the value undefined
and for whether a variable exists. In the vast majority of cases, you know when a variable exists, and typeof
will just introduce the potential for a silent failure if you make a typo in the variable name or in the string literal 'undefined'
.
var snapshot = …;
if (typeof snaposhot === 'undefined') {
// ^
// misspelled¹ – this will never run, but it won’t throw an error!
}
var foo = …;
if (typeof foo === 'undefned') {
// ^
// misspelled – this will never run, but it won’t throw an error!
}
So unless you’re doing feature detection², where there’s uncertainty whether a given name will be in scope (like checking typeof module !== 'undefined'
as a step in code specific to a CommonJS environment), typeof
is a harmful choice when used on a variable, and the correct option is to compare the value directly:
var foo = …;
if (foo === undefined) {
?
}
Some common misconceptions about this include:
that reading an “uninitialized” variable (var foo
) or parameter (function bar(foo) { … }
, called as bar()
) will fail. This is simply not true – variables without explicit initialization and parameters that weren’t given values always become undefined
, and are always in scope.
that undefined
can be overwritten. It’s true that undefined
isn’t a keyword, but it is read-only and non-configurable. There are other built-ins you probably don’t avoid despite their non-keyword status (Object
, Math
, NaN
…) and practical code usually isn’t written in an actively malicious environment, so this isn’t a good reason to be worried about undefined
. (But if you are writing a code generator, feel free to use void 0
.)
With how variables work out of the way, it’s time to address the actual question: object properties. There is no reason to ever use typeof
for object properties. The earlier exception regarding feature detection doesn’t apply here – typeof
only has special behaviour on variables, and expressions that reference object properties are not variables.
This:
if (typeof foo.bar === 'undefined') {
?
}
is always exactly equivalent to this³:
if (foo.bar === undefined) {
?
}
and taking into account the advice above, to avoid confusing readers as to why you’re using typeof
, because it makes the most sense to use ===
to check for equality, because it could be refactored to checking a variable’s value later, and because it just plain looks better, you should always use === undefined
³ here as well.
Something else to consider when it comes to object properties is whether you really want to check for undefined
at all. A given property name can be absent on an object (producing the value undefined
when read), present on the object itself with the value undefined
, present on the object’s prototype with the value undefined
, or present on either of those with a non-undefined
value. 'key' in obj
will tell you whether a key is anywhere on an object’s prototype chain, and Object.prototype.hasOwnProperty.call(obj, 'key')
will tell you whether it’s directly on the object. I won’t go into detail in this answer about prototypes and using objects as string-keyed maps, though, because it’s mostly intended to counter all the bad advice in other answers irrespective of the possible interpretations of the original question. Read up on object prototypes on MDN for more!
¹ unusual choice of example variable name? this is real dead code from the NoScript extension for Firefox.
² don’t assume that not knowing what’s in scope is okay in general, though. bonus vulnerability caused by abuse of dynamic scope: Project Zero 1225
³ once again assuming an ES5+ environment and that undefined
refers to the undefined
property of the global object.
I found this article, 7 Tips to Handle undefined in JavaScript, which is showing really interesting things about undefined
like:
The existence of undefined is a consequence of JavaScript’s permissive nature that allows the usage of:
In recent JavaScript release there is new chaining operator introduced, which is most probably best way to check if property exists else it will give you undefined
see example below
const adventurer = {
name: 'Alice',
cat: {
name: 'Dinah'
}
};
const dogName = adventurer.dog?.name;
console.log(dogName);
// expected output: undefined
console.log(adventurer.someNonExistentMethod?.());
// expected output: undefined
We can replace this old syntax
if (response && response.data && response.data.someData && response.data.someData.someMoreData) {}
with this neater syntax
if( response?.data?.someData?.someMoreData) {}
This syntax is not supported in IE, Opera, safari & samsund android
for more detail you can check this URL
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining
Object.hasOwnProperty(o, 'propertyname');
This doesn't look up through the prototype chain, however.
Source: Stackoverflow.com