Detecting an undefined object property

Question

What\'s the best way of checking if an object property in JavaScript is undefined?

Answer 1

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

Answer 2

Compare with void 0, for terseness.

if (foo !== void 0)

It's not as verbose as if (typeof foo !== 'undefined')

Answer 3

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;
}

Answer 4

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.

Answer 5

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;
}

Answer 6

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.

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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