Detecting an undefined object property


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

Use:

if (typeof something === "undefined") {
    alert("something is undefined");
}

If an object variable which have some properties you can use same thing like this:

if (typeof my_obj.someproperties === "undefined"){
    console.log('the property is not available...'); // print into console
}

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.

Correct Code

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.

Degenerate code. DO NOT USE.

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.


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. There’s a lot more to this. undefined is not a keyword in JavaScript. Instead, it’s a property on the global object with the Undefined value. However, since ES5, this property has been read-only and non-configurable. No modern browser will allow the undefined property to be changed, and as of 2017 this has been the case for a long time. Lack of strict mode doesn’t affect undefined’s behaviour either – it just makes statements like undefined = 5 do nothing instead of throwing. Since it isn’t a keyword, though, you can declare variables with the name undefined, and those variables could be changed, making this once-common pattern:

    (function (undefined) {
        // …
    })()
    

    more dangerous than using the global undefined. If you have to be ES3-compatible, replace undefined with void 0 – don’t resort to typeof. (void has always been a unary operator that evaluates to the Undefined value for any operand.)

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 === undefined³ 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. substitute void 0 otherwise.


In JavaScript there is null and there is undefined. They have different meanings.

  • undefined means that the variable value has not been defined; it is not known what the value is.
  • null means that the variable value is defined and set to null (has no value).

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)

Hope this helps!

Edit: In response to your edit, object properties should work the same way.

var person = {
    name: "John",
    age: 28,
    sex: "male"
};

alert(person.name); // "John"
alert(person.fakeVariable); // undefined

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.

So what to do?

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

Notes:

  • You can't check an object and its property at the same time. For example, this x.a === undefined or this typeof x.a == 'undefined' raises ReferenceError: x is not defined if x is not defined.
  • Variable 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.

Final fight: obj.prop === undefined vs typeof obj.prop == 'undefined'

Pluses of obj.prop === undefined:

  • It's a bit shorter and looks a bit prettier
  • The JavaScript engine will give you an error if you have misspelled undefined

Minuses of obj.prop === undefined:

  • undefined can be overridden in old browsers

Pluses of typeof obj.prop == 'undefined':

  • It is really universal! It works in new and old browsers.

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.

Update (for server-side JavaScript):

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.


The issue boils down to three cases:

  1. The object has the property and its value is not undefined.
  2. The object has the property and its value is undefined.
  3. The object does not have the property.

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).


if ( typeof( something ) == "undefined") 

This worked for me while the others didn't.


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

Crossposting my answer from related question How to 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

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)


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).


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;
}

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


'if (window.x) { }' is error safe

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.

Example: I want to know if my browser supports History API

if (window.history) {
    history.call_some_function();
}

How this works:

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.


"propertyName" in obj //-> true | false

Reading through this, I'm amazed I didn't see this. I have found multiple algorithms that would work for this.

Never Defined

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");

Defined as undefined Or never 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");

Defined as a falsy value, undefined,null, 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");
}

Compare with void 0, for terseness.

if (foo !== void 0)

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


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...


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


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 args to the rest call were incorrect as far as the user specifying the args 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 suggestions above 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.


There is a nice & 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 config 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

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 is buried in wrong answers >_<

So, for anyone who pass by, I will give you undefineds 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

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;

If you are using Angular:

angular.isUndefined(obj)
angular.isUndefined(obj.prop)

Underscore.js:

_.isUndefined(obj) 
_.isUndefined(obj.prop) 

I use if (this.variable) to test if it is defined. Simple if (variable), recommended above, fails for me. It turns out that it works only when 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 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");

It first detects that variable abc is undefined and it is defined after initialization.


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);

isUndefined1:

Try to get a property of the input value, 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

isUndefined2:

Convert input value to string to compare with "undefined" and ensure it's negative value.

isUndefined3:

In js, optional parameter works when the input value is exactly undefined.


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
}

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.


you can also use Proxy, it will work with nested calls, but 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 someting
}

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;
}

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