When is null or undefined used in JavaScript? [duplicate]

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I find that some of these answers are vague and complicated, I find the best way to figure out these things for sure is to just open up the console and test it yourself.

var x;

x == null            // true
x == undefined       // true
x === null           // false
x === undefined      // true

var y = null;

y == null            // true
y == undefined       // true
y === null           // true
y === undefined      // false

typeof x             // 'undefined'
typeof y             // 'object'

var z = {abc: null};

z.abc == null        // true
z.abc == undefined   // true
z.abc === null       // true
z.abc === undefined  // false

z.xyz == null        // true
z.xyz == undefined   // true
z.xyz === null       // false
z.xyz === undefined  // true

null = 1;            // throws error: invalid left hand assignment
undefined = 1;       // works fine: this can cause some problems

So this is definitely one of the more subtle nuances of JavaScript. As you can see, you can override the value of undefined, making it somewhat unreliable compared to null. Using the == operator, you can reliably use null and undefined interchangeably as far as I can tell. However, because of the advantage that null cannot be redefined, I might would use it when using ==.

For example, variable != null will ALWAYS return false if variable is equal to either null or undefined, whereas variable != undefined will return false if variable is equal to either null or undefined UNLESS undefined is reassigned beforehand.

You can reliably use the === operator to differentiate between undefined and null, if you need to make sure that a value is actually undefined (rather than null).

According to the ECMAScript 5 spec:

  • Both Null and Undefined are two of the six built in types.

4.3.9 undefined value

primitive value used when a variable has not been assigned a value

4.3.11 null value

primitive value that represents the intentional absence of any object value

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