JavaScript typeof, null, and undefined

JavaScript typeof, null, undefined, valueOf().

The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable.

Example

typeof "John"                // Returns string
typeof 3.14                  // Returns number
typeof false                 // Returns boolean
typeof [1,2,3,4]             // Returns object
typeof {name:'John', age:34} // Returns object

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Note  In JavaScript, an array is a special type of object. Therefore typeof [1,2,3,4] returns object. 

Null

In JavaScript null is "nothing". It is supposed to be something that doesn't exist.

Unfortunately, in JavaScript, the data type of null is an object.

Note You can consider it a bug in JavaScript that typeof null is an object. It should be null.

You can empty an object by setting it to null:

Example

var person = null;           // Value is null, but type is still an object

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You can also empty an object by setting it to undefined:

Example

var person = undefined;     // Value is undefined, type is undefined

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Undefined

In JavaScript, undefined is a variable with no value.

The typeof a variable with no value is also undefined.

Example

var person;                  // Value is undefined, type is undefined

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Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

Example

person = undefined;          // Value is undefined, type is undefined

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Difference Between Undefined and Null

typeof undefined             // undefined
typeof null                  // object
null === undefined           // false
null == undefined            // true

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