JavaScript Typeof, Null and Undefined


typeof Operator

You can use the typeof operator to detect the data type of a variable.

Example

typeof "John" // return string
typeof 3.14 // return number
typeof false // return boolean
typeof [1,2,3,4] // return object
typeof {name:'John', age:34} // return 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 means "nothing".

Null is a special type with only one value. Represents an empty object reference.

NoteUse typeof to detect null and return an object.

You can set it to null to clear the object:

Example

var person = null; // The value is null (empty), but the type is an object

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You can set it to undefined to clear the object:

Example

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

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undefined

In JavaScript, undefined is a variable with no set value.

typeof a variable with no value will return undefined .

Example

var person; // value is undefined (empty), type is undefined

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Any variable can be cleared by setting the value to undefined . The type is undefined .

Example

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

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The Difference Between undefined and null

Example

The values of null and undefined are equal, but the types are not equal:

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

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