Knowledge about JavaScript typeof, null, and undefined

JavaScript typeof, null, and undefined operators play a very important role in js. Let us explain them in detail.

typeof operator

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

Example

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

In JavaScript, an array is a special object type. So 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.

#Use typeof to detect null and return object.

You can set it to null to clear the object:

Instance

var person = null;           // 值为 null(空), 但类型为对象

You can set it to undefined to clear the object:

Instance

var person = undefined;     // 值为 undefined, 类型为 undefined

undefined

In JavaScript, undefined is a variable that has no set value.

typeof A variable with no value will return undefined.

Example

var person;                  // 值为 undefined(空), 类型是undefined

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

Instance

person = undefined;          // 值为 undefined, 类型是undefined

The difference between undefined and null

Instance

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

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

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