JavaScript typeof

JavaScript typeof, null, undefined, valueOf().

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##typeof operator

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

Example

     
  
 typeof 操作符返回变量或表达式的类型。
 
   

Tips: In JavaScript, an array is a special object type. So typeof [1,2,3,4] returns object.

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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

     
  
对象可以通过设置为 null 来清空。
 
   

Run the program to try it

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You can set it to undefined to clear the object:

Instance

     
  
对象可以设置为 undefined 来清空。
 
   

Run the program and try it

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Undefined

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

typeof A variable with no value will return undefined.

Example

     
  
变量的值如果不存在则该变量值为 undefined。
 
   

Run the program to try it

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

Instance

     
  
变量可以通过设置 undefined 来清空。
 
   

Run the program and try it

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Undefined The difference with Null

Run the program and try it