typeof

JS数据类型有7中:

  1. 基本数据类型: String,Number,Boolean, Null, Undefined,Symbol
  2. 引用数据类型: Object({},[], function)

typeof返回值有7种,且值都是字符串类型
分别是:undefined,string,number,boolean,object,function,Symbol

var a; 
typeof a;                     // "undefined" 

a = "hello world"; 
typeof a;                     // "string" 

a=42; 
typeof a;                     // "number" 

a = true; 
typeof a;                     // "boolean" 


a = null; 
typeof a;                     // "object" (注意)

a = undefined; 
typeof a;                     // "undefined" 

a={b:"c"}; 
typeof a;                     // "object"

function test(){
    return 1;
}
typeof test                    // "function"

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