typeof用法介绍

转自：https://blog.youkuaiyun.com/z18842589113/article/details/53315910

typeof 运算符把类型信息当作字符串返回。typeof 返回值有六种可能： “number,” “string,” “boolean,” “object,” “function,” 和 “undefined”。

我们可以使用typeof来获取一个变量是否存在，如if(typeof a!=”undefined”){}，而不要去使用if(a)因为如果a不存在（未声明）则会出错。

if(document.mylist.length != “undefined” ) {} //这个用法有误.
if( typeof(document.mylist.length) != “undefined” ) {}//正确的，或者
if( !isNaN(document.mylist.length) ) {}

typeof的运算数未定义,返回的就是 “undefined”
运算数为数字 typeof(x) = “number”
字符串 typeof(x) = “string”
布尔值 typeof(x) = “boolean”
对象,数组和null typeof(x) = “object”
函数 typeof(x) = “function”

对于Array,Null等特殊对象使用typeof一律返回object，这正是typeof的局限性。

Examples

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof(42) === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; 
// Despite being "Not-A-Number"

typeof Number(1) === 'number'; 
// but never use this form!



// Strings
typeof "" === 'string';
typeof "bla" === 'string';
typeof (typeof 1) === 'string'; 
// typeof always returns a string

typeof String("abc") === 'string'; 
// but never use this form!



// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(true) === 'boolean'; 
// but never use this form!



// Symbols
typeof Symbol() === 'symbol'
typeof Symbol('foo') === 'symbol'
typeof Symbol.iterator === 'symbol'


// Undefined
typeof undefined === 'undefined';
typeof declaredButUndefinedVariable === 'undefined';
typeof undeclaredVariable === 'undefined'; 


// Objects
typeof {a:1} === 'object';

// use Array.isArray or Object.prototype.toString.call

// to differentiate regular objects from arrays

typeof [1, 2, 4] === 'object';

typeof new Date() === 'object';

// The following is confusing. Don't use!

typeof new Boolean(true) === 'object'; 
typeof new Number(1) === 'object'; 
typeof new String("abc") === 'object';


// Functions
typeof function(){} === 'function';
typeof class C {} === 'function';
typeof Math.sin === 'function';


//null
// This stands since the beginning of JavaScript
typeof null === 'object';