What data types can JavaScript's typeof return?

In JavaScript, the data types that the typeof operator can return are: "undefined", "object", "boolean", "number", "string", "symbol", "function", etc.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

Use the typeof operator to return the data type of the variable.

Let’s take a look at the value of typeof corresponding to each data type:

##Symbol (new in ECMAScript 6)"symbol" Host object (provided by the JS environment, such as the browser)Implementation-dependentFunction object"function"Any other object"object"

Data type	Result
Undefined	"undefined"
Null	"object"
Boolean value	"boolean"
numeric value	"number"
Character String	"string"

Look at the specific instance:

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // 尽管NaN是"Not-A-Number"的缩写,意思是"不是一个数字"
typeof Number(1) === 'number'; // 不要这样使用!

// Strings
typeof "" === 'string';
typeof "bla" === 'string';
typeof (typeof 1) === 'string'; // typeof返回的肯定是一个字符串
typeof String("abc") === 'string'; // 不要这样使用!

// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(true) === 'boolean'; // 不要这样使用!

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

// Undefined
typeof undefined === 'undefined';
typeof blabla === 'undefined'; // 一个未定义的变量,或者一个定义了却未赋初值的变量

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

// 使用Array.isArray或者Object.prototype.toString.call方法可以从基本的对象中区分出数组类型
typeof [1, 2, 4] === 'object';

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

// 下面的容易令人迷惑，不要这样使用！
typeof new Boolean(true) === 'object';
typeof new Number(1) ==== 'object';
typeof new String("abc") === 'object';

// 函数
typeof function(){} === 'function';
typeof Math.sin === 'function';

We will find One problem is that typeof is not accurate in determining the data type. For example, the typeof return value of arrays, regular expressions, dates, and objects is all object, which will cause some errors.

So on the basis of typeof judging the type, we also need to use the

Object.prototype.toString method to further judge the data type.

Let’s take a look at the difference between the return values ​​​​of the toString method and the typeof method in the case of the same data type:

Data toStringtypeof"foo"Stringstringnew String(“foo”)Stringobject##new Number(1.2)truenew Boolean(true )new Date()new Error()##new Array(1, 2, 3)ArrayobjectRegExpRegExpYou can see that Array and Error can be correctly distinguished using the toString method , RegExp, Date and other types. So we generally use this method to verify the data type.

Number	object
Boolean	boolean
Boolean	object
Date	object
Error	object
		##/abc/g
object		new RegExp(“meow”)
object

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