Introduction to JavaScript type judgment methods (code example)

This article brings you an introduction to JavaScript type judgment methods (code examples). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

JS data type judgment

Sometimes it is necessary to judge the data type and deal with various complex logical judgments. Let’s start with the most commonly used ones.

1.typeof

The typeof operator returns a string and represents the type of the variable.

typeof oper / typeof (operand)

var testString = 'adus',
    testArray = [],
    testBoolean = true,
    testNumber = 0,
    testObject = {},
    testNull = null,
    testUndefined = undefined
    
console.log(typeof testString);//string
console.log(typeof testArray);//object
console.log(typeof testBoolean);//boolean
console.log(typeof testNumber);//number
console.log(typeof testObjec);//object
console.log(typeof testNull);//object
console.log(typeof testUndefined);//undefined

Of course, there are not only the above basic types, but also the following:

Symbol (new in ECMAScript 6) "symbol"

Host object (provided by the JS environment) Implementation-dependent

Function object ([[Call]] is implemented in the ECMA-262 clause) "function"

Hey ? Something is wrong. Why are arrays, null, etc. all objects? ? ? Please look down at the instances of

Function or Array types. They are actually extensions based on Object instances. It just has some more unique attributes.

In the original implementation of JavaScript, values ​​in JavaScript were represented by a tag representing the type and the actual data value. The object's type tag is 0.

Because null represents a null pointer (the value is 0x00 on most platforms).

Therefore, the type label of null also becomes 0, and typeof null returns "object" incorrectly. (reference)

2.instanceof

The instanceof operator is used to test whether the prototype attribute of the constructor appears anywhere in the prototype chain of the object. In layman's terms, it means whether a variable is a certain An instance of the object.

object instanceof constructor

object The object/constructor constructor to be detected

function fnc(){}
var newFnc = new fnc();
console.log(newFnc.__proto__ == fnc.prototype);//true
console.log( newFnc instanceof fnc ) //true


/*String对象和Date对象都属于Object类型和一些特殊情况*/

var simpleStr = "This is a simple string"; 
var myString  = new String();
var newStr    = new String("String created with constructor");
var myDate    = new Date();
var myObj     = {};
var myNonObj  = Object.create(null);

simpleStr instanceof String; // 返回 false, 检查原型链会找到 undefined
myString  instanceof String; // 返回 true
newStr    instanceof String; // 返回 true
myString  instanceof Object; // 返回 true

myObj instanceof Object;    // 返回 true, 尽管原型没有定义
({})  instanceof Object;    // 返回 true, 同上
myNonObj instanceof Object; // 返回 false, 一种创建对象的方法，这种方法创建的对象不是Object的一个实例

myString instanceof Date; //返回 false

myDate instanceof Date;     // 返回 true
myDate instanceof Object;   // 返回 true
myDate instanceof String;   // 返回 false

3.Object.prototype.toString()

Each object There is a toString() method that is automatically called when the object is represented as a text value, or when an object is referenced in the expected string manner. By default, the toString() method is inherited by every Object object. If this method is not overridden in a custom object, toString() returns "[object type]", where type is the type of the object.

Function.prototype.call( thisArg ) / Function.prototype.apply( thisArg )

Pass the object to be checked as the first parameter, called thisArg.

var toString = Object.prototype.toString;

toString.call(new Date); // [object Date]
toString.call(new String); // [object String]
toString.call(Math); // [object Math]

//Since JavaScript 1.8.5
toString.call(undefined); // [object Undefined]
toString.call(null); // [object Null]

Using the appropriate method in the appropriate scenario will save you worry and ensure the simplicity and robustness of the code.

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