Detailed explanation of JavaScript implementation of data types and array judgment methods

This article tells about JavaScriptHow to implement Data types and the judgment of arrays. If you don’t understand the judgment of JavaScript’s implementation of data types and arrays, or you don’t know about JavaScript’s implementation of data types and arrays If you are interested in the judgment, let’s take a look at this article. Okay, let’s stop talking and get to the point!

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1. The six major data types of js

number：数字，整数、浮点数等等，
string：单引号或者双引号来说明，
Boolean：返回true和false，这两个值不一定对应1和0
object：对象，可以执行new操作符后跟要创建的对象类型的名称来创建。
null：只有一个值得数据类型，逻辑上讲，null值表示一个空对象指针。
undefined：未定义，使用var声明变量但未对其初始化时，变量的值就是undefined。

2. Data type judgment typeof
typeof can solve the judgment of most data types, and its return value is a string, which describes the type of the operand.

//判断变量num是不是一个数字类型
if(typeof num=='number') {
    return true;
}

Return results:

var a="hling"; console.log(a); //string
var a=1; console.log(a); //number
var a=false; console.log(a); //boolean
var a; console.log(typeof a); //undfined

var a = null; console.log(typeof a); //object
var a = document; console.log(typeof a); //object
var a = []; console.log(a); //object

var a = function(){}; console.log(typeof a) 
//function除了可以判断数据类型还可以判断function类型

In addition to the four types of string, number, boolean, and undefined, null, object, and array all return object types! ! !

For function types, function is returned, such as typeof(Date), typeof(eval), etc.

3. How to determine array type in js

1) instanceof
Instanceof is used to determine whether a variable is an instance of an object. It is a ternary operator. This operator has something to do with Object-oriented in JavaScript. To understand this, you must first understand object-oriented in JavaScript. Because this operator detects whether the prototype chain of the object points to the prototype object of the constructor .

a instanceof b?alert("true"):alert("false")  
//注意b值是你想要判断的那种数据类型，是不是一个字符串，比如Array

例子：var arr = [1,2,3,1]; 
alert(arr instanceof Array); // true

2) constructor
Definition in the W3C definition: The constructor attribute returns a reference to the array function that created this object

var arr = []; 
arr instanceof Array; // true 
arr.constructor == Array; //true

Judges various types The method is:

console.log("string".constructor == String);
console.log((123).constructor == Number);
console.log(false.constructor == Boolean);
console.log([].constructor == Array);
console.log({}.constructor == Object);

General method:

function isArray(object){
    return object && typeof object==='object' &&
            Array == object.constructor;
}

3) Characteristic judgment
object.isArray() to judge, the purpose is to accurately detect whether a value is an array. IE9+, Firefox 4+, Safari 5+, Opera 10.5+ and Chrome all implement this method. However, versions before IE8 are not supported.

function isArray(object){
    return  object && typeof object==='object' &&    
            typeof object.length==='number' &&  
            typeof object.splice==='function' &&    
             //判断length属性是否是可枚举的 对于数组 将得到false  
            !(object.propertyIsEnumerable('length'));
}

4) Object.prototype.toString.call

Object.prototype.toString.call(value) == '[object Array]'

The above is all the content of this article. If you don’t know much about it, you can implement it yourself. It’s easy to master both sides!
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