Summary of distinguishing between Object and Aarry methods in JS

1. Frequently encountered problems:

When judging the type of an object in JS, typeof is usually used. At this time, the problem arises, because typeof() returns when identifying an array. object, so judging whether an object is an array in JS

requires some special processing methods. The six processing methods summarized by me are introduced below.

2. Straight to the point

To determine whether an object is an array during development, it is recommended to use the following function:

function isArray(obj){
  if(Array.isArray){
    return Array.isArray(obj);
  }else{
   return Object.prototype.toString.call(obj)==="[object Array]";
  }
}

The above function is convenient for people who are eager to solve problems. I will describe the six methods in detail below, because the examiner may need a comprehensive knowledge of you during the interview;

3. Detailed explanation of the six solutions:

(1) Method 1: Use the toString method

Try to convert the variable into a string representing its type by calling the toString() method. This method is feasible for real arrays; when the parameter object is converted to string

returns [object Arguments], the conversion will fail; in addition, the conversion will also fail for the object class containing a numeric length attribute.

The method is as follows:

nbsp;html>

 
 <title>Array的判断方法</title>
 <meta>
 <script>
	function isArrayOne(arr){
		return <span style="color:#cc0000;">Object.prototype.toString.call(arr) === "[object Array]";
	}
	var obj = {"k1":"v1"};
	var arr = [1,2];
	console.log("对象的结果："+isArrayOne(obj));
	console.log("数组的结果："+isArrayOne(arr));
 </script>
 
 
 
 

The result is as shown:

Note: It is recommended to use "===" for equals instead of "==" for equals, because it is more efficient!

(2) Method 2: Through isArray:

Using Javascript 1.8.5 (ECMAScript 5), the variable name .isArray() can achieve this purpose, provided that this function is supported. In fact, isArray is the encapsulation of

method one.

The usage is very simple:

Array.isArray(obj); //obj is the object to be detected

Returns true or false, if true, it is an array

(3) Method 3: Judge through the instanceof operator,

Note: The left side of the instanceof operator is the child object (the object to be tested), and the right side is the parent constructor (here is Array),

That is: child object instanceof parent constructor

instance: instance: any object created with new constructor () is called an instance of the constructor

I’ve been confused for a long time, so it’s better to look at the code directly:

nbsp;html>

 
 <meta>
 <meta>
 <meta>
 <meta>
 <meta>
 <title>Document</title>
 <script>
	var obj = {"k1":"v1"};
	var arr = [1,2];
	console.log("Instanceof处理对象的结果："+(obj instanceof Array));
	console.log("Instanceof处理数组的结果："+(arr instanceof Array));
 </script>
 
 
 
 

The running results are as follows:

(4) Use the isPrototypeOf() function

Principle: Detect whether an object is the prototype of Array (or is in the prototype chain, not only the direct parent object, but also all parent objects on the entire prototype chain can be detected)

Usage: parent.isPrototypeOf(child) to detect whether parent is the prototype of child;

It should be noted that the function implemented by the isPrototypeOf() function is very similar to the instancof operator;

Specific code:

Array.prototype.isPrototypeOf(arr) //true means it is an array, false means it is not an array

(5)Use the constructor

Specific code:

nbsp;html>

 
 <meta>
 <meta>
 <meta>
 <meta>
 <meta>
 <title>Document</title>
 <script>
	var obj = {&#39;k&#39;:&#39;v&#39;};
	var t1 = new Array(1);
	var t2 = t1;
	console.log(obj.constructor == Array);
	console.log(t1.constructor == Array);
	console.log(t2.constructor == Array);
 </script>
 
 
	
 

The result is as shown in the figure

(6) Use typeof(object ) + type name combined judgment:

The code is as follows:

nbsp;html>

 
 <meta>
 <meta>
 <meta>
 <meta>
 <meta>
 <title>Document</title>
 <script>
	function isArrayFour(arr){
		if(typeof(arr)==="object"){
			if(arr.concat){
				return "This is Array";
			}else{
				return "This Not Array";
			}
		}
	}
	var arr = [1];
	var obj = {&#39;k&#39;:&#39;v&#39;};
	console.log(typeof(arr));
	console.log(typeof(obj));
	console.log(isArrayFour(arr));
	console.log(isArrayFour(obj));
 </script>
 
 
 
 

The result is as follows:

This method actually has limitations Sex, some students may crack it at once, that is, what if

What if this attribute is accidentally defined in the object

var obj = {'concat':'Teast me?'};

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