JavaScript determines whether the object is an array

1. typeof

The first thing we will think of is to use typeof to detect the data type, but for basic types such as Function, String, Number, Undefined, etc. It is said that it can be detected using typeof. For example, the code is as follows:

function test(){}
console.log(typeof 1); // number
console.log(typeof test); // function 
console.log(typeof "yunxi"); // string
console.log(typeof undefined); // undefined

But for arrays or regular expressions, using typeof to detect them is not satisfactory, because when we detect arrays or regular expressions, Then the returned type will be an object object, as shown in the following code:

console.log(typeof []);  // object
console.log(typeof /\d+/g); // object

2. Instanceof

From this we can easily think of using instanceof to detect whether an object is an array For example, this detection will return a boolean. If it is an array, it will return true, otherwise it will return false. Let's take a look at the code to detect whether it is an array as follows:

console.log([] instanceof Array);  // true
console.log(/\d+/g instanceof Array); // false

You can see it as above Using instanceof can indeed determine whether it is an array element;

3. constructor attribute

In javascript, each object has a constructor attribute, which refers to the constructor that initializes the object. , such as determining the type of an unknown object, so we can write a method as follows, the code is as follows:

function isArray(obj) {
    return typeof obj == 'object' && obj.constructor == Array
}
// 测试demo
console.log(isArray([])); // true
var a = {"a":1};
console.log(isArray(a)); // false

var b = [1,2,3];
console.log(isArray(b)); // true
console.log(isArray(/\d+/g));// false

As you can see above, you can also determine whether it is an array element by calling the isArray method.

We can now see that for the second and third points, it seems that using the instanceof method and constructor attribute respectively can determine whether it is an array, but there are also exceptions, such as when using a cross-frame iframe. When using arrays in the page, it will fail because the arrays created in different iframes will not share their prototype attributes with each other; the following code test can be verified~

var iframe = document.createElement('iframe');
document.body.appendChild(iframe);
xArray = window.frames[window.frames.length-1].Array;       
var arr = new xArray("1","2","3","4","5");
//这个写法IE下是不支持的，标准浏览器firefox，chrome下有

console.log(arr);  // 打印出 ["1", "2", "3", "4", "5"]
console.log(arr instanceof Array); // false 
console.log(arr.constructor === Array); // false

We use the above method There is no way to determine whether an object is an array; but we can see from ECMA262 that you can use the Object.prototype.toString.call() method to determine whether an object is an array; the following code:

function isArray(obj) {
    return Object.prototype.toString.call(obj) == '[object Array]';
}
// 代码调用
console.log(isArray([]));  // true
console.log(isArray([1,2,3])); // true

var iframe = document.createElement('iframe');
document.body.appendChild(iframe);
xArray = window.frames[window.frames.length-1].Array;       
var arr = new xArray("1","2","3","4","5");

console.log(arr); // ["1","2","3","4","5"]
console.log(isArray(arr));  // true

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