In-depth understanding of the concat method in javascript

Before the introduction, let me ask a question: How to merge multiple arrays into one array?

The following sharing will be divided into the following sections:

1.Basic introduction to the concat method

2.Experience the concat method from examples

1.Basic introduction to the concat method

The concat method is used to merge multiple arrays . It adds the members of the new array to the end of the original array, and then returns a new array, leaving the original array unchanged.

console.log([].concat([1],[2],[3])); // [1, 2, 3]
console.log([].concat([[1],[2],[3]])); // [[1], [2], [3]]
console.log([].concat(4,[[5,6],[7]])); // [4, [5, 6], [7]]

In the above code, the first return value is to merge an empty array and three arrays [1], [2], [3] into one array, so [1, 2, 3] is returned. The second one is to merge an empty array with a two-dimensional array. The members of the two-dimensional array are [1], [2], [3], so [[1], [2], [3]] is returned. Note that what is returned is a two-dimensional array. The same goes for the third example. It is important to understand the concept here, that is, adding the members of the new array to the end of the original array.

In addition to accepting arrays as parameters, concat can also accept other types of values ​​as parameters. They will be added to the end of the array as new elements.

console.log([].concat(1,2,3)); //[1,2,3];
//等同于
console.log([].concat(1,[2,3])); //[1,2,3];
console.log([].concat([1],[2,3])); //[1,2,3];

Although there is less content here, it seems quite simple. But it’s really not easy to understand.

2. Experience the concat method from an example

After talking about the basic knowledge, let me show you a problem I encountered recently. The original question is this.

You can understand what it means by looking at the example.

One of the solutions to this question is:

var flatten = function (arr){
 return [].concat.apply([],arr);
};

This simple function can realize the function of merging elements in an array. But when I was understanding this return value, a problem arose.

Question: Why is there a difference between using the apply method and not using the apply method?

console.log([].concat.apply([],[[1],[2],[3]])); //[1, 2, 3]
console.log([].concat([[1],[2],[3]])); //[[1], [2], [3]]

In the above code, a new array is also added to the end of an empty array, and the first one returned is [1,2,3]. The second one is a two-dimensional array.

After a period of tossing, I finally understood the reasons for the differences.

First of all, when we call the instance method concat in an empty array, we pass in the parameters in concat and push them to the end of the array.

console.log([].concat(1,2,3)); //[1, 2, 3]
console.log([].concat([1],[2],[3])); //[1, 2, 3]
console.log([].concat([[1],[2],[3]])); //[[1], [2], [3]]
console.log([].concat([[[1],[2],[3]]])); // [[[1], [2], [3]]]

In the above code, it gradually changes from a single element to a one-dimensional array, a two-dimensional array, and a three-dimensional array.

Detailed explanation and summary of the call, apply, and bind methods in Javascript. In the article, it is mentioned that the apply method is similar to the call method. It also changes the this pointer in a function (the scope where the function is executed), and then in Call this function in the specified scope. The function will also be executed immediately. The only difference is that it receives an array as a parameter when the function is executed. The first parameter of the

apply method is also the object that this points to. If it is set to null or undefined or this, it is equivalent to specifying the global object. The second parameter is an array, and all members of the array are used as parameters in turn and passed into the original function when calling. The parameters of the original function must be added one by one in the call method, but in the apply method, they must be added in the form of an array.

console.log([].concat([1],[2],[3])); //[1, 2, 3]
console.log([].concat([[1],[2],[3]])); //[[1], [2], [3]]
console.log([].concat.apply([],[[1],[2],[3]])); //[1, 2, 3]

In the above code, the concat method is used directly in the first half to merge the incoming parameters into the empty array. In the second half, the instance method apply of the String object is called, pointing this inside the concat() function to [], and in the [] scope, the parameters required to call concat are passed in as an array. . Let’s look at a few more examples.

console.log([].concat([1,2,3])); //[1, 2, 3]
console.log([].concat.apply([],[[1],[2],[3]]));//[1, 2, 3]
console.log([].concat([[1],[2],[3]]));//[[1], [2], [3]]
console.log([].concat.apply([],[[[1],[2],[3]]]));//[[1], [2], [3]]
console.log([].concat([[[1],[2],[3]]]));//[[[[1], [2], [3]]]]
console.log([].concat.apply([],[[[[1],[2],[3]]]]));//[[[1], [2], [3]]]

Summary:

1. When the concat method is used alone, the members of the new array will be added to the end of the original array, and then a new array will be returned. The original array will remain unchanged; if other types of values ​​are passed in , they will be added to the end of the array as new elements.

2. Method of merging array elements:

var flatten = function (arr){
 return [].concat.apply([],arr);
};

var flatten = function (array){
 return array.reduce(function(a,b){
 return a.concat(b);
 },[])
}

If you don’t understand the apply method, you can go to this article: Detailed explanation and summary of call, apply, and bind methods in Javascript