Things to note about push methods in JavaScript arrays

The

push() method adds one or more elements to the end of thearrayand returns the new length. This article mainly introduces the usage ofJavaScript arraypush method. Friends who need it can refer to the push method of

js array. I believe everyone knows that it adds elements to the end of the array, but there are A very key point to note:

Quoted from MDN

Return value

When this method is called, the new The lengthattributevalue will be returned.

var sports = ["soccer", "baseball"]; var total = sports.push("football", "swimming"); console.log(sports); // ["soccer", "baseball", "football", "swimming"] console.log(total); // 4

After an array push, length is returned instead of a new array. If you don’t know this, you will encounter big pitfalls during use.

By the way, remember the return values of several other array methods:

pop()

pop() method fromDeletethe last element in the array and return the value of that element. This method changes the length of the array.

let a = [1, 2, 3]; a.length; // 3 a.pop(); // 3 console.log(a); // [1, 2] a.length; // 2 arr.pop()返回值从数组中删除的元素(当数组为空时返回undefined)。

shift()

shift() method removes the first element from an array and returns the value of that element. This method changes the length of the array.

let a = [1, 2, 3]; let b = a.shift(); console.log(a); // [2, 3] console.log(b); // 1 返回值 从数组中删除的元素; undefined 如果数组为空。 arr.shift()

unshift()

unshift() 方法将一个或多个元素添加到数组的开头，并返回新数组的长度。 let a = [1, 2, 3]; a.unshift(4, 5); console.log(a); // [4, 5, 1, 2, 3] arr.unshift(element1, ..., elementN) 参数列表 element1, ..., elementN 要添加到数组开头的元素。 返回值 当一个对象调用该方法时，返回其 length 属性值。

concat()

The concat() method is used to merge two or more arrays. This method does not change the existing array, but returns a new array.

var arr1 = ['a', 'b', 'c']; var arr2 = ['d', 'e', 'f']; var arr3 = arr1.concat(arr2); // arr3 is a new array [ "a", "b", "c", "d", "e", "f" ] var new_array = old_array.concat(value1[, value2[, ...[, valueN]]]) 参数 valueN 将数组和/或值连接成新数组。 返回值 新的 Array 实例。

splice()

The splice() method changes the contents of an array by removing existing elements and/or adding new elements.

Return value

An array composed of the deleted elements. If only one element was removed, an array containing only one element is returned. If no elements were removed, an empty array is returned.

slice()

The slice() method returns a shallow copy of a portion of the selected array from start to end (not including the end). A new array object, the original array will not be modified.

Return value:

A new array containing the extracted elements

Summary:

Starts with Addition at the end returns the length of the array; deletion at the beginning and end of

returns the deleted element;

splice() returns the deleted element;

concat returns New array;

slice returns the extracted array;

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