javascript - 如何在Javascript数组的开头添加新的数组元素？(How can I add new array elements at the beginning of an array in Javascript?)

Question

I have a need to add or prepend elements at the beginning of an array.

(我需要在数组的开头添加或添加元素。)

For example, if my array looks like below:

(例如，如果我的数组如下所示：)

[23, 45, 12, 67]

And the response from my AJAX call is 34 , I want the updated array to be like the following:

(我的AJAX调用的响应为34 ，我希望更新后的数组如下所示：)

[34, 23, 45, 12, 67]

Currently I am planning to do it like this:

(目前，我正在计划这样做：)

var newArray = [];
newArray.push(response);

for (var i = 0; i < theArray.length; i++) {
    newArray.push(theArray[i]);
}

theArray = newArray;
delete newArray;

Is there any better way to do this?

(有什么更好的方法吗？)

Does Javascript have any built-in functionality that does this?

(Javascript是否具有执行此操作的任何内置功能？)

The complexity of my method is O(n) and it would be really interesting to see better implementations.

(我的方法的复杂度为O(n) ，看到更好的实现将真的很有趣。)

  ask by Moon translate from so

Answer 1

Use unshift .

(使用unshift 。)

It's like push , except it adds elements to the beginning of the array instead of the end.

(就像push一样，除了将元素添加到数组的开头而不是结尾。)

• unshift / push - add an element to the beginning/end of an array

  (unshift / push -一个元素添加到一个数组的开始/结束)

• shift / pop - remove and return the first/last element of an array

  (shift / pop删除并返回数组的第一个/最后一个元素)

A simple diagram...

(一个简单的图...)

   unshift -> array <- push
   shift   <- array -> pop

and chart:

(和图表：)

          add  remove  start  end
   push    X                   X
    pop           X            X
unshift    X             X
  shift           X      X

Check out the MDN Array documentation .

(查看MDN阵列文档 。)

Virtually every language that has the ability to push/pop elements from an array will also have the ability to unshift/shift (sometimes called push_front / pop_front ) elements, you should never have to implement these yourself.

(实际上，每种能够从数组中推入/弹出元素的语言都将具有取消/移入（有时称为push_front / pop_front ）元素的能力，您永远不必自己实现这些元素。)

---

As pointed out in the comments, if you want to avoid mutating your original array, you can use concat , which concatenates two or more arrays together.

(如注释中所指出的那样，如果要避免更改原始数组，可以使用concat ，它将两个或多个数组连接在一起。)

You can use this to functionally push a single element onto the front or back of an existing array;

(您可以使用它在功能上将单个元素推到现有数组的前面或后面；)

to do so, you need to turn the new element into a single element array:

(为此，您需要将新元素转换为单个元素数组：)

const array = [ 3, 2, 1 ]

const newFirstElement = 4

const newArray = [newFirstElement].concat(array) // [ 4, 3, 2, 1 ]

concat can also append items.

(concat也可以附加项目。)

The arguments to concat can be of any type;

(concat的参数可以是任何类型。)

they are implicitly wrapped in a single-element array, if they are not already an array:

(如果它们还不是数组，则将它们隐式包装在单元素数组中：)

const array = [ 3, 2, 1 ]

const newLastElement  = 0

// Both of these lines are equivalent:
const newArray1 = array.concat(newLastElement)   // [ 3, 2, 1, 0 ]
const newArray2 = array.concat([newLastElement]) // [ 3, 2, 1, 0 ]