为什么这两个javascript 2d阵列的行为有所不同？

问题描述

在我的函数中，我定义了两个数组，第一个（ array1 ）具有预初始化的长度。我添加了第二个数组（ array2 ）仅用于测试，因为我认为第一个表现很奇怪。

In my function, I have defined two arrays, the first (array1), has a pre-initialized length. I added the second array (array2) just for testing because I thought the first was behaving strangely.

我的代码：

function test(n = 3) {
  array1 = new Array(n).fill(new Array(n));
  array2 = [
    [undefined, undefined, undefined],
    [undefined, undefined, undefined],
    [undefined, undefined, undefined]
  ];

  document.getElementById("output").innerHTML = JSON.stringify(array1) + " (array 1) <br/>" + JSON.stringify(array2) + " (array 2)<br/><br/><hr/>";


  for (i = 0; i < n; i++) {
    array1[i][0] = i;
    array2[i][0] = i;
  }

  document.getElementById("output").innerHTML += JSON.stringify(array1) + " (array 1) <br/>" + JSON.stringify(array2) + " (array 2)<br/><br/><hr/>";

}

<button onclick="test();">Press to test</button>

<br/><br/>
<div id="output"></div>

在 for 循环中，我尝试更改第二个维度的第一个值。它应输出 [[0，undefined，undefined]，[1，undefined，undefined]，[2，undefined，undefined]] ，就像第二个数组一样。

In the for loop, I try to change the first value of the second dimensions. It should output [[0, undefined, undefined], [1, undefined, undefined], [2, undefined, undefined]], like the second array does.

我的问题是：为什么会发生这种情况？并且，我如何在两个维度中创建长度为 n 的预初始化数组，其行为类似于第二个数组？

My questions are: why does this happen? And, how can I make a pre-initialized array with length n in both dimensions, that behaves like the second array?

推荐答案

因为

采用静态值并用它填充数组。因此，你会在 array1 的每个元素中获得相同的填充数组。

takes a static value and fills the array with it. Therefore you get in every element of array1 the same array of the filling.

function test(n = 3) {
    var array1 = new Array(n).fill(new Array(n)),
        array2 = [[undefined, undefined, undefined], [undefined, undefined, undefined], [undefined, undefined, undefined]],
        i;

    for (i = 0; i < n; i++) {
        array1[i][0] = i;
        array2[i][0] = i;
    }

    document.getElementById("output").innerHTML = array1 + " (array 1) <br/>" + array2 + " (array 2)<br/><br/><i>The commas with nothing in between mean undefined.</i>";
    console.log(array1);
    console.log(array2);
}

<button onclick="test();">Press to test</button><br/><br/>
<div id="output"></div>

要获得一个独立的填充数组，可以使用并使用映射值映射一个新数组。

To get an independent filled array, you could use Array.from and map a new array with mapped values.

var array = Array.from({ length: 3 }, _ => Array.from({ length: 3 }, _ => 4));

array[0][0] = 0;
console.log(array);

.as-console-wrapper { max-height: 100% !important; top: 0; }

这篇关于为什么这两个javascript 2d阵列的行为有所不同？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！