javascript - 使用数字和字符串创建新的 Array() 构造函数

Question

实际上，我正在为其他人编写代码。

在解决我发现的错误时 诸如此类。

注意:API 给出的是数字类型的产品编号。

Sometimes passing only one productNumber in new Array(). then a new bug occurs.

And when entering productNumber more than one then working fine.

Why arr1[0] is giving undefined.

var productNumber1 = 1001;
var arr1 = new Array(productNumber1);
console.log("arr1",arr1[0]);

var productNumber2 = '1001';
var arr2 = new Array(productNumber2);
console.log("arr2",arr2[0]);

能否请人解释一下这种行为。

Answer 1

在 case1 中，您传递的参数被视为数组的长度

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

A JavaScript array is initialized with the given elements, except in the case where a single argument is passed to the Array constructor and that argument is a number (see the arrayLength parameter below). Note that this special case only applies to JavaScript arrays created with the Array constructor, not array literals created with the bracket syntax.

arrayLength

If the only argument passed to the Array constructor is an integer between 0 and 2^32-1 (inclusive), this returns a new JavaScript array with its length property set to that number (Note: this implies an array of arrayLength empty slots, not slots with actual undefined values). If the argument is any other number, a RangeError exception is thrown.

所以实际上在案例 1 中，您正在创建一个长度为 1001 的数组。

但是在情况 2 中，它被视为一个元素，因为它不是数字。