javascript - 为什么在数组迭代中使用“ for…in”是个坏主意？(Why is using “for…in” with array iteration a bad idea?)

Question

I've been told not to use for...in with arrays in JavaScript.

(有人告诉我不要将for...in用于JavaScript for...in的数组。)

Why not?

(为什么不？)

  ask by lYriCAlsSH translate from so

Answer 1

The reason is that one construct:

(原因是一种构造：)

 var a = []; // Create a new empty array. a[5] = 5; // Perfectly legal JavaScript that resizes the array. for (var i = 0; i < a.length; i++) { // Iterate over numeric indexes from 0 to 5, as everyone expects. console.log(a[i]); } /* Will display: undefined undefined undefined undefined undefined 5 */ 

can sometimes be totally different from the other:

(有时可能与另一个完全不同：)

 var a = []; a[5] = 5; for (var x in a) { // Shows only the explicitly set index of "5", and ignores 0-4 console.log(x); } /* Will display: 5 */ 

Also consider that JavaScript libraries might do things like this, which will affect any array you create:

(还请注意， JavaScript库可能会执行以下操作，这会影响您创建的任何数组：)

 // Somewhere deep in your JavaScript library... Array.prototype.foo = 1; // Now you have no idea what the below code will do. var a = [1, 2, 3, 4, 5]; for (var x in a){ // Now foo is a part of EVERY array and // will show up here as a value of 'x'. console.log(x); } /* Will display: 0 1 2 3 4 foo */