为什么在数组迭代中使用“ for…in”是个坏主意？

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 */

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 */

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 */

问题来源于stack overflow