JavaScript为什么在数组迭代中使用“ for…in”是个坏主意？

JavaScript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]);}

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);}

还请注意，Javascript库可能会执行以下 *** 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 pre 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);}