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 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);}
