Question

I’ve been told not to use for...in with arrays in JavaScript. Why not?

Answer 1

The reason is that one construct:

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:

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:

// 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 */