lYriCAlsSH lYriCAlsSH - 4 months ago 7
Javascript Question

Why is using "for...in" with array iteration a bad idea?

I've been told not to use

for...in
with arrays in JavaScript. Why not?

Answer

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.
}

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
}

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'.
}