Never use for..in loop in Javascript

example.js

// Somewhere deep in your JavaScript library...
Array.prototype.foo = 1;


// While you have no idea that your code will misbehave.

var a = []; // Create a new empty array.
a[5] = 5;   // Perfectly legal JavaScript that resizes the array.

// normal loop saves you
console.log('-- normal for loop --');
for (var i = 0; i < a.length; i++) {
    // Iterate over numeric indexes from 0 to 5, as everyone expects.
    console.log(a[i]);
}

// but for in loop troubles you
console.log('-- for-in loop --');
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);
}