For Loops

for-loop.js

var a = ['cat', 'dog', 'pig', 'bird'];

// works, but you are accessing the a.length property every time.
for (var i = 0; i < a.length; i += 1) {
    console.log(a[i]);
} 

// better because it is caching length, especially for DOM collections
for (var i = 0, len = a.length; i < len; i += 1) {
    console.log(a[i]);
}