Shuffling Array Elements
You want to shuffle the elements in an array.
The Fisher-Yates shuffle is a highly efficient and completely unbiased way to randomize the elements in an array. It's a fairly simple method: Start at the end of the list, and swap the last element with a random element from earlier in the list. Go down one and repeat, until you're at the beginning of the list, with all of the shuffled elements at the end of the list. This Fisher-Yates shuffle Visualization may help you understand the algorithm.
The Wrong Way to do it
There is a common--but terribly wrong way--to shuffle an array, by sorting by a random number.
If you do a sort randomly, it should give you a random order, right? Even Microsoft used this random-sort algorithm. Turns out, this random-sort algorithm produces biased results, because it only has the illusion of shuffling. Randomly sorting will not result in a neat, tidy shuffle; it will result in a wild mass of inconsistent sorting.
Optimizing for speed and space
The following code adds the shuffle function to the Array prototype, which means that you are able to run it on any array you wish, in a much more direct manner.
Note: Although it's quite common in languages like Ruby, extending native objects is
objects you don’t own; Extending built-in native objects. Evil or not?). That being said, the code above is really quite safe to add. It only adds
Array::shuffle if it doesn't exist already, thanks to the existential assignment
?=). That way, we don't overwrite someone else's, or a native browser method.
Also, if you think you'll be using a lot of these utility functions, consider using a utility library, like Lo-dash^†. They include a lot of nifty features, like maps and forEach, in a cross-browser, lean, high-performance way.
^† Underscore is also a good alternative to Lo-dash.