array_diff in JavaScript

To get elements from the array that are not present in another array.

var a = [2, 4, 8, 16, 32],
    b = [2, 4, 32, 64, 128, 256];

// Define custom method on Array prototype
// Pass the array as parameter
Array.prototype.diff = function (arr) {

    // Merge the arrays
    var mergedArr = this.concat(arr);

    // Get the elements which are unique in the array
    // Return the diff array
    return mergedArr.filter(function (e) {
        // Check if the element is appearing only once
        return mergedArr.indexOf(e) === mergedArr.lastIndexOf(e);
    });
};

// How to call?
var diff = a.diff(b);

console.log(diff);
document.write('<pre>' + JSON.stringify(diff, 0, 4) + '</pre>');

[
    8,
    16,
    64,
    128,
    256
]

Shorter version:
var diff = array1.concat(array2).filter(function (e, i, array) {
    // Check if the element is appearing only once
    return array.indexOf(e) === array.lastIndexOf(e);
});