array-flatten.js

// Using a wrapper function to prevent global namespace pollution
(function () {
    var tempArray = [],  // the final flattened array
            idx = 0;  // the position of elements in the flattent result array

    /**
     * A helper function to actually flatten the array
     * @param array The given array as input
     * @param inner A parameter to identify whether or not the function is recursively called
     * @returns {*} Returns a flattened array
     * @private
     */
    function _flatten(array, inner) {
        //  if the input is not an array then it will return an empty array
        if (!(array instanceof Array)) return [];

        array.forEach(function (val, i) {
            if (val instanceof Array)
                tempArray.concat(_flatten(val, true));
            else
                tempArray[idx++] = val;
        });
        // create a deep copy of the flattened array
        var result = tempArray.slice(0);
        // if all elements were flattened, then clear the temporary array
        if (!inner) {
            tempArray = [];
            idx = 0;
        }
        return result;
    }

    /**
     * A prototype method on the Array object to flatten the given array.
     * Usage:
     * array.flatten()
     *
     * @returns {*} Returns a flattened array
     */
    Array.prototype.flatten = function () {
        return _flatten(this);
    };
})();



// ------------------
// Usage and Examples:
// ------------------

console.log([].flatten());
console.log([[1, 2, [3]], 4].flatten());
console.log([[1], [2], [3, 4]].flatten());