flattenArray.js

#!/usr/local/bin/node

/**
 * Converts current array to flat array
 * @returns {Array}
 */
Array.prototype.flatten = function () {

    /**
     * The result container
     * @type {Array}
     */
    var result = [];

    // Go through array items recursively and flatten them
    this.forEach(function (item) {
        if (item instanceof Array) {
            return item
                .flatten()
                .forEach(function (item) {
                    result.push(item)
                })
        }
        result.push(item)
    });

    return result
};

/**
 * The provided example
 * @type {Number[]}
 */
var exampleArray = [[1, 2, [3]], 4];

console.log(exampleArray.flatten());