Function to flatten an array

array_flatten.js

/**
 * Recursivelly flattens an array
 *
 * @param {Array} input The input array
 * @return {Array|Number} Flattened array or -1 if the input is invalid
 */
function flatten (input) {
    if(!Array.isArray(input)) {
        return -1;
    }
    return input.reduce(function (flat, flattenable) {
        if (Array.isArray(flattenable)) {
            return flat.concat(flatten(flattenable));
        } else {
            return flat.concat(flattenable);
        }
    }, []);
}

// Tests
var array1 = [[1, 2, [3]], 4];
var array2 = [5, [1, [2], [3]], 4, {n:1}];
var invalid1 = 'invalid input';

console.log(flatten(array1));
console.log(flatten(array2));
console.log(flatten(invalid1));