flatten-es6.js

/**
  * ES6 implementation of flatten function.
  * It's sorter due to syntax sugar but usually consume more memory due to larger amount of function calls.
  */
function flatten(arr) {
    return arr.reduce(
        (a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
    );
}

flatten.js

/**
  * Old style implementation of flatten function. It creates a new array and returns it.
  */
function flatten(arr) {
    var result = [];

    for (let i = 0; i < arr.length; ++i) {
	if (typeof arr[i] === "object") { // Check for an array
            if (!arr[i].length) {
	        continue; // Skip objects
	    }

	    var flattenSubArray = flatten(arr[i]);
	    for (let j = 0; j < flattenSubArray.length; ++j) {
	        result.push(flattenSubArray[j]);
	    }
	} else {
	    result.push(arr[i]);
	}
    }

    return result;
}