Justin R. Buchanan

on Software Development, Systems Administration, Networking, and Random other Stuff

Fancy For Loops – Part 1

Using libraries like underscore (or Lo-Dash) for traversing and transforming arrays or objects can be a great time saver (even though, it seems like they did it wrong). However, when helping developers with anything new, I have found the less “black box” libraries you throw at someone, the better they are able to learn what’s really going on. Performance and style discussions aside, I’d rather see a beginner JavaScript developer write tons of for or while loops before finding out what library X, Y, or Z helps them do. Not using a library is also a great learning experience about writing your own algorithms, or using polyfills because IE8 doesn’t support map() or filter().

Most of the array prototype methods can be implemented with a simple for or while loop, generally with better performance, but possibly not as elegantly or with the same level of reusability. This post is not intended to be a guide on when to use and when not to use these methods (or the underscore/Lo-Dash equivalents), but rather help understand the concepts.

Disclaimer: I am by no means a JavaScript performance or functional programming expert, but I play one at work.

Array.prototype.forEach() or “A for loop, but all functional and stuff”

Consider the following example that loops over each item in an array and logs the value to the debug console.

var values = [1,2,3,4,5,6,7,8,9,10];
for (var i = 0; i < values.length; i++){
    console.log('value at index ' + i + ': ' + values[i]);

This is pretty much the “Hello World” of for loops in JavaScript. We can write the same thing using the array prototype method .forEach(). According to the MDN, arrays have a forEach() method that accepts a callback function as the first argument, and thisArg as an optional second. We’ll ignore thisArg for this particular post.

As an aside, if you aren’t using the MDN while writing web apps, you are doing it wrong (or you have it all memorized, in which case you should be working for NASA or maybe as one of those waiters that never writes anything down to be all impressive and stuff).


The callback function will be executed once per array element, passing in the array element as the first argument, the index of the element as the second, and the array itself as the third. Using the forEach() method, we can produce the same output as the above code this way:

var values = [1,2,3,4,5,6,7,8,9,10];
values.forEach(function(value, idx, arr){
    console.log('value at index ' + idx + ': ' + value);

Array.prototype.map() or “Make all Items in an Array into Something Else”

Consider the following example that uses a for loop to produce an array of upper case letters from an array of lower case letters (without modifying the original array or its contents).

var letters = ['a','b','c','d','e','f'];
var upperLetters = new Array(letters.length);

for (var i = 0; i < items.length; i++){

// upperLetters now contains:
// ['A','B','C','D','E','F'];

We can write the same thing using map(). According to the MDN, the map() method on JavaScript arrays accepts two arguments, callback, and optionally thisArg. The first argument, callback will be executed over each item in the array. The return value of map() will be an array containing all the return values from callback. Underscore map does the same thing, but, it works reliably in browsers that do not implement native JavaScript map. The following code produces the same output as the above code.

var letters = ['a','b','c','d','e','f'];

var upperLetters = letters.map(function(value){
    return value.toUpperCase();

// upperLetters now contains:
// ['A','B','C','D','E','F'];

So why would you use the second example over the first, especially considering it is slower? The answer, of course is, “it depends”.  Using map() with a named function or function variable can be really useful for writing more concise code. Consider the following example code that creates copies of three arrays, while removing leading and trailing whitespace from the array elements:

var arr1 = [' bob', 'sally  ', '  tod', ' phil  '];
var arr2 = [' teresa ', ' julie  ', '  sandy  ', ' ron  '];
var arr3 = [' jason', ' jill  ', ' jane  ', ' sam  '];

var arr1trimmed = arr1.map(String.prototype.trim);
// ['bob', 'sally', 'tod', 'phil'] var arr2trimmed = arr2.map(String.prototype.trim);
// ['teresa', 'julie', 'sandy', 'ron'] var arr3trimmed = arr3.map(String.prototype.trim);
// ['jason', 'jill', 'jane', 'sam'];

This code reuses the trim() function, rather than passing an anonymous function into forEach() like we did earlier.  It should be noted that this simplistic trim() function example will throw an exception if any of the values in the array are undefined, null, or not a string.

Hopefully this helps un-black-box things a bit. In Part 2, we’ll look at Array.prototype.filter(), Array.prototype.reduce(), and look more at what the underscore and Lo-Dash libraries provide relating to for-loops.

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