Lazy.js it a utility library for JavaScript, similar to Underscore and Lo-Dash but with one important difference: lazy evaluation (also known as deferred execution)


Like Underscore, but lazier

Build Status Bower version NPM version

Lazy.js is a functional utility library for JavaScript, similar to Underscore and Lodash, but with a lazy engine under the hood that strives to do as little work as possible while being as flexible as possible.

It has no external dependencies, so you can get started right away with:

<script type="text/javascript" src="lazy.js"></script>

<!-- optional: if you want support for DOM event and AJAX-based sequences: -->
<script type="text/javascript" src="lazy.browser.js"></script>

Or, if you're using Node.js:

var Lazy = require('lazy.js');

Now let's look at what you can do with Lazy.js. (For more thorough information, take a look at the API Docs.)


Let's start with an array of objects representing people.

var people = getBigArrayOfPeople();

Suppose we're using this array to back some sort of search-as-you-type functionality, where users can search for people by their last names. Naturally we want to put some reasonable constraints on our problem space, so we'll provide up to 5 results at a time. Supposing the user types "Smith", we could therefore fetch results using something like this (using Underscore):

var results = _.chain(people)
  .filter(function(name) { return name.startsWith('Smith'); })

This query does a lot of stuff:

  • pluck('lastName'): iterates over the array and creates a new (potentially giant) array
  • filter(...): iterates over the new array, creating yet another (potentially giant) array
  • take(5): all that just for 5 elements!

So if performance and/or efficiency were a concern for you, you would probably not do things that way using Underscore. Instead, you'd likely go the procedural route:

var results = [];
for (var i = 0; i < people.length; ++i) {
  if (people[i].lastName.startsWith('Smith')) {
    if (results.length === 5) {

There—now we haven't created any extraneous arrays, and we did all of the work in one iteration. Any problems?

Well, yeah. The main problem is that this is one-off code, which isn't reusable or particularly readable. If only we could somehow leverage the expressive power of Underscore but still get the performance of the hand-written procedural solution...

That's where Lazy.js comes in! Here's how we'd write the above query using Lazy.js:

var result = Lazy(people)
  .filter(function(name) { return name.startsWith('Smith'); })

Looks almost identical, right? That's the idea: Lazy.js aims to be completely familiar to JavaScript devs experienced with Underscore or Lodash. Every method from Underscore should have the same name and (almost) identical behavior in Lazy.js, except that instead of returning a fully-populated array on every call, it creates a sequence object with an each method.

What's important here is that no iteration takes place until you call each, and no intermediate arrays are created. Essentially Lazy.js combines all query operations into a "sequence" that behaves quite a bit like the procedural code we wrote a moment ago. (If you ever do want an array, simply call toArray on the resulting sequence.)

Of course, unlike the procedural approach, Lazy.js lets you keep your code clean and functional, and focus on solving whatever problem you're actually trying to solve instead of optimizing array traversals.


So, Lazy.js is basically Underscore with lazy evaluation. Is that it?


Indefinite sequence generation

The sequence-based paradigm of Lazy.js lets you do some pretty cool things that simply aren't possible with Underscore's array-based approach. One of these is the generation of indefinite sequences, which can go on forever, yet still support all of Lazy's built-in mapping and filtering capabilities.

Here's an example. Let's say we want 300 unique random numbers between 1 and 1000.

  .map(function(e) { return Math.floor(e * 1000) + 1; })
  .each(function(e) { console.log(e); });

Here's a slightly more advanced example: let's use Lazy.js to make a Fibonacci sequence.

var fibonacci = Lazy.generate(function() {
  var x = 1,
      y = 1;
  return function() {
    var prev = x;
    x = y;
    y += prev;
    return prev;

// Output: [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

OK, what else?

Asynchronous iteration

You've probably seen code snippets before that show how to iterate over an array asynchronously in JavaScript. But have you seen an example with functional goodness like this?

  .async(1000) // specifies a 1-second interval between each element
  .map(function(x) { return String.fromCharCode(x + 65); })
  .each(function(char) { console.log(char); });

All right... what else?

Event sequences

With indefinite sequences, we saw that unlike Underscore and Lodash, Lazy.js doesn't actually need an in-memory collection to iterate over. And asynchronous sequences demonstrate that it also doesn't need to do all its iteration at once.

Now here's a really cool combination of these two features: with a small extension to Lazy.js (lazy.browser.js, a separate file to include in browser-based environments), you can apply all of the power of Lazy.js to handling DOM events. In other words, Lazy.js lets you think of DOM events as a sequence—just like any other—and apply the usual map, filter, etc. functions on that sequence.

Here's an example. Let's say we want to handle all mousemove events on a given DOM element, and show their coordinates in one of two other DOM elements depending on location.

// First we define our "sequence" of events.
var mouseEvents = Lazy(sourceElement).on("mousemove");

// Map the Event objects to their coordinates, relative to the element.
var coordinates = {
  var elementRect = sourceElement.getBoundingClientRect();
  return [
    Math.floor(e.clientX - elementRect.left),
    Math.floor(e.clientY -

// For mouse events on one side of the element, display the coordinates in one place.
  .filter(function(pos) { return pos[0] < sourceElement.clientWidth / 2; })
  .each(function(pos) { displayCoordinates(leftElement, pos); });

// For those on the other side, display them in a different place.
  .filter(function(pos) { return pos[0] > sourceElement.clientWidth / 2; })
  .each(function(pos) { displayCoordinates(rightElement, pos); });

Anything else? Of course!

String processing

Now here's something you may not have even thought of: String.match and String.split. In JavaScript, each of these methods returns an array of substrings. If you think about it, this often means doing more work than necessary; but it's the quickest way (from a developer's standpoint) to get the job done.

For example, suppose you wanted the first five lines of a block of text. You could always do this:

var firstFiveLines = text.split("\n").slice(0, 5);

But of course, this actually splits the entire string into every single line. If the string is very large, this is quite wasteful.

With Lazy.js, we don't need to split up an entire string just to treat it as a sequence of lines. We can get the same effect by wrapping the string with Lazy and calling split:

var firstFiveLines = Lazy(text).split("\n").take(5);

This way we can read the first five lines of an arbitrarily large string (without pre-populating a huge array) and map/reduce on it just as with any other sequence.

Similarly with String.match: let's say we wanted to find the first 5 alphanumeric matches in a string. With Lazy.js, it's easy!

var firstFiveWords = Lazy(text).match(/[a-z0-9]+/i).take(5);

Piece of cake.

Stream processing

Lazy.js can wrap streams in Node.js as well.

Given any Readable Stream, you can wrap it with Lazy just as with arrays:

  .take(5) // Read just the first 5 chunks of data read into the buffer.

For convenience, specialized helper methods for dealing with either file streams or HTTP streams are also offered. (Note: this API will probably change.)

// Read the first 5 lines from a file:

// Read lines 5-10 from an HTTP response.

In each case, the elements in the sequence will be "chunks" of data most likely comprising multiple lines. The lines() method splits each chunk into lines (lazily, of course).

This library is experimental and still a work in progress.