Native pattern matching for JavaScript

npm install sparkler
1 downloads in the last day
1 downloads in the last week
26 downloads in the last month


Sparkler is a pattern matching engine for JavaScript built using sweet.js macros, so it looks and feels like native syntax. It has no runtime dependencies and compiles down to simple ifs and fors.

Here's a small slice of what you can do with it:

function myPatterns {
  // Match literals
  case 42 => 'The meaning of life'

  // Tag checking for JS types using Object::toString
  case a @ String => 'Hello ' + a

  // Array destructuring
  case [...front, back] => back.concat(front)

  // Object destructuring
  case { foo: 'bar', x, 'y' } => x

  // Custom extractors
  case Email{ user, domain: '' } => user

  // Rest arguments
  case (a, b, => rest

  // Rest patterns (mapping a pattern over many values)
  case [...{ x, y }] =>, y)

  // Guards
  case x @ Number if x > 10 => x

You can see a slew of more examples in the pattern spec file


npm install -g sweet.js
npm install sparkler
sjs -m sparkler/macros myfile.js

How Does It Work?

Sparkler overloads the function keyword as a macro (don't worry, all your old functions will still work) but implements a slightly different syntax. There's no argument list after the name or function keyword. Instead the function body is just a set of cases with ES6 style arrow-lambdas.

function myFunc {
  // Single arguments don't need parens and simple expression 
  // bodies get an implicit `return`.
  case a @ String => 'Hello ' + a

  // Add parens and braces if you need more.
  case (x, y, z) => {
    return x + y + z;

You can also do this with anonymous functions:

DB.getResource('123', function {
  case (null, resp) => complete(resp)
  case (err) => handleError(err)

If no case matches, a TypeError('No match') is thrown.

Branch Optimization

Sparkler doesn't just try each case one at a time until one passes. That would be really inefficient. Instead, it builds up a tree of patterns to try. Take something like this:

function expensiveExtraction {
  case (MyExtractor(x, y, z), 1) => doThis()
  case (MyExtractor(x, y, z), *) => doThat()

Let's say MyExtractor is really expensive. Since these two cases share the same root argument, they are grafted together. MyExtractor is only called once, even if the first case fails in its second argument.


If your cases aren't conducive to branch optimization, Sparkler will switch over to the backtracking compiler if it thinks it will help.

function useBacktracking {
  case (MyExtractor(x), 1) => doThis()
  case (Foo(x)        , 2) => doThat()
  case (MyExtractor(x), 3) => doThisAndThat()

The MyExtractor pattern in this example can't be grafted together because a different pattern separates them. With backtracking, each unique pattern for a given argument position is guaranteed to only ever run once.

Sparkler only uses the backtracking compiler when it must, because it can be verbose compared to the simple compiler. If you can write your cases in a way that doesn't require backtracking, you'll be much happier.

Note: simple patterns like booleans and number literals are not cached, since its more efficient just to run the comparison again than bother with the caching machinery.

Argument Length

In JavaScript, you can call a function with any number of arguments. Arguments that are not provided are just set to undefined. Sparkler preserves this behavior, so you have to be careful combining ambiguous cases.

function ambiguous {
  case (a)       => 1
  case (a, b)    => 2
  case (a, b, c) => 3

The above function will always return 3 no matter how many arguments you call it with. This is because all the argument cases share the same branching (of doing nothing). It basically gets compiled to this:

function ambiguous(arg1, arg2, arg3) {
  // The longest case on a branch gets compiled first.
  var a = arg1, b = arg2, c = arg3;
  return 3;

  var a = arg1, b = arg2;
  return 2;

  var a = arg1;
  return 1;

And then optimized to this:

function ambiguous(arg1, arg2, arg3) {
  var a = arg1, b = arg2, c = arg3;
  return 3;

If you want to match on specific argument length, you need to add a guard to your case.

function argCheck {
  // Using arguments.length
  case (a, b, c) if arguments.length == 3 => 1
  // Or matching undefined
  case (a, b, undefined) => 2
  case (a) => 1

The only time Sparkler is strict with argument length is with the empty parameter list (). It will check that arguments.length is zero. This is so you can do stuff like this:

Foo.prototype = {
  // jQuery-style getter/setter
  val: function {
    case () => this._val
    case (val) => {
      this._val = val;
      return this;

If you want a catch-all, you should use a wildcard (*) instead.

Match Keyword

Sparkler exports a match infix macro for doing easy matching on an expression.

var num = 12;
var isNumber = num match {
  case Number => true
  case * => false

This works by desugaring match into a self-invoking function with num as the argument.

Custom Extractors

You can match on your own types by implementing a simple protocol. Let's build a simple extractor that parses emails from strings:

var Email = {
  // Factor out a matching function that we'll reuse.
  match: function {
    case x @ String => x.match(/(.+)@(.+)/)
    case *          => null

  // `hasInstance` is called on bare extractors.
  hasInstance: function(x) {
    return !!Email.match(x);

  // `unapply` is called for array-like destructuring.
  unapply: function(x) {
    var m = Email.match(x);
    if (m) {
      return [m[1], m[2]];

  // `unapplyObject` is for object-like destructuring.
  unapplyObject: function(x) {
    var m = Email.match(x);
    if (m) {
      return {
        user: m[1],
        domain: m[2]

Now we can use it in case arguments:

function doStuffWithEmails {
  // Calls `unapplyObject`
  case Email{ domain: '' } => ...

  // Calls 'unapply'
  case Email('foo', *) => ...

  // Calls `hasInstance`
  case Email => ...

If you don't implement hasInstance, Sparkler will fall back to a simple instanceof check.


Adt.js is a library that implements the extractor protocol out of the box, and even has its own set of macros for defining data-types.

$data Tree {
  Node {
    value : *,
    left  : Tree,
    right : Tree

function treeFn {
  case Empty => 'empty'
  case Node{ value @ String } => 'string'

Optional Extensions

Sparkler also provides a small library that extends some of the native types with convenient functions.


// Date destructuring
function dateStuff {
  case Date{ month, year } => ...

// RegExp destructuring
function regexpStuff {
  case RegExp{ flags: { 'i' }} => ...

// Partial-function composition with `orElse`
function partial {
  case Foo => 'foo'

var total = partial.orElse(function {
  case * => 'anything'

orElse is added to the prototype safely using defineProperty and isn't enumerable.


Nathan Faubion (@natefaubion)



npm loves you