Mozilla Sweet.js: Extending JavaScript with Macros

Mozilla Sweet.js enables developers to enrich JavaScript by adding new syntax to the language through the use of macros. This helps developers to customize the JavaScript syntax for their style, or to expand it by creating a new JavaScript-based DSL useful for their niche domain.

Sweet.js provides the possibility to define hygienic macros inspired by Scheme and Rust with the macro keyword. A simple example is replacing the keyword function with a shorter one, def:

macro def {
  case $name:ident $params $body => {
    function $name $params $body
  }
}

Now, functions can be defined using def:

def add (a, b) {
  return a + b;
}

A more interesting example is introducing the keyword class:

macro class {
  case $className:ident { 
    constructor $constParam $constBody
    $($methodName:ident $methodParam $methodBody) ... } => {

    function $className $constParam $constBody

    $($className.prototype.$methodName 
      = function $methodName $methodParam $methodBody; ) ...
  }
}

An example of using class:

class Person {
  constructor(name) {
    this.name = name;
  }

  say(msg) {
    console.log(this.name + " says: " + msg);
  }
}
var bob = new Person("Bob");
bob.say("Macros are sweet!");

More macro examples can be found on Mozilla/Sweet.js wiki in GitHub where the source code can also be found under a BSD license.

Sweet.js files containing macros are compiled into pure JavaScript files without any foreign syntax with sjs. require-sweet provides an AMD loader and there is a SweetJS gem for compiling Sweet.js files from Ruby.

Sweet.js currently supports declarative macro definitions, but plans are to support imperative definitions in the future, according to Tim Disney from Mozilla Research. That means macros could contain arbitrary JavaScript code that is run at compile time.