Fancy JavaScript

Usage no npm install needed!

<script type="module">
  import fancyscript from '';



Fancy JavaScript

FancyScript aims to make developing in JavaScript easier and more powerful while remaining "just JavaScript". For a completely different approach with diverse syntax rules, and in the latter case, a powerful standard library, look toward CoffeeScript or ClojureScript. FancyScript compiles to plain ES5 JavaScript. Thanks @tarebyte for the name.


npm install -g fancyscript installs the FancyScript binary fancy so it can be used globally.

fancy input.fs compiles the FancyScript source file input.fs to plain JavaScript in input.js.


  • -b, --bare Compile without a top-level function wrapper.
  • -o, --out [DIR] Write all compiled JavaScript files into the specified directory.
  • -r, --repl Start an interactive FancyScript REPL. Interrupts all other options.
  • -w, --watch Watch specified files for changes.


Strict by default

FancyScript follows strict-mode JavaScript conventions. Unless the --bare option is passed, the compiled code is wrapped in an IIFE (Immediately-Invoked Function Expression) with "use strict"; inside. For the sake of clarity, other examples will not include the function wrapper and strict mode declaration.

var a = 42;
(function () {
    "use strict";
    var a = 42;

Arrow functions

Arrow functions in Harmony are a more concise syntax for function expressions, consisting of a parameter list and an automatically-returned expression or a statement block.

[1, 2, 3, 4].map(x => x * x);
[1, 2, 3, 4].map(function (x) {
    return x * x;

Default arguments

Use default arguments to provide a default value within the function's parameter list for any optional arguments.

function foo(a, b = 42, c, d = bar(), e) { }
function foo(a, b, c, d, e) {
    if (typeof b === 'undefined') { b = 42; }
    if (typeof d === 'undefined') { d = bar(); }

Rest parameters

From the MDN page, rest parameters represent "an indefinite number of arguments as an array." A function declaration or function expression may have a rest parameter as its last argument which will absorb all the rest of the arguments when the function is called.

fn isEven (...x) {
    x.every(fn (x) { x % 2 === 0 });
function isEven() {
    var x =, 0);
    return x.every(function (x) {
        return x % 2 === 0;

isEven(2, 4, 6); // true

Spread operator

From the MDN page, the spread operator expands an expression into multiple arguments to a function or multiple elements in array literals. Currently only the former case is supported, but the latter is in progress.

var arr = ['b', 'c', 'd'];
console.log('a', ...arr, 'e');
var arr = ['b', 'c', 'd'];
console.log.apply(console.log, [].concat(['a'], arr, ['e'])); // a b c d e

Note that support for the spread operator in a call to Function.prototype.apply is neither available nor planned. A compelling use case would be needed.

Array destructuring

Array destructuring can occur in variable declarations, assignment expressions, or function parameters.

var [a, b] = [11, 42];
console.log([a, b] = [b, a]); // [42, 11]
function show([, second]) {
show([a, b]); // 11
var $tmp = [11, 42], a = $tmp[0], b = $tmp[1];
console.log(function ($tmp) {
    a = $tmp[0];
    b = $tmp[1];
    return $tmp;
}([b, a]));
function show($tmp) {
    var second = $tmp[1];
    return console.log(second);
show([a, b]);

Object destructuring

Like array destructuring, object destructuring is syntactic sugar for more easily creating objects or retrieving values from them. This can occur in variable declarations, function parameter lists, or as a shortcut in object expressions.

var { name, age: a } = getPerson(); // Declare variables "name" and "a"
var obj = { name, age: a }; // Shortcut to initialize to variable "name"
function ({ name, age: b}, cb) { }
({ name, age: a }) = getAnotherPerson();
var $tmp = getPerson(), name = $tmp['name'], a = $tmp['age'];
var obj = { name: name, age: a };
function ($tmp2, cb) {
    var name = $tmp2['name'], b = $tmp2['age'];
(function ($tmp3) {
    name = $tmp3['name'];
    a = $tmp3['age'];
    return $tmp3;

Automatic return values

If the last statement of a function is an expression contained in an ExpressionStatement, it is automatically returned.

fn isEven (x) { x % 2 === 0 }
function isEven(x) {
    return x % 2 === 0;

fn keyword

fn is an alias to the function keyword and is interchangeable.

fn hello () {
    conole.log('Hello, world!');
function hello () {
    console.log('Hello, world!');


var isEven = (x, ...r) => x % 2 === 0 && (r.length ? isEven(...r) : true)
var isEven = function (x) {
    var r =, 1);
    return x % 2 === 0 && (r.length ? isEven.apply(isEven, [].concat(r)) : true);

isEven(2, 4, 6, 7); // false
isEven(2, 4, 6, 8); // true


Licensed under the BSD 3-Clause License, the full text of which can be read in LICENSE.