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ECMAScript 6 in Node.JS

This text introduces and illustrates, with simple examples, ECMAScript 6 (ES6 for short) features natively available in Node. No transpiler or shim is required to run the code snippets. We hope the reader finds the subset of ES6 presented here interesting.

The underlying philosophy and broad direction of ES6 has mostly been agreed upon. However, implementation details are being polished until the final specification is published. Experimental ES6 in Node may not comply with the latest draft specification, available here.

We assume the unstable 0.11.x branch, which has greater ES6 support than the stable 0.10.x branch. For version control, we recommend n. To list the flags enabling experimental ES6 in Node, use node --v8-options | grep harmony.

The single --harmony flag enables most of the ES6 experimental features. As of v0.11.13, you need the --use_strict flag for the block scoping examples.

Pull requests are welcome. Enjoy.

Table of Contents

  1. Block Scoping
  2. Generators
  3. Proxies
  4. Maps and Sets
  5. Weak Maps
  6. Symbols

Block scoping

Let us start with let. Think of let as a block-scoped variation of var for variable declaration.

{ let a = 'I am declared inside an anonymous block'; }
console.log(a); // ReferenceError: a is not defined

Up until ES6, JavaScript only had function scoping, largely considered a design flaw. We illustrate improvements brought by ES6 with three examples.

The first example is about private variables.

// ES5: A convoluted function closure
var login = (function ES5() {
  var privateKey = Math.random();

  return function (password) {
    return password === privateKey;
// ES6: A simple block
  let privateKey = Math.random();

  var login = function (password) {
    return password === privateKey;

The second example has to do with variable hoisting.

// ES5: Defensive declarations at the top to avoid hoisting surprises
function fibonacci(n) {
  var previous = 0;
  var current = 1;
  var i;
  var temp;

  for(i = 0; i < n; i += 1) {
    temp = previous;
    previous = current;
    current = temp + current;

  return current;
// ES6: Variables are concealed within the appropriate block scope
function fibonacci(n) {
  let previous = 0;
  let current = 1;

  for(let i = 0; i < n; i += 1) { // Implicit block scope for the loop header
    let temp = previous;
    previous = current;
    current = temp + current;

  return current;

The third example is regarding nested for loops.

// ES5: Reusing the same loop variable name is bad
var counter = 0;
for(var i = 0; i < 3; i += 1) {
  for(var i = 0; i < 3; i += 1) {
    counter += 1;

console.log(counter); // Oops, prints "3"
// ES6: Reusing the same loop variable name is OK
var counter = 0;
for(let i = 0; i < 3; i += 1) {
  for(let i = 0; i < 3; i += 1) {
    counter += 1;

console.log(counter); // Prints "9"

The designers of ES6 conceived of a baby sister for let. The keyword const declares block-scoped constant variables.

const a = 'You shall remain constant!';

a = 'I wanna be free!'; // SyntaxError: Assignment to constant variable

On a more esoteric note, ES6 is fixing a long standing issue with block scope function definitions. The following code is not well defined in the ES5 specification.

function f() { console.log('I am outside!'); }
(function () {
  if(false) {
    // What should happen with this redeclaration?
    function f() { console.log('I am inside!'); }


Should the redeclaration of f be hoisted? Should it be ignored because the if block is not executed? Should it be scoped to the if block? Different browsers handle things differently. In ES6 function declarations are block-scoped, so the above snippet will print I am outside!.

Finally, ES6 throws a syntax error when multiple let declarations of the same variable occur in the same block. No analogous error is thrown for var redeclarations within the same scope.

  let a;
  let a; // SyntaxError: Variable 'a' has already been declared


Generators allow for function-like behaviour where execution is segmented into "pieces". Execution is paused at the end of each piece and can be resumed at the start of the next piece. The syntax for generators is similar to that of functions but the function keyword is replaced by function*. Flow control is dictated with yield statements.

function* argumentsGenerator() {
  for (let i = 0; i < arguments.length; i += 1) {
    yield arguments[i];

(Note that although the yield keyword is not a reserved keyword in ES5, the new function* syntax guarantees no ES5 function using "yield" as a variable name will break in ES6.)

Generators are useful because they return (i.e. create) iterators. In turn, an iterator, an object with a next method, actually executes the body of generators. The next method, when repeatedly called, partially executes the corresponding generator, gradually advancing through the body until a yield keyword is hit.

var argumentsIterator = argumentsGenerator('a', 'b', 'c');

// Prints "a b c"

The next method of an iterator returns an object with a value property and a done property, as long as the body of the corresponding generator has not returned. The value property refers the value yielded or returned. The done property is false up until the generator body returns, at which point it is true. If the next method is called after done is true, an error is thrown.

Alongside generators and iterators, ES6 has syntactic sugar for iteration.

// Prints "a", "b", "c"
for(let value of argumentsIterator) {

Generators are ideal for defining sequences of undetermined lengths...

function* fibonacci() {
  let a = 0, b = 1;

  while(true) {
    yield a;
    [a, b] = [b, a + b];

...which are elegantly enumerated.

// Enumerates the Fibonacci numbers
for(let value of fibonacci()) {

Generators can be used to provide an alternative to the traditional nested callback flow control, thereby avoiding callback "pyramids" and "hell". Two libraries, task.js and gen-run, aim to help write asynchronous JavaScript in a sequential style.

// task.js example
spawn(function*() {
  var data = yield $.ajax(url);
  var status = $('#status').html('Download complete.');
  yield status.fadeIn().promise();
  yield sleep(2000);

The sequential flow control also allows for meaningful try-catch statements, so the burden of explicitly passing errors through the callback chain, commonly found in Node libraries, can be alleviated in ES6.

To conclude, it is possible for a generator to yield to an iterator using a "delegated yield" with the syntax yield*.

let delegatedIterator = (function* () {
  yield 'Hello!';
  yield 'Bye!';

let delegatingIterator = (function* () {
  yield 'Greetings!';
  yield* delegatedIterator;
  yield 'Ok, bye.';

// Prints "Greetings!", "Hello!", "Bye!", "Ok, bye."
for(let value of delegatingIterator) {


A proxy is a meta-programming object for which some primitive object behaviours are replaced by function calls, called traps. The traps are methods in an associated handler object.

var random = Proxy.create({
  get: function () {
    return Math.random();

The function Proxy.create creates a proxy whose handler object is passed as the first argument. Here, we have a single get trap which overrides property reads. For each read of random, a new random value is computed at run time.

// Prints three random numbers
console.log(random.value, random.value, random.value);

Similarly, property writes can be trapped by the set trap.

var time = Proxy.create({
  get: function () {
  set: function () {
    throw 'Time travel error!';

The continue with the theme of dictating how objects should behave at the "native" level, we build arrays for which negative indexes behave as in Python.

function pythonArray(array) {
  var dummy = array;

  return Proxy.create({
    set: function (receiver, index, value) {
      dummy[index] = value;
    get: function (receiver, index) {
        index = parseInt(index);
        return index < 0 ? dummy[dummy.length + index] : dummy[index];

Now the index -1 references the last array element, -2 the penultimate, etc.

Notice set has three arguments; receiver refers to the proxy, index to the property name and value to the property value.

// Prints "gamma"
console.log(pythonArray(['alpha', 'beta', 'gamma'])[-1]);

Proxies can also be used for clean data binding. With Backbone.JS models, for example, data binding is done at the expense of having to use the model.get and model.set methods. This syntactic indirection is unnecessary with proxies.

Let's conclude proxies with a somewhat sophisticated security example. Please put your abstraction hat on.

Suppose a function f wants to share an object o to another function g and later revoke access to o. Well, f can give g a proxy p to o. The traps of p grant or deny access based on a key k private to f. Actually, a proxy q on the handler object h of p can implement the access mechanism for all the traps of p simultaneously with a single get trap.

To recap, g interfaces through p to access o. The relevant trap of p in h is called triggering the get trap of q in charge of access control based on k.

TODO: Add examples which are not get or set.

Maps and sets

A map can be thought of as a object for which the keys can be arbitrary objects. In ES5, the method toString is implicitly called on property keys before a property access, which is less than helpful for object keys given that ({}.toString()) is [object Object].

Let's illustrate...

const gods = [
  {name: 'Douglas Crockford'},
  {name: 'Guido van Rossum'},
  {name: 'Raffaele Esposito'}

let miracles = new Map();

miracles.set(gods[0], 'JavaScript');
miracles.set(gods[1], 'Python');
miracles.set(gods[2], 'Pizza Margherita');

// Prints "JavaScript"

A set is a data structure containing a finite set of elements, each occurring exactly once. The constructor is Set, and the API is simple.

// Prints [ 'constructor', 'size', 'add', 'has', 'delete', 'clear' ]

To illustrate, we have surveyed six people regarding their greatest pleasure in life...

let surveyAnswers = ['sex', 'sleep', 'sex', 'sun', 'sex', 'cinema'];

let pleasures = new Set();
surveyAnswers.forEach(function (pleasure) {

// Prints the number of pleasures in the survey, not counting duplicates

Unfortunately, support for array to set conversion, as well as set iteration is not yet supported in Node.JS.

Both maps and sets are naturally occurring data structures which can and have been implemented through object abuse. We now discuss weak maps, which are data structures which cannot be emulated with ES5.

Weak maps

Weak maps fit in a somewhat different category to that of maps and sets because weak maps fundamentally provide more than just syntax sugar. Weak maps are like maps but are intimately linked to garbage collection, providing a tool to help writing code that does not leak memory.

The JavaScript virtual machine, V8 in the case of Node, periodically frees memory allocated to objects no longer in scope. An object is no longer in scope if there is no chain of references from the current scope leading to it. If an object is held in scope, but never gets used, we say there is a memory leak. Such leaks are especially problematic when they occur periodically, as the memory allocated by the virtual machine increases over time, eventually breaking things.

By setting a key-value pair in a weak map, no reference to the property key is created. Instead, references to keys which are internal to weak maps can be thought as "weak", meaning that from the point of view of the garbage collector they are ignored. In particular, property keys of a weak map cannot be enumerated. Also, weak maps do not have a size property analogous to maps as this would expose garbage collector behaviour which should be kept hidden.

let weakMap = WeakMap();

// This is effectively a noop, the key-value pair can immediately be garbage collected
weakMap.set({}, 'noise');

Weak maps allows for "meta-data" to be associated to an object without obstructing garbage collection would the object otherwise escape the current scope.

// Expose user metadata to the rest of the program
let userMetadata = WeakMap();

server.on('userConnect', function (user) {
  userMetadata.set(user, { connectionTime: });

  server.on('userDisconnect', function () {
    // Do stuff with `user` and discard of it, automatically discarding `userMetadata`

Because literals in JavaScript are passed by value (instead of by reference), literals cannot be weak map keys.

let weakMap = WeakMap();

// TypeError: Invalid value used as weak map key
weakMap.set('example string literal', 0);

From the point of view of JavaScript code, there is no way to confirm that objects have been garbage collected. One has to have faith that the virtual machine is doing its job properly. Alternatively, the guts of the virtual machine can be inspected with a debugger, such as node-inspector.


Symbols extend the set of key values, i.e. object property identifiers. In ES5 the set of key values corresponds precisely to the set of strings.

let a = {};
let debugSymbol = Symbol();

a[debugSymbol] = 'This property value is identified by a symbol';

Symbols are unique by construction which make them useful for avoiding name collisions. By unique, we mean that two symbols created at different points in time will never refer to the same property of an object.

In ES5, name collisions avoidance is hacked by using names which are unlikely to collide. The following snippet from jQuery's source illustrates this.

  // Unique for each copy of jQuery on the page
  // Non-digits removed to match rinlinejQuery
  expando: "jQuery" + ( core_version + Math.random() ).replace( /\D/g, "" ),

Uniqueness has interesting consequences.

let a = Map();
a.set(Symbol(), 'Noise');

// Prints "1" although the one element cannot be accessed!

For informational or debugging purposes, an immutable name can be given when instantiating a symbol.

let testSymbol = Symbol('This is a test');

// Prints "This is a test"

TODO: Discuss private symbols when the get implemented.

Object observation


Typed arrays and array buffers


Concluding note

As of Node v0.11.3, a lot of the ES6 features providing new semantics to the language are available. As we saw, these include proxies, symbols and weak maps. We look forward for tasty syntax sugar yet to come, such as rest and spread operators, destructuring and classes. Yum yum.


ECMAScript 6 examples for Node.JS



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