Documentation: ECMAScript

This is intended as a quick reference for what you can use as JavaScript features.

The best support for ES6 out there

Using Babel, we provide the best support for ES6 on the market, in terms of variaty of availble language features.

Starting from Pagehop 1.3, you are be able to use features such as classes, modules, arrow functions, generators, iterators, for-of cycles and many others.

Check the current coverage of ES6 by Babel.js.

Supported ECMAScript 6 features


Arrows are a function shorthand using the => syntax. They are syntactically
similar to the related feature in C#, Java 8 and CoffeeScript. They support
both expression and statement bodies. Unlike functions, arrows share the same
lexical this as their surrounding code.

// Expression bodies
var odds = => v + 1);
var nums =, i) => v + i);

// Statement bodies
nums.forEach(v => {
if (v % 5 === 0)

// Lexical this
var bob = {
_name: "Bob",
_friends: [],
printFriends() {
this._friends.forEach(f =>
console.log(this._name + " knows " + f));


ES6 classes are a simple sugar over the prototype-based OO pattern. Having a
single convenient declarative form makes class patterns easier to use, and
encourages interoperability. Classes support prototype-based inheritance, super
calls, instance and static methods and constructors.

class SkinnedMesh extends THREE.Mesh {
constructor(geometry, materials) {
super(geometry, materials);

this.idMatrix = SkinnedMesh.defaultMatrix();
this.bones = [];
this.boneMatrices = [];
update(camera) {
static defaultMatrix() {
return new THREE.Matrix4();

Enhanced Object Literals

Object literals are extended to support setting the prototype at construction,
shorthand for foo: foo assignments, defining methods and making super calls.
Together, these also bring object literals and class declarations closer
together, and let object-based design benefit from some of the same

var obj = {
// __proto__
__proto__: theProtoObj,
// Shorthand for ‘handler: handler’
// Methods
toString() {
// Super calls
return "d " + super.toString();
// Computed (dynamic) property names
[ "prop_" + (() => 42)() ]: 42
__proto__ support comes from the JavaScript engine running your program
Although most support the now standard property, [some do not](

Template Strings

Template strings provide syntactic sugar for constructing strings. This is
similar to string interpolation features in Perl, Python and more. Optionally, a
tag can be added to allow the string construction to be customized, avoiding
injection attacks or constructing higher level data structures from string

// Basic literal string creation
`In JavaScript "\n" is a line-feed.`

// Multiline strings
`In JavaScript this is
not legal.`

// Interpolate variable bindings
var name = "Bob", time = "today";
`Hello ${name}, how are you ${time}?`

// Construct an HTTP request prefix is used to interpret the replacements and construction
Content-Type: application/json
X-Credentials: ${credentials}
{ "foo": ${foo},
"bar": ${bar}}`(myOnReadyStateChangeHandler);


Destructuring allows binding using pattern matching, with support for matching
arrays and objects. Destructuring is fail-soft, similar to standard object
lookup foo["bar"], producing undefined values when not found.

// list matching
var [a, , b] = [1,2,3];

// object matching
var { op: a, lhs: { op: b }, rhs: c }
= getASTNode()

// object matching shorthand
// binds `op`, `lhs` and `rhs` in scope
var {op, lhs, rhs} = getASTNode()

// Can be used in parameter position
function g({name: x}) {
g({name: 5})

// Fail-soft destructuring
var [a] = [];
a === undefined;

// Fail-soft destructuring with defaults
var [a = 1] = [];
a === 1;

Default + Rest + Spread

Callee-evaluated default parameter values. Turn an array into consecutive
arguments in a function call. Bind trailing parameters to an array. Rest
replaces the need for arguments and addresses common cases more directly.

function f(x, y=12) {
// y is 12 if not passed (or passed as undefined)
return x + y;
f(3) == 15
function f(x, ...y) {
// y is an Array
return x * y.length;
f(3, "hello", true) == 6
function f(x, y, z) {
return x + y + z;
// Pass each elem of array as argument
f(...[1,2,3]) == 6

Let + Const

Block-scoped binding constructs. let is the new var. const is
single-assignment. Static restrictions prevent use before assignment.

function f() {
let x;
// okay, block scoped name
const x = "sneaky";
// error, const
x = "foo";
// error, already declared in block
let x = "inner";

Iterators + For..Of

Iterator objects enable custom iteration like CLR IEnumerable or Java
Iterable. Generalize to custom iterator-based iteration with
for..of. Don’t require realizing an array, enabling lazy design patterns like

let fibonacci = {
[Symbol.iterator]() {
let pre = 0, cur = 1;
return {
next() {
[pre, cur] = [cur, pre + cur];
return { done: false, value: cur }

for (var n of fibonacci) {
// truncate the sequence at 1000
if (n > 1000)

Iteration is based on these duck-typed interfaces (using
TypeScript type syntax for exposition only):

interface IteratorResult {
done: boolean;
value: any;
interface Iterator {
next(): IteratorResult;
interface Iterable {
[Symbol.iterator](): Iterator


Generators simplify iterator-authoring using function* and yield. A function
declared as function* returns a Generator instance. Generators are subtypes of
iterators which include additional next and throw. These enable values to
flow back into the generator, so yield is an expression form which returns a
value (or throws).

Note: Can also be used to enable ‘await’-like async programming, see also ES7
await proposal.

var fibonacci = {
[Symbol.iterator]: function*() {
var pre = 0, cur = 1;
for (;;) {
var temp = pre;
pre = cur;
cur += temp;
yield cur;

for (var n of fibonacci) {
// truncate the sequence at 1000
if (n > 1000)

The generator interface is (using TypeScript type
syntax for exposition only):

interface Generator extends Iterator {
next(value?: any): IteratorResult;
throw(exception: any);


Array and generator comprehensions provide simple declarative list processing
similar as used in many functional programming patterns.

// Array comprehensions
var results = [
for(c of customers)
if ( == "Seattle")
{ name:, age: c.age }

// Generator comprehensions
var results = (
for(c of customers)
if ( == "Seattle")
{ name:, age: c.age }
Not available in Pagehop
Due to their experimental nature, these are only available if experimental support is enabled, which is not possible in Pagehop, at this moment.


Non-breaking additions to support full Unicode, including new unicode literal
form in strings and new RegExp u mode to handle code points, as well as new
APIs to process strings at the 21bit code points level. These additions support
building global apps in JavaScript.

// same as ES5.1
"𠮷".length == 2

// new RegExp behaviour, opt-in ‘u’
"𠮷".match(/./u)[0].length == 2

// new form
"\u{20BB7}" == "𠮷" == "\uD842\uDFB7"

// new String ops
"𠮷".codePointAt(0) == 0x20BB7

// for-of iterates code points
for(var c of "𠮷") {


Language-level support for modules for component definition. Codifies patterns
from popular JavaScript module loaders (AMD, CommonJS). Runtime behaviour

defined by a host-defined default loader. Implicitly async model – no code
executes until requested modules are available and processed.

// lib/math.js
export function sum(x, y) {
return x + y;
export var pi = 3.141593;
// app.js
import * as math from "lib/math";
alert("2π = " + math.sum(math.pi, math.pi));
// otherApp.js
import {sum, pi} from "lib/math";
alert("2π = " + sum(pi, pi));

Some additional features include export default and export *:

// lib/mathplusplus.js
export * from "lib/math";
export var e = 2.71828182846;
export default function(x) {
return Math.exp(x);
// app.js
import exp, {pi, e} from "lib/mathplusplus";
alert("2π = " + exp(pi, e));
Module Formatters
Babel can transpile ES6 Modules to several different formats including Common.js, AMD, System, and UMD. You can even create your own. For more details see the [modules docs]( on Babel's website.

Module Loaders

Module loaders support:

  • Dynamic loading
  • State isolation
  • Global namespace isolation
  • Compilation hooks
  • Nested virtualization

The default module loader can be configured, and new loaders can be constructed
to evaluated and load code in isolated or constrained contexts.

// Dynamic loading – ‘System’ is default loader
System.import("lib/math").then(function(m) {
alert("2π = " + m.sum(m.pi, m.pi));

// Create execution sandboxes – new Loaders
var loader = new Loader({
global: fixup(window) // replace ‘console.log’
loader.eval("console.log(\"hello world!\");");

// Directly manipulate module cache
System.set("jquery", Module({$: $})); // WARNING: not yet finalized
Additional polyfill needed
Since babel defaults to using common.js modules, it does not include the polyfill for the module loader API. Get it [here](
Using Module Loader
In order to use this, you'll need to tell babel to use the system module formatter. Also be sure to check out [System.js](

Map + Set + WeakMap + WeakSet

Efficient data structures for common algorithms. WeakMaps provides leak-free
object-key’d side tables.

// Sets
var s = new Set();
s.size === 2;
s.has("hello") === true;

// Maps
var m = new Map();
m.set("hello", 42);
m.set(s, 34);
m.get(s) == 34;

// Weak Maps
var wm = new WeakMap();
wm.set(s, { extra: 42 });
wm.size === undefined

// Weak Sets
var ws = new WeakSet();
ws.add({ data: 42 });
// Because the added object has no other references, it will not be held in the set


Proxies enable creation of objects with the full range of behaviors available to
host objects. Can be used for interception, object virtualization,
logging/profiling, etc.

// Proxying a normal object
var target = {};
var handler = {
get: function (receiver, name) {
return `Hello, ${name}!`;

var p = new Proxy(target, handler); === "Hello, world!";
// Proxying a function object
var target = function () { return "I am the target"; };
var handler = {
apply: function (receiver, ...args) {
return "I am the proxy";

var p = new Proxy(target, handler);
p() === "I am the proxy";

There are traps available for all of the runtime-level meta-operations:

var handler =
Unsupported feature
Due to the limitations of ES5, Proxies cannot be transpiled or polyfilled. See support from various [JavaScript engines](


Symbols enable access control for object state. Symbols allow properties to be
keyed by either string (as in ES5) or symbol. Symbols are a new primitive
type. Optional name parameter used in debugging - but is not part of identity.
Symbols are unique (like gensym), but not private since they are exposed via
reflection features like Object.getOwnPropertySymbols.

(function() {

// module scoped symbol
var key = Symbol("key");

function MyClass(privateData) {
this[key] = privateData;

MyClass.prototype = {
doStuff: function() {
... this[key] ...


var c = new MyClass("hello")
c["key"] === undefined

Subclassable Built-ins

In ES6, built-ins like Array, Date and DOM Elements can be subclassed.

// User code of Array subclass
class MyArray extends Array {
constructor(...args) { super(...args); }

var arr = new MyArray();
arr[1] = 12;
arr.length == 2

Math + Number + String + Object APIs

Many new library additions, including core Math libraries, Array conversion
helpers, and Object.assign for copying.

Number.isInteger(Infinity) // false
Number.isNaN("NaN") // false

Math.acosh(3) // 1.762747174039086
Math.hypot(3, 4) // 5
Math.imul(Math.pow(2, 32) - 1, Math.pow(2, 32) - 2) // 2

"abcde".includes("cd") // true
"abc".repeat(3) // "abcabcabc"

Array.from(document.querySelectorAll("*")) // Returns a real Array
Array.of(1, 2, 3) // Similar to new Array(...), but without special one-arg behavior
[0, 0, 0].fill(7, 1) // [0,7,7]
[1,2,3].findIndex(x => x == 2) // 1
["a", "b", "c"].entries() // iterator [0, "a"], [1,"b"], [2,"c"]
["a", "b", "c"].keys() // iterator 0, 1, 2
["a", "b", "c"].values() // iterator "a", "b", "c"

Object.assign(Point, { origin: new Point(0,0) })
Limited support from polyfill
Most of these APIs are supported by the babel polyfill. However, certain features are omitted for various reasons (e.g. String.prototype.normalize needs a lot of additional code to support). You can find more polyfills [here](

Binary and Octal Literals

Two new numeric literal forms are added for binary (b) and octal (o).

0b111110111 === 503 // true
0o767 === 503 // true
Only supports literal form
Babel is only able to transform 0o767 and not Number("0o767").


Promises are a library for asynchronous programming. Promises are a first class
representation of a value that may be made available in the future. Promises are
used in many existing JavaScript libraries.

function timeout(duration = 0) {
return new Promise((resolve, reject) => {
setTimeout(resolve, duration);

var p = timeout(1000).then(() => {
return timeout(2000);
}).then(() => {
throw new Error("hmm");
}).catch(err => {
return Promise.all([timeout(100), timeout(200)]);

Reflect API

Full reflection API exposing the runtime-level meta-operations on objects. This
is effectively the inverse of the Proxy API, and allows making calls
corresponding to the same meta-operations as the proxy traps. Especially useful
for implementing proxies.

var O = {a: 1};
Object.defineProperty(O, 'b', {value: 2});
O[Symbol('c')] = 3;

Reflect.ownKeys(O); // ['a', 'b', Symbol(c)]

function C(a, b){
this.c = a + b;
var instance = Reflect.construct(C, [20, 22]);
instance.c; // 42

Tail Calls

Calls in tail-position are guaranteed to not grow the stack unboundedly. Makes
recursive algorithms safe in the face of unbounded inputs.

function factorial(n, acc = 1) {
"use strict";
if (n <= 1) return acc;
return factorial(n - 1, n * acc);

// Stack overflow in most implementations today,
// but safe on arbitrary inputs in eS6
Partial support
Currently only self referencing tail recursion is supported due to the complexity of supporting tail calls globally.