v0.7.0Published 7 years ago

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Universe Ecmascript


This package lets you use new JavaScript language features that are part of the ECMAScript 2015 specification (including importing modules) but are not yet supported by all engines or browsers. Unsupported syntax is automatically translated into standard JavaScript that behaves the same way.

This video from the July 2015 Meteor Devshop gives an overview of how the package works, and what it provides.


You can use modules in plain *.js ans *.jsx files, there is no need for *.import.js(x) extension anymore. But modules from *.js, *.jsx will be auto loaded in first moment when all dependencies are loaded. It mean that:

  • If no imports and no exports was specified in file, this file will be loaded as was a before ( without module definition )
  • If no dependencies (none import) but if there is at least one export in file, the file will immediately will be loaded and it will set as a module without registering.
  • If all dependencies are loaded it will be immediately loaded without registering
  • If not all dependencies are loaded, your module will be registered and imported when all dependencies will loaded.

For more info see original universe:modules docs


To use modules in your Meteor app you need to

meteor add universe:ecmascript

This package register compilers on js and jsx extension and because of that it will not work with ecmascript and jsx core packages installed at the same time.

You may need to remove them from you app.

ecmascript is installed in all newly created apps by default and you can remove it with

meteor remove ecmascript

Our package give the same set of functionalities + modules support so it is completely safe to remove it.

Removing jsx can be more tricky as it could be implied by react "umbrella" package.

If your using react package then you need to remove it and add other dependencies back:

meteor remove react jsx
meteor add react-meteor-data //if you use it


Full install script for react apps:

meteor remove ecmascript react
meteor add react-runtime react-meteor-data universe:ecmascript

Safari and FlowRouter issue

There is one common issue with FlowRouter and Safari. You need to tell FlowRouter to wait for routes to load, otherwise it may not load correctly.

You can do it like this:

if (Meteor.isClient) {
System.import('/routesOrSomethingElse').then(() => {
    if (Meteor.isClient) {

Supported ES2015 Features


The ecmascript package uses Babel to compile ES2015 syntax to ES5 syntax. Many but not all ES2015 features can be simulated by Babel, and ecmascript enables most of the features supported by Babel.

Here is a list of the Babel transformers that are currently enabled:

  • es6.modules Language-level support for modules for component definition:
  • lib/math.js
1// lib/math.js
2export function sum(x, y) {
3return x + y;
5export var pi = 3.141593;
  • app.js
1// app.js
2import * as math from "lib/math";
3alert("2π = " + math.sum(math.pi, math.pi));

Makes it safe to use reserved keywords like catch as unquoted keys in object literals. For example, { catch: 123 } is translated to { "catch": 123 }.

Makes it safe to use reserved keywords as property names. For example, object.catch is translated to object["catch"].

Provides a shorthand for function expressions. For example, [1, 2, 3].map(x => x + 1) evaluates to [2, 3, 4]. If this is used in the body of the arrow function, it will be automatically bound to the value of this in the enclosing scope.

Adds support for binary and octal numeric literals. For example, 0b111110111 === 503 and 0o767 === 503.

Enables multi-line strings delimited by backticks instead of quotation marks, with variable interpolation:

1var name = "Ben";
2var message = `My name is:

Enables class syntax:

1class Base {
2constructor(a, b) {
3  this.value = a * b;
7class Derived extends Base {
8constructor(a, b) {
9  super(a + 1, b + 1);
13var d = new Derived(2, 3);
14d.value; // 12

Allows defining block-scoped variables that are not allowed to be redefined:

1const GOLDEN_RATIO = (1 + Math.sqrt(5)) / 2;
3// This reassignment will be forbidden by the compiler:
4GOLDEN_RATIO = "new value";

Enables the let and const keywords as alternatives to var. The key difference is that variables defined using let or const are visible only within the block where they are declared, rather than being visible anywhere in the enclosing function. For example:

1function example(condition) {
2let x = 0;
3if (condition) {
4let x = 1;
6} else {
8x = 2;
10return x;
13example(true); // logs 1, returns 0
14example(false); // logs 0, returns 2

Allows omitting the value of an object literal property when the desired value is held by a variable that has the same name as the property key. For example, instead of writing { x: x, y: y, z: "asdf" } you can just write { x, y, z: "asdf" }. Methods can also be written without the : function property syntax:

1var obj = {
2oldWay: function (a, b) { ... },
3newWay(a, b) { ... }

Default expressions for function parameters, evaluated whenever the parameter is undefined, ...rest parameters for capturing remaining arguments without using the arguments object:

1function add(a = 0, ...rest) {
2rest.forEach(n => a += n);
3return a;
6add(); // 0
7add(1, 2, 3); // 6

Allows an array of arguments to be interpolated into a list of arguments to a function call, new expression, or array literal, without using Function.prototype.apply:

1add(1, ...[2, 3, 4], 5); // 15
2new Node("name", ...children);
3[1, ...[2, 3, 4], 5]; // [1, 2, 3, 4, 5]

Provides an easy way to iterate over the elements of a collection:

1let sum = 0;
2for (var x of [1, 2, 3]) {
3sum += x;
5x; // 6

Destructuring is the technique of using an array or object pattern on the left-hand side of an assignment or declaration, in place of the usual variable or parameter, so that certain sub-properties of the value on the right-hand side will be bound to identifiers that appear within the pattern. Perhaps the simplest example is swapping two variables without using a temporary variable:

1[a, b] = [b, a];

Extracting a specific property from an object:

1let { username: name } = user;
2// is equivalent to
3let name = user.username;

Instead of taking a single opaque options parameter, a function can use an object destructuring pattern to name the expected options:

1function run({ command, args, callback }) { ... }
4command: "git",
5args: ["status", "."],
6callback(error, status) { ... },
7unused: "whatever"

Supports catch-all ...rest properties in object literal declarations and assignments:

1let { x, y, ...rest } = { x: 1, y: 2, a: 3, b: 4 };
2x; // 1
3y; // 2
4rest; // { a: 3, b: 4 }

Also enables ...spread properties in object literal expressions:

1let n = { x, y, ...rest };
2n; // { x: 1, y: 2, a: 3, b: 4 }

Allows the final parameter of a function to be followed by a comma, provided that parameter is not a ...rest parameter.

Permits the use of Flow type annotations. These annotations are simply stripped from the code, so they have no effect on the code's behavior, but you can run the flow tool over your code to check the types if desired.


The ECMAScript 2015 standard library has grown to include new APIs and data structures, some of which can be implemented ("polyfilled") using JavaScript that runs in all engines and browsers today. Here are three new constructors that are guaranteed to be available when the ecmascript package is installed:

A Promise allows its owner to wait for a value that might not be available yet. See this tutorial for more details about the API and motivation. The Meteor Promise implementation is especially useful because it runs all callback functions in recycled Fibers, so you can use any Meteor API, including those that yield (e.g. HTTP.get, Meteor.call, or MongoCollection), and you never have to call Meteor.bindEnvironment.

An associative key-value data structure where the keys can be any JavaScript value (not just strings). Lookup and insertion take constant time.

A collection of unique JavaScript values of any type. Lookup and insertion take constant time.

An implementation of the global Symbols namespace that enables a number of other ES2015 features, such as for-of loops and Symbol.iterator methods: [1,2,3][Symbol.iterator]().

  • Polyfills for the following Object-related methods:
  • Object.assign
  • Object.is
  • Object.setPrototypeOf
  • Object.prototype.toString (fixes @@toStringTag support)

Complete reference here.

  • Polyfills for the following String-related methods:
  • String.fromCodePoint
  • String.raw
  • String.prototype.includes
  • String.prototype.startsWith
  • String.prototype.endsWith
  • String.prototype.repeat
  • String.prototype.codePointAt
  • String.prototype.trim

Complete reference here.

  • Polyfills for the following Array-related methods:
  • Array.from
  • Array.of
  • Array.prototype.copyWithin
  • Array.prototype.fill
  • Array.prototype.find
  • Array.prototype.findIndex

Complete reference here.

  • Polyfills for the following Function-related properties:
  • Function.prototype.name (fixes IE9+)
  • Function.prototype[Symbol.hasInstance] (fixes IE9+)

Complete reference here.