ECMAScript

This package lets you use new JavaScript language features that are part of the ECMAScript 2015 specification 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.

Usage

The ecmascript package registers a compiler plugin that transpiles ECMAScript 2015+ to ECMAScript 5 (standard JS) in all .js files. By default, this package is pre-installed for all new apps and packages.

To add this package to an existing app, run the following command from your app directory:

meteor add ecmascript

To add the ecmascript package to an existing package, include the statement api.use('ecmascript'); in the Package.onUse callback in your package.js file:

Package.onUse((api) => {
  api.use('ecmascript');
});

Supported ES2015 Features

Syntax

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:

• es3.propertyLiterals
  Makes it safe to use reserved keywords like catch as unquoted keys in object literals. For example, { catch: 123 } is translated to { "catch": 123 }.

• es3.memberExpressionLiterals
  Makes it safe to use reserved keywords as property names. For example, object.catch is translated to object["catch"].

• es6.arrowFunctions
  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.

• es6.literals
  Adds support for binary and octal numeric literals. For example, 0b111110111 === 503 and 0o767 === 503.

• es6.templateLiterals
  Enables multi-line strings delimited by backticks instead of quotation marks, with variable interpolation:

  var name = 'Ben';
  var message = `My name is:
  ${name}`;

• es6.classes
  Enables class syntax:

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

• es6.constants
  Allows defining block-scoped variables that are not allowed to be redefined:

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

• es6.blockScoping
  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:

  function example(condition) {
    let x = 0;
    if (condition) {
      let x = 1;
      console.log(x);
    } else {
      console.log(x);
      x = 2;
    }
    return x;
  }
  
  example(true); // logs 1, returns 0
  example(false); // logs 0, returns 2

• es6.properties.shorthand
  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:

  var obj = {
    oldWay: function (a, b) { ... },
    newWay(a, b) { ... }
  };

• es6.properties.computed
  Allows object literal properties with dynamically computed keys:

  var counter = 0;
  function getKeyName() {
    return 'key' + counter++;
  }
  
  var obj = {
    [getKeyName()]: 'zero',
    [getKeyName()]: 'one',
  };
  
  obj.key0; // 'zero'
  obj.key1; // 'one'

• es6.parameters
  Default expressions for function parameters, evaluated whenever the parameter is undefined, ...rest parameters for capturing remaining arguments without using the arguments object:

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

• es6.spread
  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:

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

• es6.forOf
  Provides an easy way to iterate over the elements of a collection:

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

• es6.destructuring
  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:

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

  Extracting a specific property from an object:

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

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

  function run({ command, args, callback }) { ... }
  
  run({
    command: 'git',
    args: ['status', '.'],
    callback(error, status) { ... },
    unused: 'whatever'
  });

• es7.objectRestSpread
  Supports catch-all ...rest properties in object literal declarations and assignments:

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

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

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

• es7.trailingFunctionCommas
  Allows the final parameter of a function to be followed by a comma, provided that parameter is not a ...rest parameter.

• flow
  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.

Polyfills

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:

• Promise
  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.

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

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

• Symbol
  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.