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The WeakSet object lets you store weakly held objects in a collection.
ECMAScript6 JavaScript WeakSet


The WeakSet.prototype property represents the prototype for the WeakSet constructor.
ECMAScript6 JavaScript Property WeakSet

The property lets you detect whether a function or constructor was called using the new operator. In constructors and functions instantiated with the new operator, returns a reference to the constructor or function. In normal function calls, is undefined.
Classes ECMAScript6 JavaScript Reference


The export statement is used to export functions, objects or primitives from a given file (or module).
ECMAScript6 JavaScript Statement


The for...of statement creates a loop iterating over iterable objects (including Array, Map, Set, String, TypedArray, arguments object and so on), invoking a custom iteration hook with statements to be executed for the value of each distinct property.
ECMAScript6 JavaScript Statement


The import statement is used to import functions, objects or primitives that have been exported from an external module, another script, etc.
ECMAScript6 JavaScript Modules Statement

Strict mode

ECMAScript 5's strict mode is a way to opt in to a restricted variant of JavaScript. Strict mode isn't just a subset: it intentionally has different semantics from normal code. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don't rely on strict mode without feature-testing for support for the relevant aspects of strict mode. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.
ECMAScript5 JavaScript Strict Mode


The let statement declares a block scope local variable, optionally initializing it to a value.
ECMAScript 2015 ECMAScript6 JavaScript let Statement Variable declaration Variables

New in JavaScript 1.8.5

The following is a changelog for JavaScript 1.8.5. This version was included in Firefox 4.
ECMAScript5 Firefox 4 JavaScript JavaScript 1.8.5 Versions

Arrow functions

An arrow function expression has a shorter syntax compared to function expressions and lexically binds the this value (does not bind its own thisargumentssuper, or Arrow functions are always anonymous. These function expressions are best suited for non-method functions and they can not be used as constructors.
ECMAScript6 Functions Intermediate JavaScript Reference

Method definitions

Starting with ECMAScript 2015 (ES6), a shorter syntax for method definitions on objects initializers is introduced. It is a shorthand for a function assigned to the method's name.
ECMAScript6 Functions JavaScript Object Syntax


The @@unscopable symbol property contains property names that were not included in the ECMAScript standard prior to the ES2015 (ES6) version. These properties are excluded from with statement bindings.
Array ECMAScript6 JavaScript Property Prototype


The Array.from() method creates a new Array instance from an array-like or iterable object.
Array ECMAScript6 JavaScript Method polyfill


The Array.isArray() determines whether the passed value is an Array
Array ECMAScript5 JavaScript Method polyfill


The Array.of() method creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.
Array ECMAScript6 JavaScript Method polyfill


The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.
Array ECMAScript5 JavaScript Method Prototype


The JSON.parse() method parses a string as JSON, optionally transforming the value produced by parsing.
ECMAScript5 JavaScript JSON Method Reference


The clear() method removes all elements from a Map object.
ECMAScript6 JavaScript Map Method Prototype


The delete() method removes the specified element from a Map object.
ECMAScript6 JavaScript Map Method Prototype


The forEach() method executes a provided function once per each key/value pair in the Map object, in insertion order.
ECMAScript6 JavaScript Map Method Prototype