JavaScript: Symbol Object

By Xah Lee. Date: . Last updated: .

New in JS2015.

Symbol is the value of the property key "Symbol" of the Global Object .

console.log( window.Symbol === Symbol );

Type

Type of Symbol is a function.

// type of Symbol
console.log( typeof Symbol === "function" );

Parent

Parent of Symbol is Function.prototype.

console.log( Reflect.getPrototypeOf ( Symbol ) === Function.prototype );

Purpose

Purpose of Symbol is:

[see Symbol Tutorial]

Symbol Constructor

Properties


2017-02-10 following is work in progress.






Symbol.for ( key )

Symbol.for(string) → access shared symbols registry.

Symbol.hasInstance

Symbol.isConcatSpreadable

Symbol.iterator

Symbol.keyFor ( sym )

Symbol.match

Symbol.replace

Symbol.search

Symbol.species

Symbol.split

Symbol.toPrimitive

Symbol.toStringTag

Symbol.unscopables


Well-Known (Predefined) Symbols

Well-Known symbols are symbols predefined in JavaScript.

Well-known symbols are built-in Symbol values They are typically used as the keys of properties whose values serve as extension points. Unless otherwise specified, well-known symbols values are shared by all Code Realms.

Well-Known symbols are denoted with 2 at signs in front, like this: @@iterator

Symbol.iterator

the operator instanceof is now a method of function, with property key [Symbol.hasInstance]. someting like o instanceof f same as f[Symbol.hasInstance](o). this makes it extensible.

Symbol.unscopables

Symbol.match method for customizing str.match(obj).

Well-known Symbols

Symbol.hasInstance
A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the instanceof operator.
Symbol.isConcatSpreadable
A Boolean valued property that if true indicates that an object should be flattened to its array elements by Array.prototype.concat .
Symbol.iterator
A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
Symbol.match
A regular expression method that matches the regular expression against a string. Called by the String.prototype.match method.
Symbol.replace
A regular expression method that replaces matched substrings of a string. Called by the String.prototype.replace method.
Symbol.search
A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.
Symbol.species
A function valued property that is the constructor function that is used to create derived objects.
Symbol.split
A regular expression method that splits a string at the indices that match the regular expression. Called by the String.prototype.split method.
Symbol.toPrimitive
A method that converts an object to a corresponding primitive value.
Symbol.toStringTag
A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method Object.prototype.toString .
Symbol.unscopables
An object valued property whose own property names are property names that are excluded from the with environment bindings of the associated object.

JavaScript Symbol

BUY ΣJS JavaScript in Depth