JS: Symbol Object

What is the Keyword “Symbol”

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

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.

Reflect.getPrototypeOf ( Symbol ) === Function.prototype 

Purpose

Purpose of Symbol is:

• To create values of Symbol.
• Used as a namespace to hold static methods.
• Holds the property key "prototype". Its value is the parent of all instance of symbol.

Symbol Constructor

Properties

• prototype

• for(key)
• hasInstance
• isConcatSpreadable
• iterator
• keyFor(sym)
• match
• replace
• search
• species
• split
• toPrimitive
• toStringTag
• unscopables

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.