Introduction

Starting with ECMAScript 2015, symbol is a primitive data type, just like number and string.

symbol values are created by calling the Symbol constructor.

let sym1 = Symbol();

let sym2 = Symbol("key"); // optional string key

Symbols are immutable, and unique.

let sym2 = Symbol("key");
let sym3 = Symbol("key");

sym2 === sym3; // false, symbols are unique

Just like strings, symbols can be used as keys for object properties.

let sym = Symbol();

let obj = {
  [sym]: "value"
};

console.log(obj[sym]); // "value"

Symbols can also be combined with computed property declarations to declare object properties and class members.

const getClassNameSymbol = Symbol();

class C {
  [getClassNameSymbol](){
     return "C";
  }
}

let c = new C();
let className = c[getClassNameSymbol](); // "C"

Well-known Symbols

In addition to user-defined symbols, there are well-known built-in symbols. Built-in symbols are used to represent internal language behaviors.

Here is a list of 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 value indicating that an object should be flatten 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. Called by the ToPrimitive abstract operation.

Symbol.toStringTag

A String value that is used in the creation of the default string description of an object. Called by the built-in method Object.prototype.toString.

Symbol.unscopables

An Object whose own property names are property names that are excluded from the ‘with’ environment bindings of the associated objects.