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

symbol values are created by calling the Symbol constructor or method Symbol.for.

let sym1 = Symbol()
let sym2 = Symbol("key") // optional string key
let sym3 = Symbol.for("key") 

Symbols are immutable, and unique.

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

const 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 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. 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.