First of all, is a built-in primitive type. And it's guaranteed to be unique. Symbols are often used to add unique property keys to an object won't collide with keys that any other code might add to the object. And the properties under the Symbol key are hidden from any mechanisms other code will typically use to access the object(like , , , and etc.), but can be accessed by and (which will return both and keys). symbol for...in for...of Object.keys Object.getOwnPropertyNames JSON.stringify Object.getOwnPropertySymbols Reflect.ownKeys string symbol 13 types of built-in Symbols There are 13 types of built-in symbols to be listed as Vue3 goes. const isSymbol = (val: unknown): val is symbol => typeof val === 'symbol'

const buildInSymbols = new Set(
    Object.getOwnPropertyNames(Symbol)
        .map(key => Symbol[key])
        .filter(isSymbol)
) Symbol.hasInstance A method determining if a constructor object recognizes an object as its instance. class Foo {
  // static method
  static [Symbol.hasInstance](value) {
    return value === 'hi'
  }

  // instance method
  [Symbol.hasInstance](value) {
    return value instanceof Array
  }
}

// is conceptually equal to `Foo[Symbol.hasInstance]('hi')`, and return `true`
'hi' instanceof Foo
// is conceptually equal to `(new Foo())[Symbol.hasInstance]([1,2,3])`, and return `true`
[1,2,3] instanceof new Foo() Symbol.isConcatSpreadable A Boolean value indicating if an object should be flattened to its array elements. The default value of property of is , even if the returns , while the default value of the Array like object like is returning also. Symbol.isConcatSpreadable Array true Array.prototype[Symbol.isConcatSpreadable] undefined {length: 2, 0: 'element1', 1: 'element2'} false undefined let foo = {length: 4, 0: 'e1', 1: 'e2', 'not an array element': 'e3'}
foo[Symbol.isConcatSpreadable] = true

// result is `['hi', 'e1', 'e2', undefined, undefined]`
let result = ['hi'].concat(foo)
console.log(result.length) // 5 Symbol.iterator A method returning the default iterator for an object. Iterator offers an unified interface for different data structures to access their elements, and can be iterated over by construct. The so-called iterable object is that with property. for...of Symbol.iterator And the built-in iterable object are, , , , , , , some of array-like objects(e.g. , , etc.) and so forth. Array String Set Map generator object TypedArray arguments NodeList HTMLCollection The interface of an Iterator is like: interface Iterator<T> {
  /**
    * get the next element
    */
  next(): {
    done: boolean // indicate whether iteration finishes or not 
    value: T // element itself
  }
} And here's a good example for this: // returning object with next method
class List<T> {
  // instance method
  [Symbol.iterator]() {
    let index = 0
    // iterator duck type instance;)
    return {
      next: () => ({
          done: index + 1 >= this.xs.length
          value: this.xs[index++]
      })
    }
  }

  constructor(private xs: T[]){}
}

// by generator function
class List<T> {
  // instance method
  *[Symbol.iterator]() {
    for (let i = 0; i < this.xs.length; i++) {
      yield this.xs[i]
    }
  }

  constructor(private xs: T[]){}
} Symbol.asyncIterator It might be a good segue into which is the asynchronous version of that we mention before.
And it's a method that returns the default AsyncIterator for an object, only used by the brand new loop statement , I'll illustrate in an example: Symbol.asyncIterator Symbol.iterator for await...of // iterating over async iterables
class List<T> {
  // instance method
  [Symbol.asyncIterator]() {
    let index = 0
    // async iterator duck type instance whose `next` method returning Promise instance, and which will be fulfilled with value returning from the sync version of `next` method.
    // I'm so sorry for the above boring, verbose description :)
    return {
      next: () => {
        if (index + 1 >= this.xs.length)  {
          return Promise.resolve({done: true, value: this.xs[index++]})
        }
        else {
          return Promise.resolve({done: false, value: this.xs[index++]})
        }
      }
    }
  }

  constructor(private xs: T[]){}
}

// iterating over async generator
class List<T> {
  // instance method
  async *[Symbol.iterator]() {
    for (let i = 0; i < this.xs.length; i++) {
      yield this.xs[i]
    }

    // we are able to return an fulfilled Promise instance directly also.
    yield Promise.resolve('hi')
  }

  constructor(private xs: T[]){}
}

// show time;)
List<String> names = new List<String>(['john', 'mary'])
for await(const name of names) {
  console.log(name)
} Symbol.match A method that matches a string, also used to determine if an object may be used as a regular expression. Used as input by . String.prototype.match() Let's have an example: class LeftBracket {
  private value = '<'  

  [Symbol.match](str: string) {
    const idx = str.indexOf(this.value)
    if (idx === -1) {
      return null
    }
    else {
      let m = [this.value]
      m.index = idx
      m.input = str
      m.groups = undefined
    }
  }
}

// usage
let str = '<div>'
str.match(new LeftBracket()) // ['<', index: 0, input: '<div>', groups: undefined]
// equal to the following invocation
(new LeftBracket())[Symbol.match](str) // ['<', index: 0, input: '<div>', groups: undefined] By the way, there are two points should be notice for invocation. String.prototype.match Calling with a non- input, it's implicitly coerced into a by using . String.prototype.match RegExp RegExp new RegExp() If the input is with flag, the returning will contain the full matches with no , or properties as that without flag. RegExp /g index input groups /g let re = /t(e)st/,
    str = 'test1test2'

str.match(re) // ['test', 'e', index: 0, input: 'test1test2', groups: undefined]

re = /t(e)st/g
str.match(re) // ['test', 'test'] Symbol.matchAll A method that returns an iterator, that yields all matches of the regular expression against a string, including capturing groups. Used by .
And each match is an with extra properties and . And the matched text is as the first item of the match array, and then one item for the parenthetical capture group of the matched text. String.prototype.matchAll() Array index input const RE = {
  re: /t(e)st/g,
  *[Symbol.matchAll](str: string) {
    let m
    while (m = re.exec(str)) {
      yield m
    }
  }
}

[...'test1test2'.matchAll(RE)] // [['test', 'e', index: 0, input: 'test1test2', groups: undefined], ['test', 'e', index: 5, input: 'test1test2', groups: undefined]] For , please take attention that passing an input without flag will hit into an . String.prototype.matchAll /g TypeError Symbol.replace A method that expects two parameters and replaces matched substrings of a string. Used by as the . String.prototype.replace(searchValue, replaceValue) searchValue const foo = {
  searchValue: 'test',
  [Symbol.replace](value, replaceValue) {
    return value.replace(this.searchValue, replaceValue)
  }
}

'test11'.replace(foo) // "11" Back to the , the first argument could be either a string or RegExp, and the second one could be a string or a function to be called for each match. The arguments to the replacement function are as follows: String.prototype.replace pattern replacement , the matched substring(Corresponds to ) match $& , the nth string found by a parenthesized capture group including named capture groups. p1,p2,... , the offset of the matched substring within the whole string being examined. offset , the whole string to be examined. string , the named capturing groups object of which keys are the names of capturing group and the values are the matched portions. groups Symbol.search A method that returns the index within a string that matches the regular expression. Used by .
An innovation of which accepts a input. String.prototype.search() String.prototype.indexOf RegExp Symbol.split A method that splits a string at the indices that match a regular expression. Used by String.prototype.split() Symbol.toPrimitive A method converting an object to a primitive object. class User {
  [Symbol.toPrimitive](hint: 'string' | 'number' | 'default') {
    if (hint === 'number') {
      return 123
    }
    else if (hint === 'string') {
      return 'hi'
    }

    return true
  }
}

let user = new User()
console.log(+user) // 123
console.log(`${user}`) // "hi"
console.log(user + '') // "true" Symbol.toStringTag A string value used for the default description of an object. Used by . Object.prototype.toString() class User{}
let user = new User()
Object.prototype.toString.call(user) // [object Object]

class Account{
  get [Symbol.toStringTag]() {
    return 'Account'
  }
}
let account = new Account()
Object.prototype.toString.call(account) // [object Account] Symbol.species It's a function-valued property that the construct function that is used to create derived objects.
For example: class MyArray extend Array {}
const a = new MyArray(1,2,3)
const b = a.map(x=>x)

b instanceof MyArray // true
b instanceof Array // true

class MyArray extend Array {
  static get [Symbol.species]() {
    return Array
  }
}
const a = new MyArray(1,2,3)
const b = a.map(x=>x)

b instanceof MyArray // false
b instanceof Array // true


class T2 extends Promise {
  static get [Symbol.species]() {
    return Promise
  }
}

let t2 = new T2(r => r()).then(v=>v)
t2 instanceof T2 // true Symbol.unscopables An object value of whose own and inherited property names are excluded from the environment bindings of the associated object.
There is no other better manner to make sense than through an example: with const foo = {
  name: 'hi',
  age: 1,
  get [Symbol.unscopeables]() {
    return {
      name: true
    }
  }
}

let age = 2
let name = 'john'
with(foo) {
  console.log(age) // 1, reference to the age of foo
  console.log(name) // "john", refer to the name outside current scope
} Define your own instance symbol Create our own unique local Symbol value with code Symbol(key?: string) Note that won't coerce the string into a Symbol, it creates a brand new Symbol value each time instead. Symbol('hi') hi We can get the key by accessing property in ES2019. description Create or get the shared global Symbols by calling Symbol.for(key: string) return the global Symbol with the given value of if it can be found in th global Symbol registry. Otherwise, the new one is created, added to the global Symbol registry under the given key, and returned. key Return the key of a global symbol instance Symbol.keyFor(instance: symbol) Note that, calling with a local symbol instance will return . Symbol.keyFor undefined properties with Symbol key will not exist in , nor for...in for...of Digression - and Object.keys Object.getOwnPropertyNames will returns the enumerable own property names of the passing argument, while returns all own property names no matter it's enumerable or not. Object.keys Object.getOwnPropertyNames And the same point of both is that, all property name they return are . The ones should be got by of which the type of return value is . string Symbol Object.getOwnPropertySymbols Symbol[]

Understanding Symbol: Javascript's Newest Primitive Type

About Author

Comments

TOPICS

THIS ARTICLE WAS FEATURED IN

Related Stories

10 Things Every Beginner Should Know When Learning Javascript And React

12 Good JavaScript Shorthand Techniques

12 JavaScript Concepts That Will Level Up Your Development Skills

3 Must-learn JavaScript Concepts to Master Before Learning React

43 Stories To Learn About Es6

5 Exciting New JavaScript Features You Can Use Today

10 Things Every Beginner Should Know When Learning Javascript And React

12 Good JavaScript Shorthand Techniques

12 JavaScript Concepts That Will Level Up Your Development Skills

3 Must-learn JavaScript Concepts to Master Before Learning React

43 Stories To Learn About Es6

5 Exciting New JavaScript Features You Can Use Today

Light-Mode

Classic

Newspaper

Minty

Dark-Mode

Neon Noir

Minty

HN StartUps