What is Symbol?

JavaScriptJunior~2 min read

What `Symbol` is

Symbol is a unique and immutable value that can be used as an object property key.

Every Symbol is unique, even if it has the same description.

Example:

javascript

const id1 = Symbol('id');
const id2 = Symbol('id');

console.log(id1 === id2); // false

Despite the same 'id' description, the symbols are not equal. Every Symbol is a unique identifier.

Why `Symbol` is needed

Before symbols existed, object property keys could only be:

strings ('name', 'age');

Sometimes you need to add a "hidden" property to an object, so it does not interfere with the rest of the code (for example, libraries). This is where Symbol helps.

Example: a "hidden" property

javascript

const id = Symbol('id');

const user = {
  name: 'Alex',
  [id]: 123, // a symbol as the key
};

console.log(user);
// { name: 'Alex', [Symbol(id)]: 123 }

console.log(user[id]); // 123
console.log(Object.keys(user)); // ['name'] - the symbol is not visible

Symbol keys do not appear in for...in, Object.keys(), and they do not interfere with other properties. This is convenient for internal data or "private" properties.

A symbol as a key - example

javascript

const TOKEN = Symbol('token');

const session = {
  user: 'Tim',
  [TOKEN]: 'secret-token-123',
};

console.log(session.user);     // Tim
console.log(session[TOKEN]);   // secret-token-123
console.log(Object.keys(session)); // ['user']

So the [TOKEN] property exists, but it is "invisible" to most operations.

Getting all the symbols of an object

To get symbol properties:

javascript

const symbols = Object.getOwnPropertySymbols(session);
console.log(symbols); // [Symbol(token)]
console.log(session[symbols[0]]); // secret-token-123

Symbols do not convert to strings

javascript

const sym = Symbol('id');
console.log('My symbol: ' + sym); // TypeError

To print a symbol as a string:

javascript

console.log(sym.toString()); // Symbol(id)

Global symbols (`Symbol.for`)

Sometimes you need the same Symbol to be used in different parts of a program. There is a global symbol registry for that:

javascript

const a = Symbol.for('shared');
const b = Symbol.for('shared');

console.log(a === b); // true

Symbol.for(key) creates (or returns an already existing) global symbol. You can get it by name through Symbol.keyFor():

javascript

console.log(Symbol.keyFor(a)); // 'shared'

Well-known symbols

JS defines several built-in symbols that let you override the behavior of objects. For example:

Symbol	What it is used for
`Symbol.iterator`	Lets an object be iterable (`for...of`)
`Symbol.toPrimitive`	Controls the conversion of an object to a primitive
`Symbol.toStringTag`	Sets the name used by `Object.prototype.toString`
`Symbol.hasInstance`	Defines the behavior of the `instanceof` operator
`Symbol.species`	Controls the creation of subclasses
`Symbol.asyncIterator`	For asynchronous iterators

Example: `Symbol.iterator`

javascript

const numbers = {
  data: [1, 2, 3],
  [Symbol.iterator]() {
    let i = 0;
    const arr = this.data;
    return {
      next() {
        return i < arr.length
          ? { value: arr[i++], done: false }
          : { done: true };
      }
    };
  }
};

for (const n of numbers) {
  console.log(n); // 1, 2, 3
}

Thanks to Symbol.iterator, the object became "iterable".

Comparing `Symbol` with other types

Type	Can be an object key	Uniqueness	Appears in `for...in`
`string`	Yes	No	Yes
`number`	No (converted to a string)	No	-
`Symbol`	Yes	Yes	No

Practical use

Use Symbol when:

you need to create a unique identifier that will not collide with other properties;
you need to add a "private" property to an object or class;
you need to customize the behavior of objects using well-known symbols (Symbol.iterator, Symbol.toPrimitive, and so on).

Quick summary

What	Description
Type	A primitive (`symbol`)
Main feature	Uniqueness
Creation	`Symbol('description')`
Object keys	Can be used as a key (unique and hidden)
Global symbols	`Symbol.for()` / `Symbol.keyFor()`
Well-known symbols	Let you override JS's behavior

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