What is a Symbol?

Learn to create a unique, immutable primitive value with Symbol.

What is a Symbol?

In JavaScript, Symbol is the seventh primitive type; like numbers, strings and booleans, it is immutable. Calling Symbol() returns a unique value unlike any other.

Here is the interesting part: calling Symbol('id') twice produces different symbols each time. The text in parentheses is just a description label for debugging; it does not affect identity. typeof Symbol() returns 'symbol'.

The description label

The text in parentheses is a description label. Symbol('user').description returns 'user'; if no description is given, it returns undefined.

The label is only for debugging: console.log shows the symbol as Symbol(user). Even when two symbols share the same description, their identities remain different. The description plays no role in equality comparison.

Exercise Create two symbols and compare them

Create two symbols in the variables first and second. Give both the description 'id'. Log that first is 'symbol' using typeof. Then log the result of first !== second.

Show example solution

const first = Symbol('id');
const second = Symbol('id');
console.log(typeof first === 'symbol');
console.log(first !== second);

Test yourself

Which statement about Symbol is true?
- Symbol('x') === Symbol('x') returns false
- Symbol('x') === Symbol('x') returns true
- Symbol() returns an object
- typeof Symbol() returns 'object'
Show answer

Symbol('x') === Symbol('x') returns false — Correct. Each Symbol() call produces a unique value; even with the same description, two symbols are never equal.

Show that two symbols are different

const different = Symbol('x') ____ Symbol('x');

Show answer

const different = Symbol('x') !== Symbol('x');

Match the Symbol concepts

Symbol()

Creates a unique value
description

Debugging label
typeof Symbol()

Returns 'symbol'
Symbol('x') === Symbol('x')

Returns false

Next lesson: Symbol.for and Symbol.keyFor

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