Symbol
- Node.js course
- Beginner
- Lesson 16/116
- 5 min
Learn unique identifiers with Symbol.
Symbol
A Symbol is a unique and immutable primitive type in JavaScript. Each call to Symbol() creates a new, unique value.
Example: const a = Symbol('id') and const b = Symbol('id'). Even though the descriptions match, a === b returns false; they are two different symbols.
Symbols are usually used to add private keys to objects; other code cannot accidentally overwrite them.
Symbol uniqueness
Two Symbols are never equal to each other, even if their descriptions match. This makes them 'clash-free keys'.
Example: const a = Symbol('x'), const b = Symbol('x'). a === b returns false. The description is just a label for debugging; it does not affect uniqueness.
If you really want to share the 'same' Symbol, you can use Symbol.for('key'); this looks up a global registry. But this is an advanced topic.
Practice Compare two Symbols
Define two separate Symbols: const a = Symbol('id') and const b = Symbol('id'). Check equality with console.log(a === b); also print its type with console.log(typeof a).
Starter code
// Define two separate Symbols (with the same description)
// Log a === b and typeof aSample solution
const a = Symbol("id");
const b = Symbol("id");
console.log(a === b);
console.log(typeof a);Expected output
false symbol
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is Symbol used for?
- For unique keys Correct answer
- To create arrays
- To concatenate strings
- To round numbers
Explanation: Symbol creates unique keys.
2 Match the Symbol properties
| Term | Meaning |
|---|---|
Symbol() |
Unique value |
Symbol('id') |
Symbol with description |
typeof s |
'symbol' |
s === s |
Only equal to itself |
3 Define a Symbol
const id = ____;const id = Symbol('id');Explanation: Symbol('id') creates a new unique symbol.