Symbol
is a built-in object whose constructor returns a symbol
primitive — also called a Symbol value or just a Symbol — that's guaranteed to be unique. Symbols are often used to add unique property keys to an object that won't collide with keys any other code might add to the object, and which are hidden from any mechanisms other code will typically use to access the object. That enables a form of weak encapsulation, or a weak form of information hiding.
Example: Using Symbol
as Unique Property Keys
javascript Copy code // Create unique symbols const id = Symbol('id'); const name = Symbol('name'); // Create an object with symbol-based properties const user = { [id]: 101, [name]: 'Alice', age: 25 }; // Access the symbol properties console.log(user[id]); // 101 console.log(user[name]); // "Alice" // Add another property using a symbol const email = Symbol('email'); user[email] = 'alice@example.com'; console.log(user[email]); // "alice@example.com" // Symbols are not enumerable in for...in loops or Object.keys for (const key in user) { console.log(key); // Only "age" is logged } console.log(Object.keys(user)); // ["age"] // You can still access all properties if you know the symbol keys console.log(Object.getOwnPropertySymbols(user)); // [Symbol(id), Symbol(name), Symbol(email)]
Example: Using Symbol.iterator
to Make an Object Iterable
javascript Copy code const collection = { items: ['apple', 'banana', 'cherry'], [Symbol.iterator]() { let index = 0; const items = this.items; return { next() { if (index < items.length) { return { value: items[index++], done: false }; } else { return { done: true }; } } }; } }; // Iterate over the object for (const item of collection) { console.log(item); } // Output: // apple // banana // cherry
This example shows how Symbol.iterator
allows a custom object to be iterated using for...of
.
Symbols usable in popular npm library :
Example: Using Symbol
in a Popular npm Library
A notable example of Symbol
usage is in the Express.js framework, a widely used web application framework for Node.js. In Express.js, Symbol
is employed to define unique property keys, preventing potential conflicts with user-defined properties.
Code Snippet from Express.js:
javascript Copy code // In Express.js, a symbol is used to define a unique property key const app = express(); app[Symbol('router')] = router;
In this snippet, Symbol('router')
creates a unique symbol that serves as a property key for the app
object. This approach ensures that the router
property is distinct and does not interfere with other properties that might be added to the app
object.
Benefits of Using Symbol
in Libraries:
- Uniqueness: Symbols guarantee that property keys are unique, reducing the risk of accidental overwrites.
- Immutability: Once created, symbols cannot be changed, providing a stable identifier.
- Non-enumerability: Properties keyed by symbols are not enumerable in loops, which can be beneficial for internal properties that should not be exposed.
Top comments (0)