Lesson 85 - JavaScript Symbols – Creating Unique Identifiers

Symbols are a primitive data type introduced in ES6. They are unique and immutable, and are often used to create unique object keys that won’t collide with other keys or properties.

What is a Symbol?

A Symbol is a unique and immutable value. Even if two symbols have the same description, they are not equal.

const sym1 = Symbol('id');
const sym2 = Symbol('id');

console.log(sym1 === sym2); // false

Creating Symbols

const uniqueKey = Symbol();
const anotherKey = Symbol('description'); // optional description

Using Symbols as Object Keys

const user = {
  name: "Alice",
  age: 30
};

const uid = Symbol("userID");
user[uid] = 101;

console.log(user);        // Symbol won’t appear in normal enumeration
console.log(user[uid]);   // 101

Symbols and Property Privacy

Symbols can help create semi-private object properties:

const password = Symbol("password");

const user = {
  name: "Bob",
  [password]: "secret123"
};

console.log(user.password);       // undefined
console.log(user[password]);      // "secret123"

Symbol Keys are Not Enumerable

Symbol properties are not included in for...in or Object.keys():

for (let key in user) {
  console.log(key); // only logs "name"
}

console.log(Object.keys(user));      // ["name"]
console.log(Object.getOwnPropertySymbols(user)); // [Symbol(password)]

Use Cases for Symbols

• Creating unique constants
• Avoiding naming collisions in object keys
• Implementing internal object properties
• Creating semi-private fields

Summary

• Symbols are unique, immutable primitive values.
• Use them as unique object keys.
• They are not included in standard property enumeration.
• Great for avoiding key collisions and creating internal/private properties.

Try Your Hand

Create a user object with a symbol-based key called userSecret and store a secret string. Then write code to print the secret using the symbol.