JavaScript Symbol Type: How it Works with Examples

JavaScript Symbol Type gives a way to make unique values. A Symbol never matches any other value and this helps in many cases.

What is JavaScript Symbol Type

A Symbol in JavaScript is a new primitive type. It makes unique values that never match with other values in memory.

Here is an example:

let sym = Symbol("id");

A Symbol can take a description as text. That text only helps for debugging and does not affect the value.

Use Symbols as object keys

Symbols help in making hidden keys in objects. They also help to define parts that stay safe from name clashes.

For example:

let id = Symbol("id");
let user = { name: "Ali" };
user[id] = 100;
console.log(user[id]);

The Symbol makes a safe key that never collides with other keys in the object.

You can use a Symbol as a property key in any object. The key does not appear in normal loops but still holds the value.

Here is an example:

let sym = Symbol("secret");
let obj = { name: "Sara" };
obj[sym] = "hidden";
console.log(obj[sym]);

The Symbol key holds the word “hidden”. A normal loop skips it but you can still get it by the Symbol.

For another example:

let sym = Symbol("data");
let obj = {};
Object.defineProperty(obj, sym, { value: 42 });
console.log(obj[sym]);

The Symbol works as a safe property key. It avoids clash with any normal string key inside the object.

Global Symbol Registry with Symbol.for()

JavaScript also has a global Symbol registry. Symbol.for() searches that registry and returns the same Symbol for the same key.

For example:

let sym1 = Symbol.for("shared");
let sym2 = Symbol.for("shared");
console.log(sym1 === sym2);

This returns true. Both variables hold the same Symbol because they came from the global registry.

Examples

Symbol as unique key:

let user = {};
let id = Symbol("id");
user[id] = 123;
console.log(user[id]);

Here the Symbol id makes a unique key for the object. No other property can replace or break this value because the key is unique.

Symbol.for in registry:

let sym1 = Symbol.for("app");
let sym2 = Symbol.for("app");
console.log(sym1 === sym2);

Both lines return the same Symbol because they come from the global registry. This helps in cases where you need shared identity.

Hide property with Symbol:

let sym = Symbol("secret");
let obj = { name: "Omar", age: 22 };
obj[sym] = "token";
for (let key in obj) console.log(key);

The loop prints only name and age. The Symbol key does not appear, but you can still read it with the Symbol.

Symbol with defineProperty:

let sym = Symbol("meta");
let obj = {};
Object.defineProperty(obj, sym, { value: "data", enumerable: false });
console.log(Object.keys(obj));

The Symbol key stays hidden from normal property checks. This helps to store data that must not mix with user keys.

Wrapping Up

In this article you learned what a Symbol is and how it works with objects. Here is a quick recap:

Symbols make unique values, they act as safe object keys, and they also exist in the global registry.

FAQs

let sym = Symbol("id");
console.log(typeof sym); // "symbol"

let sym1 = Symbol();
let sym2 = Symbol("user");
console.log(sym1, sym2);

let user = {};
let id = Symbol("id");
user[id] = 123;
console.log(user[id]); // 123

let sym1 = Symbol("key");
let sym2 = Symbol("key");
console.log(sym1 === sym2); // false

let g1 = Symbol.for("key");
let g2 = Symbol.for("key");
console.log(g1 === g2); // true

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