JavaScript Symbol Types

Introduction to Symbol Types in JavaScript

In the realm of JavaScript, Symbol types stand as a unique and powerful feature introduced in ECMAScript 2015 (ES6). Symbols are immutable, primitive data types that can be used as identifiers for object properties. Each Symbol value is distinct, ensuring that there are no conflicts with other properties, even if they share the same name.

Understanding the Basics of Symbols

let mySymbol = Symbol(); let anotherSymbol = Symbol(); console.log(mySymbol === anotherSymbol); // Outputs: false

Explanation: This example demonstrates the fundamental characteristic of Symbol types in JavaScript - uniqueness. Two symbols are created using Symbol(). Despite having no descriptions and being created in a similar manner, they are not equal (mySymbol === anotherSymbol results in false). This uniqueness is the core feature of Symbols, making them ideal for unique identifiers.

Creating Symbols with Descriptions

While Symbols are unique, adding a description can aid in debugging and code readability.

Code Example: Symbols with Descriptions

let sym1 = Symbol('description'); let sym2 = Symbol('description'); console.log(sym1 === sym2); // Outputs: false console.log(sym1.description); // Outputs: 'description'

Explanation: Here, two Symbols are created with identical descriptions. The key takeaway is that even when Symbols have the same description, they remain unique (sym1 === sym2 is false). The description is merely a way to provide readable identification for debugging purposes, as shown by sym1.description.

Symbols as Object Keys

One of the primary uses of Symbols is as keys for object properties, ensuring property uniqueness and avoiding accidental property overwrites.

Code Example: Using Symbols as Object Keys

const SIZE = Symbol(); let square = { [SIZE]: 10, area: function() { return this[SIZE] ** 2; } }; console.log(square.area()); // Outputs: 100

Explanation: In this example, a Symbol SIZE is used as a key for an object property. This ensures that the SIZE property is unique and won't conflict with any other property named SIZE. The area method of the square object uses this unique key to calculate the area of a square, demonstrating how Symbols can be used to avoid property name clashes.

System Symbols in JavaScript

JavaScript predefines a set of system symbols available under Symbol.* which serve various specific purposes.

Example: Using Well-Known Symbols

let myArray = [1, 2, 3]; let iterator = myArray[Symbol.iterator](); console.log(; // Outputs: 1

Explanation: This snippet shows the use of a well-known Symbol, Symbol.iterator, which is part of JavaScript's built-in set of Symbols. The Symbol.iterator is used to define the default iterator for an object. Here, it's used to iterate over an array, illustrating how predefined Symbols serve specific purposes in JavaScript's standard library.

Symbol.for and Symbol.keyFor Methods

The Symbol.for(key) method searches for existing symbols with the given key and returns it if found; otherwise, it creates a new symbol. Symbol.keyFor(symbol) retrieves a shared symbol key from the global symbol registry.

Code Example: Symbol.for and Symbol.keyFor

let globalSym = Symbol.for('global'); let localSym = Symbol('local'); console.log(Symbol.keyFor(globalSym)); // Outputs: 'global' console.log(Symbol.keyFor(localSym)); // Outputs: undefined

Explanation: In this code, Symbol.for('global') searches for a Symbol with the key 'global' in the global symbol registry. If it doesn't exist, it creates and stores it in the registry. This makes globalSym a reusable symbol across different parts of the code. Symbol.keyFor() is used to retrieve the key of a global symbol. It returns the key for globalSym but undefined for localSym, as localSym is not a global symbol but a locally created one.


Symbols in JavaScript offer a robust way to create unique identifiers, making them an essential tool for developers looking to manage object properties effectively and implement advanced coding patterns. Their unique nature and the ability to create property keys that are immune to accidental overwrites or clashes with other properties make them an invaluable addition to the JavaScript language.

Practice Your Knowledge

What are some characteristics of JavaScript Symbol type?

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