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Trevor I. Lasn

Staff Software Engineer, Engineering Manager

Exploring JavaScript Symbols

Deep dive into JavaScript Symbols - what they are, why they matter, and how to use them effectively

I remember the first time I encountered Symbols in JavaScript. It was 2015, and like many developers, I thought, “Great, another primitive type to worry about.”

But as I’ve grown in my career, I’ve come to appreciate these quirky little primitives. They solve some interesting problems in ways that strings and numbers just can’t match.

Symbols stand apart from other JavaScript primitives because they’re guaranteed to be unique.

When you create a Symbol with Symbol('description'), you’re getting something that will never equal any other Symbol, even one created with the same description. This uniqueness is what makes them powerful for specific use cases.

The real power of Symbols emerges when working with objects. Unlike strings or numbers, Symbols can be used as property keys without any risk of colliding with existing properties. This makes them invaluable for adding functionality to objects without interfering with existing code.

When you use a Symbol as a property key, it won’t show up in Object.keys() or normal for...in loops.

You can still access these properties through Object.getOwnPropertySymbols(), but it requires intentional effort. This creates a natural separation between an object’s public interface and its internal state.

The global Symbol registry adds another dimension to Symbol usage. While normal Symbols are always unique, sometimes you need to share Symbols across different parts of code. That’s where Symbol.for() comes in:

JavaScript provides built-in Symbols that let you modify how objects behave in different situations. These are called well-known Symbols, and they give us hooks into core language features.

One common use case is making objects iterable with Symbol.iterator. This lets us use for...of loops with our own objects, just like we do with arrays:

Another powerful well-known Symbol is Symbol.toPrimitive. It lets us control how objects convert to primitive values like numbers or strings. This becomes useful when objects need to work with different types of operations:

Inheritance Control with Symbol.species

When working with arrays in JavaScript, we sometimes need to restrict what kind of values they can hold. This is where specialized arrays come in, but they can cause unexpected behavior with methods like map() and filter()

A normal JavaScript array that can hold any type of value:

An array that has special rules or behaviors - like only accepting certain types of values:

Think of it like this: a regular array is like an open box that accepts anything, while a specialized array is like a coin slot that only accepts specific items (in this case, numbers).

The problem Symbol.species solves is: when you use methods like map() on a specialized array, do you want the result to be specialized too, or just a regular array?

We can fix this by telling JavaScript to use regular arrays for derived operations. Here’s how Symbol.species solves this:

Note: Symbol.species is under discussion for potential removal from JavaScript.

Symbols Limitations and Gotchas

Working with Symbols isn’t always straightforward. One common confusion arises when trying to work with JSON. Symbol properties completely disappear during JSON serialization:

String coercion of Symbols leads to another common pitfall. While you might expect Symbols to work like other primitives, they have strict rules about type conversion:

Memory handling with Symbols can be tricky, especially when using the global Symbol registry. Regular Symbols can be garbage collected when no references remain, but registry Symbols stick around:

Symbol sharing between modules shows an interesting pattern. When using Symbol.for(), the Symbol becomes available across your entire application, while regular Symbols stay unique:

When To Use Symbols

Symbols shine in specific situations. Use them when you need truly unique property keys, like adding metadata that won’t interfere with existing properties. They’re perfect for creating specialized object behaviors through well-known Symbols, and the registry Symbol.for() helps share constants across your application.

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This article was originally published on https://www.trevorlasn.com/blog/symbols-in-javascript. It was written by a human and polished using grammar tools for clarity.