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Readonly<T[]> vs ReadOnlyArray<T>

Written byPhuoc Nguyen
24 Aug, 2023
TypeScript offers two ways to create arrays that cannot be modified: `ReadonlyArray<>` and `Readonly<>`. While they both serve the same purpose, their syntax and behavior differ. In this post, we'll dive into the distinctions between `ReadonlyArray<>` and `Readonly<>`.


TypeScript has a handy built-in generic type called `ReadonlyArray<>`. This type gives you an array that can't be changed once it's created. It's defined like this:
interface ReadonlyArray<T> {
readonly [n: number]: T;
readonly length: number;
It has two properties: `length` and an indexed property that is also read-only. The indexed property is defined using square brackets and `n: number` notation, where `n` is the index of the element in the array. The `T` is the type of the elements in the array.
Here's an example that shows how to create a `ReadonlyArray<>`:
const cars: ReadonlyArray<string> = ['Audi', 'BMW', 'Mercedes'];
This creates a read-only array of strings with the values `Audi`, `BMW`, and `Mercedes`. You can still access the elements of the array, but you can't modify the array itself.
If you try to modify it, the code will not compile. For instance, the following code will not work:
// Error: Property 'push' does not exist
// on type 'ReadonlyArray<string>'


`Readonly<>` is another handy tool that allows you to create a read-only version of any type. In other words, you can use it to prevent accidental modifications to your data.
You can use it to create a read-only array simply by passing an array type as an argument. Check out this example:
const cars: Readonly<string[]> = ['Audi', 'BMW', 'Mercedes'];
The syntax for creating a read-only array is pretty similar to `ReadonlyArray<T>`. But there's a key difference: `Readonly<>` can be used to create a read-only version of any type, not just arrays.
Once you've created a `Readonly<>` object, it's locked in place. If you try to modify it, your code won't even compile. Check out this example of how it won't work:
// Error: Property 'push' does not exist
// on type 'readonly string[]'


To sum up, TypeScript has two built-in ways of creating immutable arrays: `ReadonlyArray<>` and `Readonly<>`.
The former is a generic type that creates a read-only array, while the latter is a utility type that can make any type, including arrays, read-only. These tools are incredibly handy for developing immutable data structures in TypeScript.

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