How can you specify a tuple type in TypeScript for an array that is empty?

I'm attempting to convert a const object into a class in order to create readonly properties. The goal is to ensure that the values in these properties never change.

Specifically, I'm trying to create a module property in my class that is defined but always empty.

I've experimented with a few different approaches:

public modules: Readonly<[]> = []; // Causes an error: A tuple type element list cannot be empty.
public readonly modules: IModule[] = []; // Prevents re-assignment but still allows modification with methods like push and pop.

interface EmptyTuple {
  length: 0;
}

// Encounter an error stating types of property 'length' are incompatible, where 'number' is not assignable to '0'.
public modules: Readonly<EmptyTuple> = [];

The most recent attempt doesn't seem logical, as 0 is essentially a number. I'm feeling a bit lost here, so any guidance would be greatly appreciated.

I searched through https://github.com/Microsoft/TypeScript/issues/13126 in hopes of finding a solution, but unfortunately came up empty-handed.

Is there anyone who knows how to create an unmodifiable empty array?

typescript

Answer №1

Is there a way to create an immutable empty array in JavaScript?

To achieve this, you can define the array field using the ReadonlyArray type and mark it as readonly:

class Example {
  readonly items: ReadonlyArray<{}> = [];
}

const example = new Example();

example.items = []; //error
example.items.push({}) //error

Answer №2

TypeScript 3.0 has recently introduced enhanced support for tuple types:

type EmptyTuple = []

const empty: EmptyTuple = [] // This is valid

// Encountering an error here:
//   Type '[number]' is not compatible with type '[]'.
//     The lengths of the types are not matching.
//       Type '1' cannot be assigned to type '0'.
const err1: EmptyTuple = [1]

Reference - TypeScript 3.0 Announcement

Answer №3

To prevent these errors, you can implement strategies like the following:

    public modulesList: any[] = []; // This declaration assigns an empty array to avoid the error.
    public readonly modulesList: any[] = []; // This declaration prevents re-assignment while permitting array methods like push and pop.

Implementing these techniques can help you avoid common errors.

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