The parameter cannot be assigned the readonly type 'X' in this context

Question

The parameter cannot be assigned the readonly type 'X' in this context

I encountered an issue with a third-party library where the class structure changed. Initially, it was defined as:

export class Foo {
  field: X[];
  ….
}

In my code, I was working with this type:

print(foo.field)

After updating to a new version, the class was updated to:

export class Foo {
  readonly foo: readonly X[];
  ….
}

This change caused my code to throw an error:

Argument of type 'readonly X' is not assignable to parameter of type 'X'.

Here is the function in question:

function print(foo: Foo[]): string {
  return //process foo, return some form of string
}

I am seeking guidance on how to resolve this error.

typescript types typescript-typings

Answer 1

Answer №1

If you want to ensure that the array you pass to a function is not changed, you can specify it as readonly:

function display(data: readonly Data[]): string {
  return //process data and return a string
}

Using readonly type[] in TypeScript indicates that the array should remain unchanged. This restricts modifications by allowing only read-only operations on the array:

const dataArray: readonly number[] = [];
dataArray[0] = 5;
// Error: Index signature in type 'readonly number[]' only permits reading.
dataArray.push(10);
// Error: Property 'push' does not exist on type 'readonly number[]'.

readonly type[] is similar to ReadonlyArray<type>. More information can be found in the documentation on TypeScript's website.

Answer 2

If you want to ensure that the array you pass to a function is not changed, you can specify it as readonly:

function display(data: readonly Data[]): string {
  return //process data and return a string
}

Using readonly type[] in TypeScript indicates that the array should remain unchanged. This restricts modifications by allowing only read-only operations on the array:

const dataArray: readonly number[] = [];
dataArray[0] = 5;
// Error: Index signature in type 'readonly number[]' only permits reading.
dataArray.push(10);
// Error: Property 'push' does not exist on type 'readonly number[]'.

readonly type[] is similar to ReadonlyArray<type>. More information can be found in the documentation on TypeScript's website.

The parameter cannot be assigned the readonly type 'X' in this context

Answer №1

Similar questions

Unexpected errors are encountered when using ng serve

Numerous toggle classes available

Change an ISO date format to DD:MM:YYYY HH:MM using Angular

Protractor Browser Instance Failure

Converting a String variable to a String Literal Type in Typescript: A step-by-step guide

I am encountering difficulties in accessing my component's property within the corresponding template while working with Angular 5

The importation of TypeScript source modules is not compiled accurately in the distribution folder

Utilizing a mutual RxJS subject for seamless two-way data binding in Angular 2

How to toggle visibility of multiple div elements in ReactJS

Explore the functionalities of RxJS filter operators by applying them to a stream of arrays

Having trouble deciding between flatMap and concatMap in rxJs?

Ways to address the issue of duplicated names in input fields

Developing an asynchronous function to retrieve data from an external API utilizing Await/Async strategy

The ES6 import feature conceals the TypeScript definition file

The BehaviorSubject will consistently emit identical values to each subscription

Double up on your calls to the subscribe function in Angular to ensure successful login

getting TypeScript configured with webpack

Error: The script cannot be compiled with the '--isolatedModules' flag as it is recognized as a global script file

Using TypeScript with Node.js: the module is declaring a component locally, but it is not being exported

Ways to turn off Typescript alerts for return statements