Typescript Class implements Interface does not respect readonly modifier

Question

I want to create class base on interface but it looks link it does not respect readonly modifier.

code below works without compiler error:

interface I {
  readonly a: string
}

class C implements I{
  a= ""
}
const D = new C
D.a = "something"

in order to make property 'a' really readonly I should make it readonly in class definition too! so what's the use case on readonly modifier in interface definition?

in other word how can I make sure when I am creating a class by implementing interface I creating it with right modifier?

Evert · Accepted Answer · 2020-04-12 04:08:32Z

2

Even more concerning and surprising, is that the opposite is also true. An object with a readonly property can satisfy the type of an object where that property is not read-only.

I found this issue: https://github.com/microsoft/TypeScript/issues/18770

It's open, this could suggest that there is interest in fixing this somehow and somewhat acknowledged as a problem.

answered Apr 12, 2020 at 4:08

Evert

101k18 gold badges136 silver badges221 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Saeid Ostad Over a year ago

I added my comment in that issue, hopefully they will put that in their roadmap.

Michael Tidwell · Accepted Answer · 2021-05-30 19:40:10Z

2

You would need to define the type of "D" early, note I made "D" into lowercase "d"

interface I {
  readonly a: string
}

class C implements I{
  a= ""
}
let d: I // THIS IS NEEDED
d = new C
d.a = "something" // NOW THIS WON'T WORK

answered May 30, 2021 at 19:40

Michael Tidwell

3913 silver badges12 bronze badges

1 Comment

ebbishop Over a year ago

Yep, this is it!

VRoxa · Accepted Answer · 2020-04-12 03:47:36Z

0

The main idea of readonly keyword in an interface is the constraint in case of declaring an object of the interface type.

interface ITest {
    readonly a: string;
}

const t: ITest = {
    a: 'readonly'
}

t.a = 'another value'; // -> Compiler error

When implementing an interface in a class, the class must redeclare the access attributes of the properties inferred

interface ITest {
    readonly a: string;
    b: string;
    c: string;
}

class ATest implements ITest {
    a: string = ``;
    constructor(public b: string, public c: string) { }
}

const t = new ATest('b', 'c');
t.a = 'another value'; // This is OK

answered Apr 12, 2020 at 3:47

VRoxa

1,0519 silver badges27 bronze badges

2 Comments

Saeid Ostad Over a year ago

I understand that but back to my question, how can I make sure when I am creating a class by implementing interface I am creating it with right modifier?

VRoxa Over a year ago

As I pointed, classes must redeclare the access attributes of their properties. There is no inheritance of property accesors from interfaces to classes. The only constraint is properties inferred from interfaces must be public, indeed.

Collectives™ on Stack Overflow

Typescript Class implements Interface does not respect readonly modifier

3 Answers 3

1 Comment

1 Comment

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related