Typescript interfaces do not guarantee that properties required by interfaces are gettable or settable

[jasonbaker]

TypeScript Version: 3.0.0-dev.201xxxxx

Search Terms:
typescript interface getter setter

Code

interface Blah {
  a: string;
}

class SettableBlah implements Blah{
  // No error for not having a getter
  set a(val: string) {
  }
}

class GettableBlah implements Blah{
  // No error for not having a setter
  get a() {
    return 'a';
  }
}

const s: Blah = new SettableBlah();
// No error for SettableBlah not having a getter
s.a;

const g: Blah = new GettableBlah();
// No error for GettableBlah not having a setter
g.a = 'a';

Expected behavior:

My expectation is that if an interface specifies a property a, it is read-write. If that interface specifies readonly a, that property should be readonly. However typescript appears to guarantee neither readability nor writability (only that the property is either readable or writable). So I would expect this code to not compile for a couple of reasons:

1. SettableBlah does not have a getter for a
2. GettableBlah does not have a setter and the field is not marked readonly.

Actual behavior:

The code compiles and breaks at runtime for reasons that might be difficult for the programmer to track down.

Playground Link: http://www.typescriptlang.org/play/#src=interface%20Blah%20%7B%0D%0A%20%20a%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Aclass%20SettableBlah%20implements%20Blah%7B%0D%0A%20%20%2F%2F%20No%20error%20for%20not%20having%20a%20getter%0D%0A%20%20set%20a(val%3A%20string)%20%7B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aclass%20GettableBlah%20implements%20Blah%7B%0D%0A%20%20%2F%2F%20No%20error%20for%20not%20having%20a%20setter%0D%0A%20%20get%20a()%20%7B%0D%0A%20%20%20%20return%20'a'%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aconst%20s%3A%20Blah%20%3D%20new%20SettableBlah()%3B%0D%0A%2F%2F%20No%20error%20for%20SettableBlah%20not%20having%20a%20getter%0D%0As.a%3B%0D%0A%0D%0Aconst%20g%3A%20Blah%20%3D%20new%20GettableBlah()%3B%0D%0A%2F%2F%20No%20error%20for%20GettableBlah%20not%20having%20a%20setter%0D%0Ag.a%20%3D%20'a'%3B

Related Issues:

This is related to #11596. This bug references behavior that allows a write-only property (which is specified to be unsupported in 11596) to be treated as read-write behavior by the compiler.

[mhegazy]

#11481 tracks the issue.

I could see us erroring on set-only class implementation since the getter type is undefined.

[jasonbaker]

Does this mean that there's no way the compiler can guarantee that a field that is not marked readonly has both a getter and a setter?

[mhegazy]

An interface is a contract. if the contract has a non-readonly property, it means you can assign to it, and you can read its value (nothing in the contract says that they both have to be the same).

A get-only property can still be written to. so that satisfies the contract. A set-only property can still be read from, but it has the value undefined at runtime. so the compiler should flag this as a violation of the contract, and it is not doing this today.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.