Unexpectedly report error on correct code: 'field' is declared but its value is never read.(6133)'.

[vczh]

TypeScript Version: 3.7.2

Search Terms: Put the error message but I don't recognize any similar issue by reading titles.

Code

export class A {
    private field: {} = {};

    public constructor() {
        // empty
    }

    public setField(value: {}): void {
        this.setFieldInternal('field', value);
    }

    private setFieldInternal(name: 'field', value: {}): void {
        this[name] = value;
    }
}

Expected behavior: No error is reported

Actual behavior: 'field' is declared but its value is never read.(6133).

Private member field could be assigned by assigning to this[name] while name has the type 'field' (or any string literal union that contains 'field'). So I believe this error message is incorrect.

If I add readonly to this field, this[name] = value; will report another error: Cannot assign to 'field' because it is a read-only property.

The compiler leaves me no space for writing code like this.

Playground Link: N/A

Related Issues: N/A

[ajafff]

Are you sure this comes from the compiler? This looks more like a linter error to me.

[dragomirtitian]

@ajafff It does come form the compiler if noUnusedLocals is used.

Playground Link

[ajafff]

@dragomirtitian the OP mentions Private member variable 'field' is never reassigned; mark it as 'readonly'. and that's definitely not a compiler error.

The error you get with noUnusedLocals ('field' is declared but its value is never read.) is correct, because the property is only written and never read.

[RyanCavanaugh]

One of these errors comes from tslint

[vczh]

I don't think so, since https://www.typescriptlang.org/play/ gives me this error too. Unless it has tslint turned on.

[ajafff]

@vczh Look at the error message. It's different in the TypeScript playground

[vczh]

@ajafff playground shows 'field' is declared but its value is never read.(6133), there is still an error. OK let me change the title of this issue.

[dragomirtitian]

@vczh Yes but as @ajafff pointed out, that error is actually correct in the context of noUnusedLocals. Add a read of the field and the error goes away as expected.

Playground Link