Typescript compiler error when comparing mutable enum member variables

Question

I am getting the error:

TS2365: Operator '===' cannot be applied to types 'ExampleState.Unsaved' and 'ExampleState.Saving'.

When comparing an enum against a mutable member variable:

enum ExampleState {
    Unset = -1,
    Unsaved = 0,
    Saving = 1,
    Saved = 2
}

class Example {

    private state : ExampleState = ExampleState.Unset;

    public Save() {
        if (this.state === ExampleState.Unsaved) {
            this.BeginSaving();
            while (this.state === ExampleState.Saving) {  // !error!
                this.CommitSave();
            }
        }
    }

    private BeginSaving() {
        this.state = ExampleState.Saving;
    }

    private CommitSave() {
        this.state = ExampleState.Saved;
    }
}

The real example is an async method that performs multiple attempts at saving - this has been simplified to just illustrate the error.

Typescript doesn't seem to understand that this variable is mutable and overly aggressively assuming that it is not being changed. Why is this happening and what is the work-around?

You could use while (this.state as any === ExampleState.Saving) as a workaround. But I suggest you to submit the case to the TypeScript team. — Paleo
– Paleo, Commented Oct 11, 2017 at 9:13
Same suggestion, although I'd rather do: while (this.state as ExampleState === ExampleState.Saving) — Mikael Couzic
– Mikael Couzic, Commented Oct 11, 2017 at 12:07
There is a open issues about that: github.com/Microsoft/TypeScript/issues/9998 — Markus
– Markus, Commented Oct 11, 2017 at 12:15
In the issue posted by @Magu, there is another workaround. I think it's the best solution. — Paleo
– Paleo, Commented Oct 11, 2017 at 13:36

Markus · Accepted Answer · 2017-10-11 14:58:13Z

2

This is an know issus from the Control flow analysis.

As another workaround you can create a wrapper method for the state property. (Thanks @Paleo)

Visit the Playground.

enum ExampleState {
    Unset = -1,
    Unsaved = 0,
    Saving = 1,
    Saved = 2
}

class Example {

    private state : ExampleState = ExampleState.Unset;

    private State() { 
        return this.state;
    }

    public Save() {
        if (this.State() === ExampleState.Unsaved) {
            this.BeginSaving();
            while (this.State() === ExampleState.Saving) { 
                this.CommitSave();
            }
        }
    }

    private BeginSaving() {
        this.state = ExampleState.Saving;
    }

    private CommitSave() {
        this.state = ExampleState.Saved;
    }
}

answered Oct 11, 2017 at 14:58

Markus

4,2094 gold badges25 silver badges27 bronze badges

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

Collectives™ on Stack Overflow

Typescript compiler error when comparing mutable enum member variables

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related