Don't emit 'variable x is used before being assigned', when the variable is assigned in every possible case of a switch statement.

[evert]

Search Terms

• used before assigned switch

Suggestion

The following source returns Variable 'hi' is used before being assigned.:

type Foo = 'a' | 'b';

function test(foo: Foo) {
    let hi: string;
    switch (foo) {
        case 'a':
            hi = 'hello';
            break;
        case 'b':
            hi = 'world';
            break;
    }
    console.log(hi);
}

It seems like every branch assigns the variable, but maybe I there's a secret branch I can't see.

Use Cases

I'm forced to add a ! or a default. Neither are really optimal. If type Foo changes later to include a third case, at that point I would like typescript to complain about this error. Effectively I would like to be statically guarded to the Foo type changing.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[jack-williams]

Duplicate of #18362 or #30000?

[evert]

Those two seem similar, but they both deal with enum. I don't know TS enough to say if it's the exact same problem.

[j-oliveras]

This works with version 3.7-beta.

[jack-williams]

Good spot @j-oliveras

@RyanCavanaugh I added fixed labels to the associated issues I posted above --- mainly because one was locked and I could only label it. Just letting you know so you can amend my labels.

[evert]

Wow awesome! excellent news

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.