Indirect type narrowing via `const`

[gasi]

TypeScript Version: 2.0.3

Code

interface Square {
    kind: "square";
    size: number;
}

interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}

type Shape = Square | Rectangle;

function area(s: Shape) {

    // // Doesn’t compile: Using a `const` as a type guard:
    // // `error TS2339: Property 'size' does not exist on type 'Shape'.`

    // const isSquare = s.kind === 'square';
    // if (isSquare) {
    //     return s.size * s.size;
    // }

    // // `s` is not narrowed to type `Rectangle`

    // Compiles: Directly using `s` in the type guard:
    if (s.kind === 'square') {
        return s.size * s.size; // `s` has type `Square`
    }

    // `s` is narrowed to type `Rectangle`

    return s.width * s.height;
}

Expected behavior:

s’s type can be narrowed indirectly via a const.

Actual behavior:

s’s type can only be narrowed via direct access.

[ahejlsberg]

This would require us to track what effects a particular value for one variable implies for other variables, which would add a good deal of complexity (and associated performance penalty) to the control flow analyzer. Still, we'll keep it as a suggestion.

[gasi]

Thanks, @ahejlsberg, I appreciate the feedback and understand the complexities.

[electricessence]

I believe. :|