Use before declaration errors when using let statements within a switch case

TypeScript Version: 2.5.3

Code

function test(value) {
    switch (value) {
        case 1:
            let foo = 10;
            break;
        case 2:
            foo = 20; // Runtime ReferenceError: foo is not defined
            break;
        default:
    }
}

test(2);

tsc --target es2015 (or higher)

Expected behavior: This should probably produce a compile time error: Block-scoped variable 'pen' used before its declaration. on reference to foo in case 2.

Actual behavior:

The code type-checks and compiles fine, however, when running the es2015 transpiled output, you will receive a ReferenceError error at runtime. Ideally this type of error could be caught at compile time.

[RyanCavanaugh]

I’m trying to think of a reasonably-efficient algorithm to detect this and not coming up with many good ideas. switch is goto 😕

Update: Have something reasonable

[DanielRosenwasser]

Okay, I misread. Here’s one that would’ve been more obvious to me.

let sHasBeenSet = false;
switch (n) {
    case 0:
        let s = "ok";
        sHasBeenSet = true;
    case 1:
        if (sHasBeenSet) {
            s; // ok by construction
        }
        break;
}