Null inference of constants within callbacks

[ntrrgc]

TypeScript Version:

1.9.0-dev.20160528-1.0

Code

function printString() {
    const myString: string|null = 'Hello';
    if (myString == null) {
        return;
    }
    setTimeout(() => {
        console.log('String length was ' + myString.length);
    }, 100);
}
printString();

Actual behavior:

When compiled with --strictNullChecks, this code fails:

testcase.ts(7,44): error TS2531: Object is possibly 'null'.

Expected behavior:

Null inference works fine within the immediate printString scope, but it's disabled inside closures.

This behavior makes sense for variables, since otherwise I could write something like this, that would make myString actually become null when it's read and fail:

function printString() {
    let myString: string|null = 'Hello';
    if (myString == null) {
        return;
    }
    setTimeout(() => {
        console.log('String length was ' + myString.length);
    }, 100);
    myString = null;
}
printString();

On the other hand, constants don't allow this, so their non-nullity can be guaranteed if they were checked before the closure was declared.

[ivogabe]

This will be fixed with #8849

[ntrrgc]

Cool! 😃