2.0: Type guards not working inside callbacks

[jeffreymorlan]

function f(x: {}) {
    if (typeof x === 'string') {
        x.charAt(0); // OK
        [1, 2].map(i => x.charAt(i)); // OK in 1.8, error in current master
    }
}

This appears to have started with commit dad0564

[Arnavion]

The new behavior is correct, since TS cannot know when the callback will execute. Consider:

var x: {} = "a";
if (typeof x === "string") {
    setTimeout(() => console.log(x.charAt(0)), 0);
}
x = 5;

It is wrong to assume x is a string when x.charAt() is called, as indeed it isn't.

The fix is to bind it to a scoped local:

var x: {} = "a";
if (typeof x === "string") {
    let y = x;
    [1, 2].map(i => y.charAt(i));
    setTimeout(() => console.log(y.charAt(0)), 0);
}

[mquandalle]

Also instead of scoped locals, we can use const. This works just fine:

function f(x: {}) {
    const y = x;
    if (typeof y === 'string') {
        y.charAt(0);
        [1, 2].map(i => y.charAt(i));
    }
}

Having to rename function parameters to use const is quite cumbersome though, I hope TS will provide a compiler parameter to assume const automatically for all functions parameters as suggested in #9491 (comment).

I even think that useConstParameters should be the default in TS 2.0; I realize it would be a breaking change, but basically this issue and the dozen of similar instances are also breaking changes—and quite serious ones, it breaks whenever we use a function parameter inside a callback, and the error message is confusing. I suspect the number of functions that re-assign their parameters symbols in their body is much lower, and that the error message would be way easier to understand.