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);
}