Passing bound callback to function taking unbound callback

[danielrentz]

TypeScript Version: 3.7.2

Search Terms: bound unbound callback this parameter

Code

class C { public prop = 1; }
const c = new C();
 
function callUnbound(fn: () => void): void {
    fn();
}

function callBound(fn: (this: C) => void): void {
    //fn(); // compiler error as expected
    fn.call(c); // passes as expected
    callUnbound(fn); // passes unexpectedly (`callUnbound` does not call with context)
}

callBound(function () { console.log(this.prop); });

Expected behavior:
Last line in callBound causes a compiler error.

Actual behavior:
Last line in callBound passes the compiler, and fn will be called without this context.

Playground Link:

Related Issues:

[jcalz]

Duplicate of #7968

[fatcerberus]

The callBound() call on the last line likely wouldn't be an error even with stricter this typing, because its argument function has no this annotation at all. It's the same principle that you can pass a function with fewer parameters in place of one with more. The callUnbound() call is where I would expect the compiler error, if one were to be produced.

edit: Disregard, I was stupid and misread the issue.

[danielrentz]

The callBound() call on the last line likely wouldn't be an error even with stricter this typing, because its argument function has no this annotation at all. It's the same principle that you can pass a function with fewer parameters in place of one with more. The callUnbound() call is where I would expect the compiler error, if one were to be produced.

That's what I wanted to express with "Last line in (the function) callBound". :)

[fatcerberus]

Oops, you’re right, that was a brain fart. I read it as “the last line, callBound, ...” 😬