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StrictNullChecks and throw/return with incorrect when used with closure scope #13572
Description
TypeScript Version: 2.1.5
Code
function other() {
return new Promise((resolve, reject) => {
resolve("OK!");
});
}
function test(hosts: {whatever: boolean}[]) {
let host = hosts.filter((f) => f.whatever).shift();
if (!host) {
throw new Error("Crap!");
}
return other().then(() => {
console.log(`${host.whatever} selected`);
});
}Expected behavior:
Do not emit an error about host possibly being null, since the method exits early if that's the case.
Actual behavior:
Typically when using strictnullchecks Typescript is intelligent enough to understand that when you throw an error from a method based on a type check/guard it narrows the type after the statement. As indicated in this short example:
function test(hosts: {whatever: boolean}[]) {
let host = hosts.filter((f) => f.whatever).shift();
// Correctly calls out hose as possibly being null
console.log(`${host.whatever} selected`);
if (!host) {
throw new Error("Crap!");
}
// Does not complain here because of the above check/throw
console.log(`${host.whatever} selected`);
}For some reason when you add the return of the function and use host through a closure it starts complaining.
I'm sure it's the closure, but I don't see how this could error.
Activity
tlong-dev commentedon Jan 19, 2017
Note: re-assigning the variable to a new variable after the throw seems to re-bind the type information and solves the issue... Seems like a bug.
HerringtonDarkholme commentedon Jan 19, 2017
#9998 / #11498
Changing
let hosttoconst hostalso solves this.tlong-dev commentedon Jan 19, 2017
Wow, the perfect comment!
Found the threads I knew existed by couldn't locate in search, and offered clean workaround!
Thanks. Closing in favor of #9998 and #11498