assert incorrectly narrows type

This code:

import { assert } from "tsafe";
const x: string | undefined = Math.random() < 0.5 ? undefined : "wtf";
assert(x);
let y: string = x;
console.log(y);

should not compile, but it does. Half the time it will console.log undefined and half the time will console.log wtf. Yet the type of y is supposedly a string.

It’s not obvious why, so to avoid the spoiling the fun the explanation is base64 encoded: QXNzZXJ0IGlzIGRlZmluZWQgd2l0aCBgY29uZGl0aW9uOiBhbnkgPSB0cnVlYCB3aGljaCBjb21waWxlcyB0byAiaWYgY29uZGl0aW9uIGlzIHVuZGVmaW5lZCwgbWFrZSBpdCB0cnVlIiB3aGlsZSB0aGUgdHlwZSBzaWduYXR1cmUgZG9lc24ndCByZWZsZWN0IHRoaXMuIA==

[husseinhere]

@garronej The example is actually quite relevant.

What do you think the outcome of my example is?

Presumedly you think it will do this:

When you write assert(x) you say to typescript “Hey listen, trust be I know x will be truthy, if ever it isn’t the case at runtime an exception will be thrown here so, please, assume for the rest of the execution that x is truthy”.

But it doesn’t. Run it and see ;P It does not throw a runtime exception, even though it narrows the type as if it did.

[garronej]

Hi @husseinhere,
I fixed the code after merging it. The condition deeded to be optional and that was it.