tsc --strict is allowing me to use an assigned var.

[StokeMasterJack]

TypeScript Version: 3.4.5

Search Terms:

Code

let aaa: string | null;
function getAaa(): string {
    if (aaa === null) {  aaa = 'hello'; }
    return aaa;
}
console.log(getAaa().length);

Expected behavior:
I expected a compile error (when using --strict). Something like this:
error TS2454: Variable 'aaa' is used before being assigned.

Actual behavior:
No compile error. But got this runtime error:
Cannot read property 'length' of undefined

Playground Link: https://www.typescriptlang.org/play/#src=let%20aaa%3A%20string%20%7C%20null%3B%0D%0A%0D%0Afunction%20getAaa()%3A%20string%20%7B%0D%0A%20%20%20%20if%20(aaa%20%3D%3D%3D%20null)%20%7B%0D%0A%20%20%20%20%20%20%20%20aaa%20%3D%20'hello'%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20return%20aaa%3B%0D%0A%7D%0D%0A%20%0D%0A%0D%0Aconsole.log(getAaa().length)%3B

[nmain]

See #28013

[typescript-bot]

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