Strict mode: Object is possibly 'null' when it really isn't · Issue #16285 · microsoft/TypeScript (original) (raw)

TypeScript Version: 2.3.4

Code

/* tsconfig.json { "compilerOptions": { "target": "ES2015", "module": "commonjs", "strict": true } } */

let maybeNumber = Math.random() > 0.5 ? 2 : null; let ones = [1, 1, 1]; let twoes; if (maybeNumber) { twoes = ones.map(x => x * maybeNumber); } console.log(maybeNumber, twoes);

Expected behavior:
Compilation passes, TS should know that .map function will be executed immediately and so maybeNumber will not be null

Actual behavior:
Compilation error: error TS2531: Object is possibly 'null'

Weird:
When I change let maybeNumber to const maybeNumber compilation passes for some reason, even though type of maybeNumber should be still number | null