guard-indexed-type.js

type NumberDict = {[key: string]: number};
type NullableObject = {} | null;

function nullableObjectIsNumberDict(x: NullableObject): x is NumberDict {
    return !!x && Object.keys(x).every(key => typeof x[key] === 'number');
}

function use(x: NullableObject) {
    if (!nullableObjectIsNumberDict(x)) {
        return;
    }
    x // I expected NumberDict; TS says {}
}