TS 4.7 Beta: Object access should be narrowable after check #48802
Description
If I read the release blog post for v4.7 Beta correctly, the following test case should work, but it still produces an error.
interface Foo { [key: string]: number | string; } declare function generateKey(): string; function fn(foo: Foo) { const key = generateKey(); if (typeof foo[key] === 'number') { foo[key].toExponential(2); // Error: Property 'toExponential' does not exist on type 'string | number'. } }Is this a bug?
TS Playground Link v4.7.0-dev.20220420
If this only works for object access with Symbol, it should be clarified in the blog post.
FWIW, here's the example from the blog post (which works):
const key = Symbol(); const numberOrString = Math.random() < 0.5 ? 42 : "hello"; let obj = { [key]: numberOrString, }; if (typeof obj[key] === "string") { let str = obj[key].toUpperCase(); }Originally posted by @kyliau in #45974 (comment)