typeof something === 'function' No longer narrow certain generic typed variable to Function #54522
Description
Bug Report
typeof something === 'function' seems no longer narrow something to a Function in 5.1.3, when something has a generic type under certain circumstance.
🔎 Search Terms
typeof function
🕗 Version & Regression Information
The following code failed to compile on 5.1.3, while typescript prior to 5.0.4 works.
- This changed between versions 5.0.4 and 5.1.3
Updates:
The following code pass tsc before 4.7.4
fnLength1 is rejected by tsc since 4.8
methodFnLength is rejected by tsc since 5.1
⏯ Playground Link
💻 Code
export function methodFnLength<T extends {}, K extends keyof T>(
obj: T,
methodKey: K,
): number {
const fn = obj[methodKey];
if (typeof fn !== 'function') {
return 0;
}
return fn.length; // Since Typescript 5.1: fn has type T[K], instead of Function
}
export function fnLength1<T extends object>(
fn: T
): number {
if (typeof fn !== 'function') {
return 0;
}
return fn.length; // Since Typescript 4.8: fn is narrowed to never
}
export function fnLength2<T>(
fn: T
): number {
if (typeof fn !== 'function') {
return 0;
}
return fn.length; // okay
}
🙁 Actual behavior
It does not narrow fn to Function in all cases.
🙂 Expected behavior
It does narrow fn to Function in all cases as expected.