let callback1: null | (() => void) = () => {};
let callback2: null | (() => void) = null;
let callback3: null | (() => void) = null;
if (callback1 != null)
callback1(); // callable
callback2 = () => {}
if (callback2 != null)
callback2(); // callable
new Promise<void>(() => callback3 = () => {});
if (callback3 != null)
callback3(); // not callable
// ^^^
// This expression is not callable.
// Type 'never' has no call signatures.
It seems like TypeScript is doing some static analysis, but wrongly, as the code works fine in JS:
let callback3 = null;
new Promise(() => callback3 = () => { console.log("It Works"); });
if (callback3 != null)
callback3();
// output: It Works
Since you init callback3 with a literal null and you don't modify it, TS infers its type as null.
with if (callback3 != null) you're narrowing it down to never hence the error.
This doesn't happen in this kind of case :
declare const maybeCallback: null | (() => void);
callback3 = maybeCallback;
if(callback3 !== null) {
// () => void not never
}