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Extending global Function interface breaks instanceof Type Guard #26159
Description
TypeScript Version: 3.1.0-dev.201xxxxx
Search Terms:
global Function interface
instanceof
Type Guard
Code
declare global {
interface Function {
now(): string;
}
}
Function.prototype.now = function () {
return "now"
}
class X {
static now() {
return {}
}
why() {
}
}
class Y {
}
console.log(X.now()) // works as expected
console.log(Y.now()) // works as expected
export const x: X | number = Math.random() > 0.5 ? new X() : 1
if (x instanceof X) {
x.why() // should compile
}Expected behavior:
Code compiles
Actual behavior:
Compilation failed
"Property 'why' does not exist on type 'number'."
Playground Link: Playground
Related Issues:
#7271
Activity
kobiburnley commentedon Aug 2, 2018
Andy (Andrewkraft) (@Andy-MS) You're right, I forgot to emphasize on this. It happens for classes that their static fields override some new added properties to the Function interface.
But this works as expected though: