Inspired by #10421
The in operator could trivially be seen as a type guard:
interface A {
x: number;
}
interface B {
y: string;
}
let q: A | B = ...;
if ('x' in q) {
// q: A
} else {
// q: B
}
Basically, for a n in x where n is a string literal or string literal type and x is a union type, the "true" arm narrows to types which have an optional or required property n, and the "false" arm narrows to types which have an optional or missing property n.
yortus, acrazing, nippur72, yahiko00, aravindarun and 44 morejeffreymorlan, demensky and ethanresnick
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