Inside the test function, b type does not narrow to string when a type is string.
Why? I don’t understand why it behaves this way.
type Foo = {
a: string;
b: string;
};
type Bar = {
a: number;
b: number;
};
type Uinon = Foo | Bar;
function test({ a, b }: Uinon) {
if (typeof a === "string") {
a;
b;
}
}
playground
If a is type literal as shown below, this is fine, but if it is not type literal, why is it not type narrowing?
type Foo = {
a: "A";
b: string;
};
type Bar = {
a: "B";
b: number;
};
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The feature you are using is not type narrowing, but rather a discriminated union. From (apparently outdated) documentation:
Discriminating Unions
A common technique for working with unions is to have a single field which uses literal types which you can use to let TypeScript narrow down the possible current type. For example, we’re going to create a union of three types which have a single shared field.
And this discrimination is only implemented for literal types.