If we write the simple construct, we have:

for(const a of ["a", "b"]) {
    type b = typeof a; // string
}

Is there a way to have?

for(const a of ["a", "b"]) {
    type b = typeof a; // "a" | "b"
}

without repeating all the strings twice?

for(const a of ["a", "b"] as (("a" | "b")[])) {
    type b = typeof a; // "a" | "b"
}

CodePudding user response：

Declare the array as const so it doesn't get widened to string[].

for(const a of ["a", "b"] as const) {
    type b = typeof a; // "a" | "b"
}