Narrowing string to string literal union

Question

I want to narrow a string to a string literal union. In other words, I want to check if the string is one of the possible values of my literal union, so that this will work (if the operator couldbe existed).

type lit = "A" | "B" | "C";
let uni: lit;
let str = "B";
if(str couldbe lit){
    uni = str;
} else {
    doSomething(str);
}

How can I achieve this?

I tried using if (str instanceof lit), but that doesn't seem to work. Using keyof to iterate over the string union doesn't work either, because the allowed values aren't keys per se.

One way would be to use switch with one case for each possible value, but that could lead to subtle errors if lits allowed values change.

Answer 1

You can use User-Defined Type Guards.

type lit = "A" | "B" | "C";
let uni: lit;
let str = "B";

function isLit(str: string): str is lit {
    return str == "A" || str == "B" || str == "C";
}
function doSomething(str: string) {

}

if (isLit(str)) {
    uni = str;
}
else {
    doSomething(str);
}

ADD:

To avoid duplicated edit, class can be used both for compile-time and run-time. Now all you have to do is to edit just one place.

class Lit {
    constructor(public A = 0, public B = 0, public C = 0) {}
}
type lit = keyof Lit;
let uni: lit;

function isLit(str: string): str is lit {
    let lit = new Lit();
    return (str in lit) ? true : false;
}