typescript - Checking validity of string literal union type at runtime?

Question

I have a simple union type of string literals and need to check it's validity because of FFI calls to "normal" Javascript. Is there a way to ensure that a certain variable is an instance of any of those literal strings at runtime? Something along the lines of

type MyStrings = "A" | "B" | "C";
MyStrings.isAssignable("A"); // true
MyStrings.isAssignable("D"); // false

Answer 1

Since Typescript 2.1, you can do it the other way around with the keyof operator.

The idea is as follows. Since string literal type information isn't available in runtime, you will define a plain object with keys as your strings literals, and then make a type of the keys of that object.

As follows:

// Values of this dictionary are irrelevant
const myStrings = {
  A: "",
  B: ""
}

type MyStrings = keyof typeof myStrings;

isMyStrings(x: string): x is MyStrings {
  return myStrings.hasOwnProperty(x);
}

const a: string = "A";
if(isMyStrings(a)){
  // ... Use a as if it were typed MyString from assignment within this block: the TypeScript compiler trusts our duck typing!
}