Union types

The basic definitions of the previous section may be enough for plenty of code, but as you start working with more complex problems, you'll need some more advanced Flow features, and you may want to define types separately so that you can reuse them elsewhere. Due to this, in this and the following sections, we'll look at more advanced types. 

In JS, it's common that a variable may have, at different times, different data types. For that situation, you can use union types:

// Source file: src/types_advanced.js

let flag: number | boolean;
flag = true; // OK
flag = 1; // also OK
flag = "1"; // error: wrong type

let traffic: "red" | "amber" | "green"; // traffic is implicitly string
traffic = "yellow"; // error: not allowed

type numberOrString = number | string;
function addTwo(x: numberOrString, y: numberOrString) {
    return x + y;
}

// continues...