Union Types in TypeScript

typescript-union-types.ts

function print(text: string | string[]): string {
    if (typeof text === 'string') {
        return text;
    }

    // compiler now knows that you can use join
    // and that variable type is definitely string[]

    return text.join(' ');
}

let x = print('hello text');
let y = print(['hello', 'text', 'array']);

// let z = print(5); // Error: Argument of type '5' is not assignable to type 'string | string[]'

console.log(x);
console.log(y);