unions and type guard

unions.ts

interface IMessage {
    name: string;
}

function itemFn(pipIn: string[]) {
    return pipIn;
}

function baseFn(pipIn: string) {
    return [pipIn];
}

function main(pipIn: string|string[]) {
    if (pipIn instanceof Array) {
        return itemFn(pipIn);
    } else {
        return baseFn(pipIn);
    }
}