Discriminated Union TypeScript example using instanceof. It's nice for web APIs

discriminated_union.ts

class Video {
    id: number;
    
    constructor(id: number) {
        this.id = id;
    }
}

class Error {
    message: string;
    
    constructor(message: string) {
        this.message = message;
    }
}

// This function takes a discriminated union, it is either an Error or Video
// This allows us to API response types pretty clean 
function processResult (input: Video | Error) {

    var el = document.createElement("p");
    if (input instanceof Error) {
        var err = input; 
        el.innerText = "error" + input.message;     
    } else if (input instanceof Video) {
        el.innerText = "thing:" + input.id;     
    }
    document.body.appendChild(el);
}

processResult(new Video(12));