example.js

let a = new Ok('uno')

let result1 = 
  a.then(i => i + " wat")
  .then(i => `yeah man: ${i}`)
  .match({ 
    Ok: i => i, 
    Err: reason => { throw new Error(reason} } 
  })
console.log(result1) // yeah man: uno wat

let result2 = 
  a.then(i => i + " wat")
  .then(() => Err('failed')
  .then(i => `yeah man: ${i}`)
  .match({ 
    Ok: i => i, 
    Err: reason => { throw new Error(reason} } 
  })
// Error: failed 

Result.js

class Ok {
    constructor(data) {
        this.data = data
        this._type = "Ok"
        this.then.bind(this)
        this.match.bind(this)
    }

    then(func) {
        const result = func(this.data)
        if(result && result._type === 'Ok') {
            return result
        } else if(result && result._type === 'Err') {
            return result
        } else {
            return new Ok(result)
        }
    }

    match(obj) {
        return obj.Ok(this.data)
    }
}

class Err {
    constructor(reason) {
        this.reason = reason
        this._type = 'Err'
        this.then.bind(this)
        this.match.bind(this)
    }

    then() {
        return this
    }

    match(obj) {
        return obj.Err(this.reason)
    }
}