form2obj.ts

let user = new User(filterByProps(form, [
  'username',
  'displayname'
]))
  
function filterByProps(obj: any, props: string[]): any {
  if (!props) return obj
  let ret = {}
  Object.keys(obj).forEach(key => {
    if (props.some(prop => prop === key)) {
      ret[key] = clone(obj[key])
    }
  })
  return ret
}

export interface user {
  username: email | username
  firstname?: string
  lastname?: string
  displayname?: string
}

export class User implements user {
  username: email | username
  firstname?: string
  lastname?: string
  displayname?: string

  constructor(user?: user) {
    Object.assign(this, user)
  }
}

export function clone(obj: any): any {
  return JSON.parse(JSON.stringify(obj))
}