Simple Maybe<T> approximation in typescript

maybe.ts

type Maybe<T> = T | undefined;
function maybeMap<T, U>(mv: Maybe<T>, f: (v:T) => U) : Maybe<U> {
  if(mv !== undefined) {
    return f(mv);
  }else{
    return undefined;
  }
}
function maybeBind<T, U>(mv: Maybe<T>, f: (v:T) => Maybe<U>) : Maybe<U> {
  if(mv !== undefined) {
    return f(mv);
  }else{
    return undefined;
  }
}
function withDefault<T>(mv : Maybe<T>, defaultValue : T) : T {
  if(mv === undefined) {
    return defaultValue;
  }else{
    return mv;
  }
}
function maybeFromNullable<T>(v : T | null) : Maybe<T> {
  if(v === null) {
    return undefined;
  }else{
    return v;
  }
}