Generic objet builder in typescript

builder.ts

export type Builder<T> = {
  build(): Partial<T>;
  append<K extends keyof T>(key: K, value: T[K]): Builder<T>;
};

export function createBuilder<T>(): Builder<T> {
  const obj: Partial<T> = {};

  const builder: Builder<T> = {
    build: () => ({ ...obj }),
    append(key, value) {
      obj[key] = value;
      return builder;
    },
  };

  return builder;
}

main.ts

import { createBuilder } from "./builder.ts";

type Pokemon = {
  name: string;
  number: number;
  type: string; // lazy to write a enum
  level: number;
};

const umbreon = createBuilder<Pokemon>()
  .append("name", "Umbreon")
  .append("type", "dark")
  .append("number", 197)
  .append("level", 100)
  .build();

console.log(umbreon);
// { name: "Umbreon", type: "dark", number: 197, level: 100 }