Builder pattern in TypeScript (https://stackoverflow.com/a/53982272/10325032)

builder-pattern.ts

/**
 * The shape we are building.
 */
interface IPoint {
  x: number;
  y: number;
  z?: number;
}

class Point implements IPoint {
  x: number;
  y: number;
  z?: number;

  constructor(point: IPoint) {
    this.x = point.x;
    this.y = point.y;
    this.y = point.y;
  }
}

/**
 * Accumulates data, and only lets you `build()` when all requirements are met.
 */
class PointBuilder implements Partial<IPoint> {
  x?: number;
  y?: number;
  z?: number;

  withX(value: number): this & Pick<IPoint, 'x'> {
    return Object.assign(this, { x: value });
  }

  withY(value: number): this & Pick<IPoint, 'y'> {
    return Object.assign(this, { y: value });
  }

  withZ(value: number): this & Required<Pick<IPoint, 'z'>> {
    return Object.assign(this, { z: value });
  }

  build(this: IPoint) {
    return new Point(this);
  }
}
  
/**
 * The `z` property doesn't have to be provided.
 */
new PointBuilder()
  .withX(1)
  .withY(1)
  .build();

/**
 * The `.build()` method cannot be called — we are still missing `y`.
 */
new PointBuilder()
  .withX(1)
  .withZ(1);

/**
 * The `z` property is correctly recognized as `number` (as opposed to `number | undefined`)
 */
new PointBuilder()
  .withX(1)
  .withZ(1)
  .z