Builder Variations in TypeScript

Generic Builder

typescript
1class Builder<T> {
2  private data: Partial<T> = {};
3
4  set<K extends keyof T>(key: K, value: T[K]): this {
5    this.data[key] = value;
6    return this;
7  }
8
9  build(): T {
10    return this.data as T;
11  }
12}
13
14const shipment = new Builder<Shipment>()
15  .set('sender', senderAddress)
16  .set('recipient', recipientAddress)
17  .build();

Immutable Builder

typescript
1class ImmutableBuilder<T> {
2  constructor(private readonly data: Partial<T> = {}) {}
3
4  set<K extends keyof T>(key: K, value: T[K]): ImmutableBuilder<T> {
5    return new ImmutableBuilder({ ...this.data, [key]: value });
6  }
7
8  build(): T {
9    return Object.freeze(this.data) as T;
10  }
11}

Builder with Validation

typescript
1class ValidatingBuilder {
2  private errors: string[] = [];
3
4  from(address: Address): this {
5    if (!address.postalCode) {
6      this.errors.push('Sender postal code required');
7    }
8    this.data.sender = address;
9    return this;
10  }
11
12  build(): Shipment {
13    if (this.errors.length > 0) {
14      throw new ValidationError(this.errors);
15    }
16    return new Shipment(this.data);
17  }
18}

Async Builder

typescript
1class AsyncShipmentBuilder {
2  async from(address: Address): Promise<this> {
3    await this.validateAddress(address);
4    this.data.sender = address;
5    return this;
6  }
7
8  async build(): Promise<Shipment> {
9    const rates = await this.fetchRates();
10    return new Shipment({ ...this.data, rates });
11  }
12}
13
14// Usage
15const shipment = await new AsyncShipmentBuilder()
16  .then(b => b.from(sender))
17  .then(b => b.to(recipient))
18  .then(b => b.build());

Summary