default-values-in-typescript.md

Applying default values

If our dependencies look like this:

declare class Repository<T> {
  find<T>(options: FindOptions): Promise<T>;
}

interface FindOptions {
  limit: number;
  offset?: number;
}

we can apply defaults in a number of ways:

Using short-circuit evaluation

class Foo {
  constructor(private readonly repo: Repository<any>) {}

  find(options: Partial<FindOptions>) {
    return this.repo.find({
      ...options,
      limit: options.limit || 10,
    });
  }
}

Using default parameters

class Foo {
  constructor(private readonly repo: Repository<any>) {}

  find({ limit = 10, ...rest }: Partial<FindOptions>) {
    return this.repo.find({
      limit,
      ...rest,
    });
  }
}

Using a complete set of defaults

declare const defaults: FindOptions;

class Foo {
  constructor(private readonly repo: Repository<any>) {}

  find(options: Partial<FindOptions>) {
    return this.repo.find(
      update(defaults, options)
    );
  }
}

Where:

const update = <T extends object>(source: T, patch: Partial<T>): T =>
  Object.assign({}, source, patch);

Using partial defaults

type Defaults = Pick<FindOptions, 'limit'>;
declare const defaults: Defaults;

class Foo {
  constructor(private readonly repo: Repository<any>) {}

  find(options: Omit<FindOptions, keyof Defaults>) {
    return this.repo.find(
      merge(defaults, options),
    );
  }
}

Where:

const merge = <T extends object, U extends object>(source: T, patch: U): Overwrite<T, U> =>
  Object.assign({}, source, patch);
  
type Overwrite<T, U> = Omit<T, Extract<keyof T, keyof U>> & U;

type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;