Functional Programming in JavaScript

# Part IV: Implementing Method Chaining with OOP

# Example of method chaining

Implementations in defferent languages:

collect([1, 2, 3])
  .map((v: number) => v * 2)
  .filter((v: number) => v < 5)
  .toArray();

# Create test case for "map" function

Create an index.test.ts file in src folder.

import { test, expect } from 'vitest';
import { collect } from './index';

test('map', () => {
  const actual = collect([1, 2, 3, 4, 5])
    .map((v: number) => v * 2)
    .toArray();
  const expected = [2, 4, 6, 8, 10];

  expect(actual).toStrictEqual(expected);
});

Run test command.

npm run test

🔴 FAIL  src/index.test.ts > method chaining
TypeError: collect is not a function

# Implement "map" function

Create an index.ts file in src folder, and create a Collection class.

class Collection {
  private items;

  constructor(items: Array<any>) {
    this.items = items;
  }
}

Implement a map function for the class, and return the class itself.

import {
  map,
} from './modules';

class Collection {
  private items;

  constructor(items: Array<any>) {
    this.items = items;
  }

  map(callable: Function) {
    this.items = map(this.items, callable);
    return this;
  }
}

Implement a toArray function for the class, and return the array data.

class Collection {
  // ...

  toArray() {
    return this.items;
  }
}

Create a collect helper function, and return a Collection instance.

// ...

const collect = (items: Array<any>): Collection => new Collection(items);

export {
  collect,
};

Run test command.

npm run test

🟢 PASS  Waiting for file changes...

# Implement more functions

Implement more functions for the class with TDD:

  • map ✅
  • every
  • filter
  • find
  • forEach
  • includes
  • reduce
  • reject
  • size
  • some

Finally, run coverage command.

npm run coverage

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