Counter with private state

Write makeCounter(start), where start defaults to 0. It returns an object with four methods: increment() and decrement() change the count by one and return the new count, reset() sets it back to start and returns it, and value() returns the current count.

The count is private: none of the object's properties holds it, so code outside can only change it through the methods. Every counter is independent, and the methods keep working when they're called on their own, without the object in front.

const counter = makeCounter(5);
counter.increment(); // 6
counter.increment(); // 7
counter.decrement(); // 6
counter.reset();     // 5
counter.count;       // undefined
 
const { increment, value } = counter;
increment();         // 6
value();             // 6

What the tests check

1. 1starts at 0 by default

   expect(makeCounter().value()).toBe(0);
   

2. 2increment and decrement return the new count

   const counter = makeCounter(5);
   expect(counter.increment()).toBe(6);
   expect(counter.increment()).toBe(7);
   expect(counter.decrement()).toBe(6);
   expect(counter.value()).toBe(6);
   

3. 3reset goes back to start and returns it

   const counter = makeCounter(5);
   counter.increment();
   counter.increment();
   expect(counter.reset()).toBe(5);
   expect(counter.value()).toBe(5);
   

4. 4counters are independent

   const a = makeCounter();
   const b = makeCounter(10);
   a.increment();
   a.increment();
   b.decrement();
   expect(a.value()).toBe(2);
   expect(b.value()).toBe(9);
   expect(a.reset()).toBe(0);
   expect(b.reset()).toBe(10);
   

5. 5no property holds the count

   const counter = makeCounter(3);
   counter.increment();
   const notMethods = Reflect.ownKeys(counter).filter((key) => typeof counter[key] !== "function");
   expect(notMethods).toEqual([]);
   

6. 6the methods work on their own, without the object

   const { increment, decrement, reset, value } = makeCounter(1);
   increment();
   increment();
   decrement();
   expect(value()).toBe(2);
   expect(reset()).toBe(1);
   

Learn more

• ClosuresLesson
• Closure patternsLesson
• Write makeCounter(start): a counter with increment, decrement, reset, and value methods and private state.Interview question