closures pracitse:

Here’s a curated list of 20 closure-related JavaScript questions, progressing from easy to hard, covering fundamental to advanced concepts:

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Easy (1-7) - Basic Understanding

1. What is a closure? (Definition)

2. Write a function counter() that returns a function which increments and returns a count each time it's called.

    const count = counter(); 
    count(); // 1 
    count(); // 2
   

3. Fix the loop to print 1, 2, 3 after 1s each:

    for (var i = 1; i <= 3; i++) {
      setTimeout(() => console.log(i), 1000);
    }
   

4. Predict the output:

    function outer() {
      let x = 10;
      function inner() {
        console.log(x);
      }
      return inner;
    }
    const foo = outer();
    foo(); // ?
   

5. Create a private counter using closures where the variable can't be accessed directly.

6. What will this log?

    let a = 1;
    function outer() {
      let a = 2;
      function inner() {
        console.log(a);
      }
      return inner;
    }
    const func = outer();
    func(); // ?
   

7. Write a function multiplier(x)that returns a function to multiply its argument by x.

    const double = multiplier(2);
    double(5); // 10
   

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Medium (8-14) - Practical Applications

8. Memoization: Create a memoize(func) that caches results of expensive function calls.

9. Function Factory: Write a createGreeting(greeting) that returns a function to greet a name.

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    const sayHi = createGreeting("Hi");
    sayHi("Alice"); // "Hi, Alice"
   

10. Currying: Convert add(a, b, c) into add(a)(b)(c).

11. Predict the output:

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    function outer() {
      let count = 0;
      return function inner() {
        count++;
        return count;
      };
    }
    const c1 = outer();
    const c2 = outer();
    console.log(c1(), c1(), c2()); // ?
    

12. Debouncing: Implement debounce(func, delay) to delay function execution.

13. Throttling: Implement throttle(func, limit) to restrict function calls.

14. Module Pattern: Create a calculator module with private total and methods add, subtract, getTotal.

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Hard (15-20) - Advanced Scenarios

15. Partial Application: Write a partial(func, ...args) that fixes some arguments.

16. Closure Chain: Predict the output:

    function outer() {
      let x = 1;
      return function middle() {
        let y = 2;
        return function inner() {
          console.log(x + y);
        };
      };
    }
    outer()()(); // ?
    

17. Closure + Async: Fix this to log 0, 1, 2:

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    for (var i = 0; i < 3; i++) {
      setTimeout(() => console.log(i), 1000);
    }
    ``` *(Using closures)*
    

18. Closure in Event Listeners: Why does this log the last i? Fix it.

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    for (var i = 0; i < 3; i++) {
      document.getElementById(`btn-${i}`).addEventListener('click', () => console.log(i));
    }
    

19. Closure + Prototypes: Explain how closures differ from class private fields (#privateVar).

20. Closure Leak: How can closures cause memory leaks? Give an example.

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Bonus (Expert Level)

• Implement once(func) (allows function to run only once).

• Closure + Generators: Create a closure-based iterator.

• Closures in React: How do hooks like useState use closures?

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These questions cover variable scope, encapsulation, functional programming, optimization (debouncing/throttling), and memory management.