You can find all the code in this post at repo Github.

Closure related challenges

Hello World

/**
 * @return {Function}
 */

function createHelloWorld() {
  return function (...args) {
    return "Hello World";
  };
}

// Usage example
const output = createHelloWorld();
console.log(output()); // => "Hello World"

add

/**
 * @param  {...any} args
 * @return {Function | number}
 */

function add(...args) {
  let sum = args.reduce((acc, val) => acc   val, 0);

  function innerAdd(...moreArgs) {
    sum  = moreArgs.reduce((acc, val) => acc   val, 0);
    return innerAdd;
  }
  innerAdd.getValue = function () {
    return sum;
  };

  return innerAdd;
}

// Usage example
console.log(add(1).getValue()); // => 1
console.log(add(1)(2).getValue()); // => 3
console.log(add(1)(2)(3).getValue()); // => 6
console.log(add(1)(2, 3).getValue()); // => 6
console.log(add(1, 2)(3).getValue()); // => 6
console.log(add(1, 2, 3).getValue()); // => 6

Sum

/**
 * @param {number} num
 */

function sum(num) {
  const func = function (num2) {
    return num2 ? sum(num   num2) : num;
  };

  func.valueOf = () => num;
  return func;
}

// Usage example
const sum1 = sum(1);
console.log(sum1(2) == 3); // => true
console.log(sum1(3) == 4); // => true
console.log(sum(1)(2)(3) == 6); // => true
console.log(sum(5)(-1)(2) == 6); // => true

Counter

/**
 * @param {number} initialValue
 * @return {Function}
 */

function makeCounter(initialValue = 0) {
  let count = initialValue - 1;

  return function (...args) {
    count  = 1;
    return count;
  };
}

// Usage example
const counter = makeCounter(0);
console.log(counter()); // => 0
console.log(counter()); // => 1
console.log(counter()); // => 2

//------------------------------
// return an object
/**
 * @param {number} initialValue
 * @return {{get: Function, increment: Function, decrement: Function, reset: Function }}
 */
function makeCounter(initialValue = 0) {
  let count = initialValue;

  return {
    get: () => count,
    increment: () =>   count,
    decrement: () => --count,
    reset: () => (count = initialValue),
  };
}

// Usage example
const counterObj = makeCounter(0);
console.log(counterObj.get()); // => 0
counterObj.increment();
console.log(counterObj.get()); // => 1
counterObj.decrement();
counterObj.reset();
console.log(counterObj.get()); // => 0

Cycle

/**
 * @template T
 * @param  {...T} values
 * @returns () => T
 */

function cycle(...values) {
  let index = -1;

  return function (...args) {
    index = (index   1) % values.length;
    return values[index];
  };
}

// Usage example
const helloFn = cycle("hello");
console.log(helloFn()); // => "hello"
console.log(helloFn()); // => "hello"

const onOffFn = cycle("on", "off");
console.log(onOffFn()); // => "on"
console.log(onOffFn()); // => "off"
console.log(onOffFn()); // => "on"

Limit

/**
 * @param {Function} func
 * @param {Number} count
 * @return {Function}
 */

function limit(fn, max) {
  let count = 0;
  let value;

  return function (...args) {
    if (count 







  
  
  Once




/**
 * @param {Function} fn
 * @return {Function}
 */

function once(fn) {
  let ranOnce = false;
  let value;

  return function (...args) {
    if (!ranOnce) {
      value = fn.call(this, ...args);
      ranOnce = true;
    }

    return value;
  };
}

// Usage example
function func(num) {
  return num;
}

const onced = once(func);
console.log(onced(1)); // => 1, func called with 1
console.log(onced(2)); // => 1, even 2 is passed, previous result is returned








  
  
  To be or not to be




/**
 * @param {any} val
 * @return {true | Error}
 */

function expect(val) {
  return {
    toBe: function (arg) {
      if (val === arg) {
        return true;
      } else {
        throw new Error("Not Equal");
      }
    },
    notToBe: function (arg) {
      if (val !== arg) {
        return true;
      } else {
        throw new Error("Equal");
      }
    },
  };
}

// Usage example
expect(5).toBe(5); // Passes
expect(5).notToBe(6); // Passes

try {
  expect(5).toBe(6); // Throws an error
} catch (error) {
  console.log(error.message); // Not Equal
}








  
  
  Reference

• 148. create a counter object - BFE.dev
• 2620. Counter - LeetCode
• 2665. Counter II - LeetCode
• 2665. Counter II - LeetCode
• GreatFrontEnd
• 2667. Create Hello World Function - LeetCode
• 23. create a sum() - BFE.dev
• 46. implement _.once() - BFE.dev
• 2704. To Be Or Not To Be - BFE.dev
• 161. toBe() or not.toBe() - BFE.dev
• "Hello, World!" program - Wikipedia.org