Issue with Array.prototype.reduce

[loia5tqd001]

Your GreatFrontEnd account email (optional)

loia5tqd001@gmail.com

Problematic section

• Question Description
• Question Skeleton
• Question Solution
• Test Cases (Missing/Incorrect/Unclear)

Type of question

• JavaScript
• User Interface
• System Design
• Quiz

Describe the issue

Missing testcase. The code below passed all the tests but it behaves differently with the original reduce function. Pls add a testcase to catch this bug.

Code you used to submit (where relevant)

/**
 * @template T, U
 * @param {(previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U} callbackFn
 * @param {U} [initialValue]
 * @return {Array<U>}
 */
Array.prototype.myReduce = function (callbackFn, initialValue) {
  const hasInitial = initialValue !== undefined;
  if (!hasInitial && !this.length) throw new TypeError();

  let result = hasInitial ? initialValue : this[0];
  const startIndex = hasInitial ? 0 : 1;
  for (let i = startIndex; i < this.length; i++) {
    if (Object.hasOwn(this, i)) {
      result = callbackFn(result, this[i], i, this);
    }
  }
  return result;
};

console.log(
  [,2,3].myReduce((a, b) => a + b), 
  [,2,3].reduce((a, b) => a + b)
);

Edit: Later I realized that this code is similar to the solution 😃, so I marked Question Solution as the problem as well.

[loia5tqd001]

Another code that passed all the testcases but behaves differently with the original reduce

/**
 * @template T, U
 * @param {(previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U} callbackFn
 * @param {U} [initialValue]
 * @return {Array<U>}
 */
Array.prototype.myReduce = function (callbackFn, initialValue) {
  if (initialValue === undefined && !this.length) throw new TypeError();

  let result = initialValue;
  for (let i = 0; i < this.length; i++) {
    if (!Object.hasOwn(this, i)) continue;
    if (result === undefined) {
      result = this[i];
      continue;
    }
    result = callbackFn(result, this[i], i, this);
  }
  return result;
};

console.log(
  [,2,3,4,5,6].myReduce((a, b, i) => i === 2 ? undefined : a + b, 0), 
  [,2,3,4,5,6].reduce((a, b, i) => i === 2 ? undefined : a + b, 0)
);