JavaScript: Apply a function against an accumulator and each key in the object

JavaScript fundamental (ES6 Syntax): Exercise-118 with Solution

Write a JavaScript program to apply a function against an accumulator and each key in the object (from left to right).

  • Use Object.keys() to iterate over each key in the object.
  • Use Array.prototype.reduce() to apply the specified function against the given accumulator.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const transform = (obj, fn, acc) => Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);
  { a: 1, b: 2, c: 1 },
  (r, v, k) => {
    (r[v] || (r[v] = [])).push(k);
    return r;

Sample Output:



flowchart: Apply a function against an accumulator and each key in the object

Live Demo:

See the Pen javascript-basic-exercise-118-1 by w3resource (@w3resource) on CodePen.

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JavaScript: Tips of the Day

How to remove a specific item from an array?

Find the index of the array element you want to remove using indexOf, and then remove that index with splice.

The splice() method changes the contents of an array by removing existing elements and/or adding new elements.

const array = [2, 5, 9];


const index = array.indexOf(5);
if (index > -1) {
  array.splice(index, 1);

// array = [2, 9]

The second parameter of splice is the number of elements to remove. Note that splice modifies the array in place and returns a new array containing the elements that have been removed.

For the reason of completeness, here are functions. The first function removes only a single occurrence (i.e. removing the first match of 5 from [2,5,9,1,5,8,5]), while the second function removes all occurrences:

function removeItemOnce(arr, value) {
  var index = arr.indexOf(value);
  if (index > -1) {
    arr.splice(index, 1);
  return arr;

function removeItemAll(arr, value) {
  var i = 0;
  while (i < arr.length) {
    if (arr[i] === value) {
      arr.splice(i, 1);
    } else {
  return arr;
console.log(removeItemOnce([2,5,9,1,5,8,5], 5))
console.log(removeItemAll([2,5,9,1,5,8,5], 5))

Ref: https://bit.ly/2N9nKRp