JavaScript: Split values into two groups according to a predicate function

JavaScript fundamental (ES6 Syntax): Exercise-58 with Solution

Write a JavaScript program to split values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to.

If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.

  • Use Array.prototype.reduce() and Array.prototype.push() to add elements to groups, based on the value returned by fn for each element.
  • If fn returns a truthy value for any element, add it to the first group, otherwise add it to the second group.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const bifurcateBy = (arr, fn) =>
  arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]);
console.log(bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'));

Sample Output:



flowchart: Split values into two groups according to a predicate function

Live Demo:

See the Pen javascript-basic-exercise-58-1 by w3resource (@w3resource) on CodePen.

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Previous: Write a JavaScript program to compute the average of an array, after mapping each element to a value using the provided function.
Next: Write a JavaScript program to create a function that invokes fn with a given context, optionally adding any additional supplied parameters to the beginning of the arguments.

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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