JavaScript: Add an event listener to an element with the ability to use event delegation

JavaScript fundamental (ES6 Syntax): Exercise-95 with Solution

Write a JavaScript program to add an event listener to an element with the ability to use event delegation.

  • Use EventTarget.addEventListener() to add an event listener to an element.
  • If there is a target property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correct this context.
  • Omit opts to default to non-delegation behavior and event bubbling.
  • Returns a reference to the custom delegator function, in order to be possible to use with off.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const on = (el, evt, fn, opts = {}) => {
  const delegatorFn = e => e.target.matches(opts.target) && fn.call(e.target, e);
  el.addEventListener(evt, opts.target ? delegatorFn : fn, opts.options || false);
  if (opts.target) return delegatorFn;

const fn = () => console.log('!');
console.log(on(document.body, 'click', fn));  
console.log(on(document.body, 'click', fn, { target: 'p' }));  
console.log(on(document.body, 'click', fn, { options: true }));

Sample Output:



flowchart: Return the difference between two arrays

Live Demo:

See the Pen javascript-basic-exercise-95-1 by w3resource (@w3resource) on CodePen.

Improve this sample solution and post your code through Disqus

Previous: Write a JavaScript program to move the specified amount of elements to the end of the array.
Next: Write a JavaScript program to pick the key-value pairs corresponding to the given keys from an object.

What is the difficulty level of this exercise?

Test your Programming skills with w3resource's quiz.

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