30-seconds-of-code/snippets/on.md

title, tags

title	tags
on	browser,event,intermediate

Adds 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.

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('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' });
// logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true });
// use capturing instead of bubbling