How to listen for changes in DOM elements using the MutationObserver API in JavaScript?

Published September 1, 2020

What is Mutation Observer?

Mutation Observer is a DOM API which helps us to listen for changes to elements in DOM and react according to it.

How to listen using Mutation Observer API?

To observe or listen to a specific element in DOM first, we have to make the MutationObserver.

// make mutation observer
const observer = new MutationObserver();
  • Here we have to pass a callback function inside the MutationObserver() constructor function because this callback function is fired whenever there is a change to that element.
// make mutation observer
// and passing a callback function
// this callback function is triggered
// whenever there is a chnage to the element
const observer = new MutationObserver((event) => {
  console.log(event);
});

Yeah, I know that we haven't said the Mutation observer which element to look for changes.

That is what we are going to do in the next step.

To attach an element to the observer we have to use the observe() method on the observer object.

// make mutation observer
// and pass a callback function
// this callback function is triggered
// whenever there is a chnage to the element
const observer = new MutationObserver((event) => {
  console.log(event);
});

// get reference to the h1 tag
const h1 = document.querySelector("h1");

// attach an h1 element tag to the observer
// to look for changes
observer.observe(h1);
  • The observe() method accepts a valid reference to a DOM element as the first argument.
  • It accepts a configuration object as the second argument. - The configuration object is used to tell the observer what type of changes to be observed on a specified element, in our case we have an h1 tag and we want to look for changes such as a change in attribute, change in the list of children inside the h1 tag and also any change in the sub-tree of the h1 tag. The config object looks like this,
const config = {
  attributes: true,
  childList: true,
  subtree: true,
};

Let's pass this config object to the observer.

// make mutation observer
// and pass a callback function
// this callback function is triggered
// whenever there is a chnage to the element
const observer = new MutationObserver((event) => {
  console.log(event);
});

// get reference to the h1 tag
const h1 = document.querySelector("h1");

// attach an h1 element tag to the observer
// to look for changes
const config = {
  attributes: true,
  childList: true,
  subtree: true,
};
observer.observe(h1, config);

Now the observer is ready and looking for changes and whenever there is a change in those specified configs, The observer calls the callback function and the details are passed into the callback function.

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