Best Practices for Efficient DOM Manipulation in JavaScript

The DOM (document object model) is an API for web pages, and it provides a way for JavaScript to access and manipulate the content and structure of HTML documents. There are some tips that can be utilized when manipulating the DOM to improve performance and reduce the risk of errors or bugs.

Use CSS Classes Instead of Inline Styles

When styling DOM elements, it's better and advisable to use CSS classes instead of inline styles. This is because CSS classes can be easily modified and reused, while inline styles can be harder to manage and maintain.

For example, instead of this:

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document.getElementById('button').style.backgroundColor = 'red';

You can style the element this way:

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.button-style {

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  background-color: red;

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}

And then apply the style to the element like this:

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let cardButton = document.getElementById('button');

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cardButton.classList.add('button-style');

This is a better way of styling DOM elements.

Cache Selectors

Selector caching is another method for enhancing the efficiency of your DOM manipulation code. You can store the result of the selector in a variable and reuse it later rather than constantly querying the DOM for the same element. This can speed up your code and help reduce the number of DOM queries.

For example, instead of doing this:

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document.querySelector('#button').addEventListener('click', function() {

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    // do something

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});

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document.querySelector('#button').className = "enabled";

Do this instead:

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const myButton = document.querySelector('#button');

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myButton.addEventListener('click', function() {  

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    // do something

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});

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myButton.className = "enabled";

This is called caching a selector.

Use Event Delegation

Event delegation is a technique that involves attaching an event listener to a parent element and then handling events that occur on its child elements. If you have a lot of child elements, this may be more effective than individually attaching event listeners to each one. Delegating events can also help your code become cleaner and easier to maintain.

Let's say you have some buttons inside a div container that you want to add event listeners to. Instead of adding the event listener to the individual buttons, you can attach it to the parent element, the div element.

Consider this example:

Here, instead of attaching a click event on each button, we create a single click event for the parent container. Inside that event handler we use the event.target property to get the button that was actually clicked.

It doesn't make a different if there are only a few elements that need event handlers, but if we are attaching event handlers to a large number of elements—for example in a table with thousands of rows—this technique could save a lot of time.

Use Selectors Wisely

A common way of accessing the DOM elements on a web page is by using selectors. Selectors such as getElementByClassName, getElementById, querySelector, querySelectorAll, and so on, are used to select elements in order to access and manipulate them. For example, in choosing which to use, the getElementById or the querySelector method to select elements from the web page. It is important to know how each method selects elements and when they can be used. 

The querySelector method is used to select an element based on a CSS selector. This means that you can grab an ID, a class, or any other selector using this method. querySelector grabs the first element that meets the criteria.

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const textHead = document.querySelector("#title")

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const textHead = document.querySelector("#title h1")

In the first example, the # is used to specify that you're looking for an ID with the name title. In the second example, you're grabbing the h1 element that follows the ID with the name title

The getElementById is more straightforward. It is only used to retrieve or select an element by its ID.

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const content = document.getElementById("heading")

Here you don't have to specify that you're looking for an ID with the # symbol as it's already in the function name.

If you need to pick elements using more complex rules that can be represented using a CSS selector, you should opt for the querySelector method. However, use getElementById if you want to select an element based on its ID.

Avoid Using the innerHTML Method to Manipulate HTML Elements

The innerHTML, though an effective way of manipulating the DOM HTML elements can be unsafe and vulnerable to cross-site scripting (XSS) attacks. An XSS attack using innerHTML works by injecting malicious code into your website, which it then uses to carry out an attack. innerHTML can also be a slower method of dynamically updating HTML content because it requires the browser to parse and render the element's whole HTML content from raw text. 

There are however some alternatives to using this method: the createElement and textContent methods.

The createElement method can be used to create new HTML elements and add them to the DOM using the appendChild() property.

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const newText = document.createElement('p');

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const parentElement = document.getElementById('heading');

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parentElement.appendChild(newText);

The textContent property can be used to set or get an element's text content.

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const elementOne = document.getElementById('textOne');

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elementOne.textContent = 'New text added!';

Changing the innerHTML property of an element can be much easier to code than creating multiple nodes using createElement and appendChild. Using innerHTML however, can be less secure and more prone to attacks.

Avoid Nesting Elements in Selectors

In some circumstances, nesting components within selectors can be a bad idea when manipulating the DOM. This is because it makes it harder to reuse the selector in other parts of your code since you will need to override the specific selector. It can also make it harder to maintain your code and implement new features because if your HTML structure changes, the selector may no longer match the element you intend it to.

For example, instead of nesting elements this way:

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document.querySelector('#parent .child');

Do this:

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const parent = document.querySelector('#parent');

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const child = parent.querySelector('.child');

You can reuse the parent variable in other parts of your code, which makes it more flexible and simpler to maintain, and there is less chance that it will break if the HTML structure changes.

Here's another example of bad and good practice for nesting elements in selectors:

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// BAD PRACTICE

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let menuHead = document.querySelector('header > nav > ul.menu');

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// GOOD PRACTICE

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let menuHead = document.querySelector('.menu');

Of course, there may be circumstances in which it is required or appropriate to nest elements in selectors.

Limit the Number of DOM Manipulations

The DOM can be slow to manipulate, especially if you're modifying the page frequently. Try to reduce the number of DOM operations you perform to optimize performance. For instance, if you need to alter both the text of a paragraph and the background color of a button, it is better to make both changes at the same time rather than separately. 

Conclusion

Effective DOM manipulation is essential for creating web apps that are fast and offer a seamless user experience. Developers can speed up their code and lower the possibility of performance problems by sticking to best practices including reducing the number of DOM changes, and utilizing event delegation. It is also crucial to test and evaluate code in order to find problems and fix them as necessary.