Gabriel Mayta

Posted on Jan 25, 2019 • Updated on Jan 28, 2019

Javascript DOM Manipulation to improve performance

#frontend #javascript #html #webdev

I wrote this article to show you with examples, how avoid performance issues when develop Web Applications with Vanilla Javascript.

Use selector instead nested elements

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

// GOOD
let menu = document.querySelector('.menu');

Avoid DOM manipulations inside loops

// BAD
for (let i = 0; i < 10; i++) {
    document.querySelector('.numbers').innerText += i;
}

// GOOD
let numbers = '';
for (let i = 0; i < 10; i++) {
    numbers += i;
}
document.querySelector('.numbers').innerText = numbers;

Don't use DOM values inside loops

// BAD
let nodes = document.querySelectorAll('.menu-items');
for (let i = 0; i < node.length; i++) { ... }

// GOOD
let nodes = document.querySelectorAll('.menu-items');
const size = nodes.length;
for (let i = 0; i < size; i++) { ... }

Use css classes instead inline styles

// BAD 
let card = document.querySelector('.card');
card.style.width = '400px';
card.style.color = '#f0f0f0';
card.style.marginTop = '10px';

// GOOD 
let card = document.querySelector('.card');
card.style.cssText = ''.concat(
    'width       : 400px;',
    'color       : #f0f0f0;',
    'margin-top  : 10px;'
);

// BETTER 
.card-custom {
    width: 400px;
    color: #f0f0f0;
    margin-top: 10px;
}

let card = document.querySelector('.card');
card.classList.add('card-custom');

Appending DOM is the most expensive operation, choose the right approach

// BAD
let container = document.querySelector('.container');
for (let i = 0; i < 1000; i++) {
    let a = document.createElement('a');
    a.text = `Row N° ${i}`;
    container.appendChild(a);
}

// GOOD
let container = document.querySelector('.container');
let html = '';
for (let i = 0; i < 1000; i++) {
    html = html.concat(`<a>Row N° ${i}</a>`);
}
container.innerHTML = html;

// BETTER
let a = document.createElement('a');
let container = document.querySelector('.container');
for (let i = 0; i < 1000; i++) {
    let cloneA = a.cloneNode(true);
    cloneA.text = `Row N° ${i}`;
    container.appendChild(cloneA);
}

// BETTER^2
let a = document.createElement('a');
let container = document.querySelector('.container');
let documentFragment = document.createDocumentFragment();
for (let i = 0; i < 1000; i++) {
    let cloneA = a.cloneNode(true);
    cloneA.text = `Row N° ${i}`;
    documentFragment.appendChild(cloneA);
}
container.appendChild(documentFragment);

Use innerHTML only for first rendering and then use DOM methods

// BAD
let container = document.querySelector('.container');
container.innerHTML = '<input type="text" name="example" value="Hi DEVS!">';
container.innerHTML = '<input type="text" name="example" value="Bye DEVS!">';

// GOOD
let container = document.querySelector('.container');
container.innerHTML = '<input type="text" name="example" value="Hi DEVS!">';
let input = container.querySelector('input');
input.value = 'Bye DEVS!';

If you have some tips, please leave your comment.
Keep calm and code!

Top comments (29)

Jason Miller 🦊⚛ • Jan 26 '19

FWIW DocumentFragment rarely provides much of a performance benefit these days. Also, in most cases imperative DOM construction is much faster than innerHTML and it's variants (insertAdjacentHTML etc), especially since the HTML parser's performance can be severely impacted by browser extensions like ad blockers. The only trick to remember is that when building DOM up imperatively, you should always build a detached tree first, then apped the fully constructed tree to the document.

Max Cerrina • Jan 27 '19

The only trick to remember is that when building DOM up imperatively, you should always build a detached tree first, then apped the fully constructed tree to the document.

Why? I'm assuming it's because a smaller tree traverses faster, yeah?

Jason Miller 🦊⚛ • Feb 25 '19

Mutating a disconnected tree is cheaper than a connected one because it has no chance of causing side effects like layout. So you build up the largest possible disconnected tree, then "commit" all those mutations by attaching its root.

yhorobey • Sep 12 '19 • Edited

Newdays DOM is smart enough to not launch reflow after each DOM change.
The trick is "do not ask DOM for values which are not available without reflow".

For example:
You have some container, div, you add to it a couple of other elements and after each addition you ask DOM "what is height of the parent div". Most likelly this will cause a reflow and this is very costly operation.

Comment deleted

Jason Miller 🦊⚛ • Apr 9 '21 • Edited

A detached tree (or disconnected tree) is a DOM tree that hasn't been added to the document yet. Mutating DOM nodes that are not currently appended to a document is extremely cheap.
Example:

Slow Version
Items are appended to the list, which is already "connected" because it has been inserted into document.body:

let list = document.createElement('ul');
document.body.appendChild(list);
for (let i = 0; i < 1000; i++) {
  let item = document.createElement('li');
  item.textContent = `Item ${i}`;
  list.append(item);
}

Fast Version
All we have to change here is to move the document.body.appendChild(list) to the end.
Because items are appended to the list before the list is appended to document.body, building up each item and inserting it into the list is very cheap. Rendering can only happen after the very last line of code executes.

let list = document.createElement('ul');
for (let i = 0; i < 1000; i++) {
  let item = document.createElement('li');
  item.textContent = `Item ${i}`;
  list.append(item);
}
document.body.appendChild(list);  // we just moved this to the end

Andrei Gatej • Jan 26 '19

For styles, you could also use
Object.assign(elem.style,
{
prop1: “value1”,
/* ... */
})

Thank you for sharing! I’ve certainly learnt a few useful things.

Fabio Biondi • Jan 26 '19

or by using spread syntax {...elem.style, prop1: "val1" }

Gabriel Mayta • Jan 26 '19

Spread operator, great, thanks for your comment Fabio.

Matias Trujillo • Jan 26 '19

It is convenient to generate the least amount of mutation in the Dom, if you use an object the mutation will be as many properties as the object has, when using a text the mutation will only be one

Andrei Gatej • Jan 27 '19

It doesn't make much sense to me. In the end, the same changes are applied to the element, no matter if you use an object or text.
Could you please elaborate on your statement ?

Matias Trujillo • Jan 28 '19

Hi!, the use of object generates a mutation of the style for each property associated to it, an object of 10 properties generates 10 mutations to the DOM when modifying style, forcing the browser to interpret the style property 10 times, instead using a string you will generate a single mutation to the element and therefore a single interpretation of the style.
This should not be a problem for you, since this should only inconvenience the user experience when multiple elements are modified concurrently in a record time.

Another benefit of the use of string, is that your style does not have dirty properties.

Andrei Gatej • Jan 28 '19 • Edited

Wow! Thank you very much! Now it makes sense.
Could you refer me to other resources so I can read more about that? It’s a very interesting topic.

Gabriel Mayta • Jan 26 '19

Thanks you, I will update the article.

Tobias SN • Jan 26 '19 • Edited

I’d just like to note a few mistakes you’ve made here:

for (let i = 0; i < 10; i++) {
    document.querySelector('.counter').innerHTML += i;
}

will output “0123456789”, since innerHTML is a string, and thus you’re not adding i, but appending it.

let counter = 0;
for (let i = 0; i < 10; i++) {
    counter += i;
}
document.querySelector('.counter').innerHTML = counter;

will however output “45” instead, since both counter and i are numbers.

Also, you’re using innerHTML to insert text in quite a few of your snippets. This is however not recommended, as it goes through the HTML parser even though it doesn’t contain any HTML (Even then, you should use DOM functions to insert elements, not innerHTML). Thus, you should use innerText instead.

Gabriel Mayta • Jan 26 '19

Hi Tobias,

you're right. I made a mistake. I upgraded the article. Thanks!

yhorobey • Sep 12 '19

45, not 55

Tobias SN • Sep 12 '19 • Edited

Thanks, I fixed it.

Ivan • Jan 28 '19

it would be nice to have codepens or similar for the examples, this way we can see it running and you will be able to check if the code you are presenting works or not

The last example:

container.value = 'Bye DEVS!';

you are not updating the input but the container, this will update the input

container.querySelector("input").value = 'Bye DEVS!';

Thanks for sharing

Gabriel Mayta • Jan 28 '19

You're right. I will create a codepen with the examples. Thanks.

howardjesstrupanion • Feb 1 '19

innerHTML is pretty darn fast: browsers have been optimized for its use, because programmers seemed to like it. See codepen.io/jwhooper/pen/GzKwMV

But unless you're building massive DOM structures (scores of thousands of elements), you're unlikely to actually see the difference in performance. I'd rather worry about writing clear, maintainable code.