Is the ID selector the quickest method in jQuery and CSS?

Question

Is the ID selector the quickest method in jQuery and CSS?

Which is the optimal choice in jQuery/javascript for speed?

$('#myID .myClass')

or

$('.myClass')

What is the preferred option to utilize in CSS?

#myID .myClass{}

or

.myClass{}

In hindsight, I realize my explanation was insufficient. My apologies!

Undoubtedly, utilizing an ID is faster as a selector in both CSS and JavaScript. However, there are occasions where using classes becomes necessary due to multiple selectors.

Consider this scenario: within a large HTML document lies the following markup:

<div id="myID">

<a class="myClass">link1</a>

<a class="myClass">link1</a>

<a class="myClass">link1</a>

</div>

If the goal is to target all instances of "myClass", would it be more efficient to first target the ID before addressing the classes? (thus avoiding traversing the entire HTML structure) For instance:

Would the following perform better:

$('#myID').find('.myClass')

Than simply doing:

$('.myClass')

javascript jquery css jquery-selectors css-selectors

Answer 1

Answer №1

After conducting testing on newer internet browsers, it appears that the most efficient approach would be to use either of the following methods:

$('#id').find('.class') // or
$('.class')

However, it is advised against using the method below:

$('#id .class')

This recommendation stems from the fact that modern browsers now utilize a hash implementation for getElementsByClassName, resulting in consistently fast lookups based on class names. The definition of "modern browser" can vary depending on interpretation.

Answer 2

After conducting testing on newer internet browsers, it appears that the most efficient approach would be to use either of the following methods:

$('#id').find('.class') // or
$('.class')

However, it is advised against using the method below:

$('#id .class')

This recommendation stems from the fact that modern browsers now utilize a hash implementation for getElementsByClassName, resulting in consistently fast lookups based on class names. The definition of "modern browser" can vary depending on interpretation.

Answer 3

Answer №2

Most modern browsers handle class-names similarly since they are internally hashed. The main difference arises in older browsers that lack a .getElementsByClassName or similar method, causing .myClass to be parsed internally by jQuery. This results in every element in the dom being traversed and checked for the classname (or utilizing XPath when applicable).

It is advisable to use #myID .myClass whenever possible as it enables jQuery to directly navigate to #myID and traverse from there as needed.

Answer 4

Most modern browsers handle class-names similarly since they are internally hashed. The main difference arises in older browsers that lack a .getElementsByClassName or similar method, causing .myClass to be parsed internally by jQuery. This results in every element in the dom being traversed and checked for the classname (or utilizing XPath when applicable).

It is advisable to use #myID .myClass whenever possible as it enables jQuery to directly navigate to #myID and traverse from there as needed.

Answer 5

Answer №3

Let's approach this situation with pure logic for a moment, assuming we have no knowledge of how browsers work internally or access the DOM. Imagine that whatever process they follow is inherently logical.

In this scenario, wouldn't it make sense that the narrower selector would yield results more quickly?

We are presented with two selectors, which can be roughly translated to English as:

Any element with the class myClass that is a child of an element with the ID of myID
Any element with the class myClass

As for determining "What is best to use in CSS," this is entirely subjective and depends on whether you want to target all instances of .myClass or only those within #myID.

Answer 6

Let's approach this situation with pure logic for a moment, assuming we have no knowledge of how browsers work internally or access the DOM. Imagine that whatever process they follow is inherently logical.

In this scenario, wouldn't it make sense that the narrower selector would yield results more quickly?

We are presented with two selectors, which can be roughly translated to English as:

Any element with the class myClass that is a child of an element with the ID of myID
Any element with the class myClass

As for determining "What is best to use in CSS," this is entirely subjective and depends on whether you want to target all instances of .myClass or only those within #myID.

Answer 7

Answer №4

Great question, indeed.

Let's consider a scenario where you have analyzed a DOM with N elements at a maximum depth of D and CSS with S number of rules. The task of determining styles for all elements roughly has a computational complexity of O(N*D*S).

It's important to note that not all CSS selectors have the same level of computational complexity.

For instance, the li.item selector and li[class ~= "item"] are equivalent in terms of CPU resources needed. The selector li[class = "item"] can be computed faster since it doesn't require scanning white spaces.

When it comes to selectors like:

#myID .myClass{} /* #1 */
.myClass{} /* #2 */

more CPU resources are required because the work amount is similar to case #2 plus the necessity to scan the parent/child chain (of max D elements) to locate the element with "myID".

This discussion pertains solely to pure CSS selectors.

In situations involving jQuery or similar libraries, things may vary slightly. Theoretically, jQuery's engine could utilize document.getElementById() to minimize the lookup set (thus reducing the N number), but this approach might not align perfectly with CSS behavior. For example, check out this demonstration: http://jsfiddle.net/dnsUF/. Here, jQuery indicates one element with #foo while there are actually two such elements present.

To sum up:

In the case of CSS, #2 is quicker
In the context of jQuery, #1 could be faster (though not necessarily correct in a CSS sense)

Furthermore, here's an article addressing the complexity of CSS selectors: And another discussing how to enhance performance using style sets:

Answer 8

Great question, indeed.

Let's consider a scenario where you have analyzed a DOM with N elements at a maximum depth of D and CSS with S number of rules. The task of determining styles for all elements roughly has a computational complexity of O(N*D*S).

It's important to note that not all CSS selectors have the same level of computational complexity.

For instance, the li.item selector and li[class ~= "item"] are equivalent in terms of CPU resources needed. The selector li[class = "item"] can be computed faster since it doesn't require scanning white spaces.

When it comes to selectors like:

#myID .myClass{} /* #1 */
.myClass{} /* #2 */

more CPU resources are required because the work amount is similar to case #2 plus the necessity to scan the parent/child chain (of max D elements) to locate the element with "myID".

This discussion pertains solely to pure CSS selectors.

In situations involving jQuery or similar libraries, things may vary slightly. Theoretically, jQuery's engine could utilize document.getElementById() to minimize the lookup set (thus reducing the N number), but this approach might not align perfectly with CSS behavior. For example, check out this demonstration: http://jsfiddle.net/dnsUF/. Here, jQuery indicates one element with #foo while there are actually two such elements present.

To sum up:

In the case of CSS, #2 is quicker
In the context of jQuery, #1 could be faster (though not necessarily correct in a CSS sense)

Furthermore, here's an article addressing the complexity of CSS selectors: And another discussing how to enhance performance using style sets:

Answer 9

Answer №5

Identifiers are the most efficient method of accessing an element due to their distinctiveness.

Answer 10

Identifiers are the most efficient method of accessing an element due to their distinctiveness.

Answer 11

Answer №6

Indeed, the id attribute is known for being one of the quickest ways to access an element on a webpage. For a demonstration, take a look at this speed test: .

Answer 12

Indeed, the id attribute is known for being one of the quickest ways to access an element on a webpage. For a demonstration, take a look at this speed test: .

Answer 13

Answer №7

#myID .myClass is certainly a more efficient method for targeting the element, especially when dealing with multiple elements that have the .myClass attribute.

Answer 14

#myID .myClass is certainly a more efficient method for targeting the element, especially when dealing with multiple elements that have the .myClass attribute.

Answer 15

Answer №8

Update - 2021 - Visit this link for more details
https://jsperf.com/id-vs-class-vs-tag-selectors/2

In Summary:

According to my tests, using ID selector $("#foo") is approximately 4 times faster than using CSS class selector $(".bar") in Chrome version 90 on Windows 10.

Answer 16

Update - 2021 - Visit this link for more details
https://jsperf.com/id-vs-class-vs-tag-selectors/2

In Summary:

According to my tests, using ID selector $("#foo") is approximately 4 times faster than using CSS class selector $(".bar") in Chrome version 90 on Windows 10.

Is the ID selector the quickest method in jQuery and CSS?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

Answer №8

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