Guidelines for writing code examples

The guidelines described in this article apply to the styling and formatting of code examples, irrespective of the language. For guidelines about what content to include while writing the code examples, see the writing style guide.

For technology-specific guidelines, see the following articles:

General best practices

This section provides the best practices for creating an understandable minimal code example to demonstrate the usage of a specific feature or function.

Code examples that you add to MDN Web Docs should be:

  • simple enough to be understandable, but
  • complex enough to do something interesting, and preferably useful.

There is one overarching consideration that you need to keep in mind: Readers will copy and paste the code sample into their own code and may put it into production.

Therefore, you should make sure that the code example is usable, follows generally accepted best practices, and does not do anything that will cause an application to be insecure, grossly inefficient, bloated, or inaccessible. If the code example is not runnable or production-worthy, be sure to include a warning in a code comment and in the explanatory text; for example, if it is only a snippet and not a full example, make this clear. This also means that you should provide all of the information necessary to run the example including any dependencies and setup information.

Code examples should be as self-contained and easy to understand as possible. The aim is not necessarily to produce efficient, clever code that impresses experts and has great functionality, but rather to produce reduced working examples that can be understood as quickly as possible.

Some more general best practices include:

  • The code example should be short and ideally show only the feature you are immediately interested in.
  • Only include code that is essential for the example. A large amount of non-relevant code can easily distract or confuse the reader. If you want to provide a full, more lengthy example, put it in one of our GitHub repos (or a JSBin, Codepen, or similar) and then provide the link to the full version above or below the sample.
  • Don't include unnecessary server-side code, libraries, frameworks, preprocessors, and other such dependencies. They make the code less portable and harder to run and understand. Use vanilla code where possible.
  • Don't assume readers' knowledge of any libraries, frameworks, preprocessors, or other non-native features. For example, use class names that make sense within the example rather than class names that make sense to BEM or Bootstrap users.
  • Write your code to be as clean and understandable as possible, even if it is not the most efficient way to write it.
  • Be inclusive in your code examples; consider that MDN readers come from all over the world, and are diverse in their ethnicities, religions, ages, genders, etc. Ensure text in code examples reflects that diversity and is inclusive of all people.
  • Don't use bad practices for brevity (such as presentation elements like <big> or document.write()); do it correctly.
  • In the case of API demos, if you are using multiple APIs together, point out which APIs are included and which features come from where.

Guidelines for formatting

Opinions on correct indentation, whitespace, and line lengths have always been controversial. Discussions on these topics are a distraction from creating and maintaining content.

On MDN Web Docs, we use Prettier as a code formatter to keep the code style consistent (and to avoid off-topic discussions). You can consult our configuration file to learn about the current rules, and read the Prettier documentation.

Prettier formats all the code and keeps the style consistent. Nevertheless, there are a few additional rules that you need to follow.

These MDN Web Docs guidelines for formatting code examples are also good practices when you are coding.

Choosing a syntax language

To ensure proper formatting and syntax highlighting of code blocks, writers must specify the language of the code block they are writing in. See Example code blocks in MDN Markdown for a list of languages supported by MDN, as well as details on how to request a new language.

If the code block is pseudocode, the output of a command, or otherwise not a programming language, explicitly set the language to plain.

Warning: if the desired language is not yet supported by MDN, do not set the language of a code block to a similar language, as doing so may have unintended side effects with Prettier formatting and syntax highlighting.

Code line length

  • Code lines shouldn't be so long that they require horizontal scrolling to read.
  • As a recommended practice, keep code lines up to a maximum of 80 characters long (64 for interactive examples).
  • Break long lines at natural breaking points for the sake of readability, but not at the expense of best practices.

For example, this is not great:

let tommyCat =
  "Said Tommy the Cat as he reeled back to clear whatever foreign matter may have nestled its way into his mighty throat. Many a fat alley rat had met its demise while staring point blank down the cavernous barrel of this awesome prowling machine.";

This is better, but somewhat awkward:

const tommyCat =
  "Said Tommy the Cat as he reeled back to clear whatever foreign " +
  "matter may have nestled its way into his mighty throat. Many a fat alley rat " +
  "had met its demise while staring point blank down the cavernous barrel of " +
  "this awesome prowling machine.";

Even better is to use a template literal:

const tommyCat = `Said Tommy the Cat as he reeled back to clear whatever foreign
  matter may have nestled its way into his mighty throat. Many a fat alley rat
  had met its demise while staring point blank down the cavernous barrel of
  this awesome prowling machine.`;
if (
  obj.CONDITION ||
) {
  /* something */

const toolkitProfileService = Components.classes[

Code block height

Code blocks should be as long as they need to be, but no longer. Ideally, aim for something short, like 15-25 lines. If a code block is going to be a lot longer, consider just showing the most useful snippet, and link to the complete example on a GitHub repo or CodePen, say.

Inline code formatting

Use inline code syntax (`) to mark up function names, variable names, and method names. For example: "the frenchText() function".

Method names should be followed by a pair of parentheses: for example, doSomethingUseful(). The parentheses help differentiate methods from other code terms.

Guidelines for proper rendering

These guidelines should be followed to ensure that the code examples you write display properly on MDN Web Docs. You should also consider responsiveness making writing code examples so that they are also useful on mobile devices.

Size of the rendered code example

  • Set the width to 100%: The main content pane on MDN Web Docs is about 700px wide on desktop, so the embedded code examples must look OK at that width.
  • Set height below 700px: We recommend keeping this height for the rendered code example width for maximum onscreen legibility.

Color in the rendered code example

  • Use keywords for primary and other "basic" colors, for example:
    color: black;
    color: white;
    color: red;
  • Use rgb() for more complex colors (including semi-transparent ones):
    color: rgb(0 0 0 / 50%);
    color: rgb(248 242 230);
  • For hex colors, use the short form where relevant:
    color: #058ed9;
    color: #a39a92c1;
    color: #ff0;
    color: #fbfa;
    color: #ffff00;
    color: #ffbbffaa;

Mark rendered examples as good or bad

You'll notice on this page that the code blocks that represent good practices to follow are rendered with a green check mark in the right corner, and the code blocks that demonstrate bad practices are rendered with a white cross in a red circle.

You can follow the same style while writing code examples. You don't need to use this style everywhere — only on pages where you want to specifically call out good and bad practices in your code examples.

To get this rendering, use "code fences" to demarcate the code block, followed by the language info string. For example:

function myFunc() {

To represent the code block as a good or bad example, add example-good or example-bad after the language string, like so:

```html example-good

```html example-bad

These will be rendered as:

<p class="brush: js example-good"></p>
<p class="brush: js example-bad"></p>