Library API

Highlight.js exports a few functions as methods of the hljs object.

highlight(languageName, code, ignore_illegals, continuation)

Core highlighting function. Accepts a language name, or an alias, and a string with the code to highlight. The ignore_illegals parameter, when present and evaluates to a true value, forces highlighting to finish even in case of detecting illegal syntax for the language instead of throwing an exception. The continuation is an optional mode stack representing unfinished parsing. When present, the function will restart parsing from this state instead of initializing a new one. This is used internally for sublanguage support.

Note: continuation is NOT intended to support line-by-line highlighting because there is no requirement that a grammar handle linebreaks in any special way. It’s quite possible for a grammar to have a single mode/regex that matches MANY lines at once. This is not discouraged and entirely up to the grammar.

Returns an object with the following properties:

  • language: language name, same as the name passed in languageName, returned for consistency with highlightAuto
  • relevance: integer value representing the relevance score
  • value: HTML string with highlighting markup
  • top: top of the current mode stack
  • illegal: boolean representing whether any illegal matches were found
  • code: the original raw code

highlightAuto(code, languageSubset)

Highlighting with language detection. Accepts a string with the code to highlight and an optional array of language names and aliases restricting detection to only those languages. The subset can also be set with configure, but the local parameter overrides the option if set.

Returns an object with the following properties:

  • language: detected language
  • relevance: integer value representing the relevance score
  • value: HTML string with highlighting markup
  • second_best: object with the same structure for second-best heuristically detected language (may be absent)


Post-processing of the highlighted markup. Currently consists of replacing indentation TAB characters and using <br> tags instead of new-line characters. Options are set globally with configure.

Accepts a string with the highlighted markup.


Applies highlighting to a DOM node containing code.

This function is the one to use to apply highlighting dynamically after page load or within initialization code of third-party Javascript frameworks.

The function uses language detection by default but you can specify the language in the class attribute of the DOM node. See the class reference for all available language names and aliases.


Configures global options:

  • tabReplace: a string used to replace TAB characters in indentation.
  • useBR: a flag to generate <br> tags instead of new-line characters in the output, useful when code is marked up using a non-<pre> container.
  • classPrefix: a string prefix added before class names in the generated markup, used for backwards compatibility with stylesheets.
  • languages: an array of language names and aliases restricting auto detection to only these languages.
  • languageDetectRe: a regex to configure how CSS class names map to language (allows class names like say color-as-php vs the default of language-php, etc.)
  • noHighlightRe: a regex to configure which CSS classes are to be skipped completely.

Accepts an object representing options with the values to updated. Other options don’t change

  tabReplace: '    ', // 4 spaces
  classPrefix: ''     // don't append class prefix
                      // … other options aren't changed


Applies highlighting to all <pre><code>...</code></pre> blocks on a page.


Attaches highlighting to the page load event.

registerLanguage(languageName, languageDefinition)

Adds new language to the library under the specified name. Used mostly internally.

  • languageName: a string with the name of the language being registered
  • languageDefinition: a function that returns an object which represents the language definition. The function is passed the hljs object to be able to use common regular expressions defined within it.

registerAliases(alias|aliases, {languageName})

Adds new language alias or aliases to the library for the specified language name defined under languageName key.

  • alias|aliases: a string or array with the name of alias being registered
  • languageName: the language name as specified by registerLanguage.


Returns the languages names list.


Looks up a language by name or alias.

Returns the language object if found, undefined otherwise.


Looks up a language by name or alias.

This should be used when one language definition depends on another. Using this function (vs getLanguage) will provide better error messaging when a required language is missing.

Returns the language object if found, raises a hard error otherwise.


Enables debug/development mode. This mode purposely makes Highlight.js more fragile! It should only be used for testing and local development (of languages or the library itself). By default “Safe Mode” is used, providing the most reliable experience for production usage.

For example, if a new version suddenly had a serious bug (or breaking change) that affected only a single language:

  • In Safe Mode: All other languages would continue to highlight just fine. The broken language would appear as a code block, but without any highlighting (as if it were plaintext).
  • In Debug Mode: All highlighting would stop when an error was encountered and a JavaScript error would be thrown.