set highlight language with highlightLanguage option

README.md

@@ -385,6 +385,8 @@ The HTML output accepts a Javascript object with configuration. Possible options
   For example: `{ "tag-file-changed": "<span class="d2h-tag d2h-changed d2h-changed-tag">MODIFIED</span>" }`
   > For more information regarding the possible templates look into
   > [src/templates](https://github.com/rtfpessoa/diff2html/tree/master/src/templates)
+- `highlightLanguage`: language used for highlighting, default is `''` to select the language based on the diffed
+  filename extension.
 
 ### Diff2Html Browser
 

src/ui/js/diff2html-ui-base.ts

@@ -9,6 +9,7 @@ export interface Diff2HtmlUIConfig extends Diff2HtmlConfig {
   highlight?: boolean;
   fileListToggle?: boolean;
   fileListStartVisible?: boolean;
+  highlightLanguage?: string;
   /**
    * @deprecated since version 3.1.0
    * Smart selection is now enabled by default with vanilla CSS
@@ -23,6 +24,7 @@ export const defaultDiff2HtmlUIConfig = {
   highlight: true,
   fileListToggle: true,
   fileListStartVisible: false,
+  highlightLanguage: '',
   /**
    * @deprecated since version 3.1.0
    * Smart selection is now enabled by default with vanilla CSS
@@ -141,8 +143,15 @@ export class Diff2HtmlUI {
     files.forEach(file => {
       // HACK: help Typescript know that `this.hljs` is defined since we already checked it
       if (this.hljs === null) return;
-      const language = file.getAttribute('data-lang');
-      const hljsLanguage = language ? getLanguage(language) : 'plaintext';
+      const hljsLanguage = (() => {
+        const highlightLanguage = this.config.highlightLanguage;
+        if (highlightLanguage === '') {
+          const language = file.getAttribute('data-lang');
+          return language ? getLanguage(language) : 'plaintext';
+        } else {
+          return highlightLanguage;
+        }
+      })();
 
       // Collect all the code lines and execute the highlight on them
       const codeLines = file.querySelectorAll('.d2h-code-line-ctn');