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Pranom Vignesh
Pranom Vignesh

Posted on • Updated on

Extend Language Configuration in Monaco Editor

Monaco editor is an online editor with syntax highlighting. It offers syntax highlighting support for many languages by default.
But we might need custom syntax highlighting to match our real life use-cases.
Unfortunately, there is no API available to extend the language configuration, Refer this comment

As per the advice, I have overwritten the output of the built-in tokenizer

Table of contents

How I Approached

  1. I took all the language configurations that is available in the monaco editor using the API monaco.languages.getLanguages()
  2. Then i filtered out my desired language (in my case, I took javascript)
  3. There will a method named loader(), which will be available for most of the registered languages
  4. On executing the loader, it will return an object containing 2 keys named ,the configuration and language
  5. This language will hold the tokenizer data
  6. I took this tokenizer and modified the certain parts with my custom tokens
  7. The modification is done in such a way, that the base object reference is unaffected

Actual Code

const allLangs = await monaco.languages.getLanguages();
const { conf, language: jsLang } = allLangs.find(({ id }) => id ==='javascript').loader();
for (let key in customTokenizer) {
  const value = customTokenizer[key];
  if (key === 'tokenizer') {
    for (let category in value) {
      const tokenDefs = value[category];
      if (!jsLang.tokenizer.hasOwnProperty(category)) {
        jsLang.tokenizer[category] = [];
      if (Array.isArray(tokenDefs)) {
        jsLang.tokenizer[category].unshift.apply(jsLang.tokenizer[category], tokenDefs)
  } else if (Array.isArray(value)) {
    if (!jsLang.hasOwnProperty(key)) {
      jsLang[key] = [];
    jsLang[key].unshift.apply(jsLang[key], value)
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  1. In monaco-editor, the javascript worker provides excellent code completions, If we create a new language tokenizer, we might lose this suggestions. This method avoids the need for a new language, thus preserves the code completions
  2. This custom tokenizer follows the monaco editor’s monarch pattern, So if you have already written custom tokenizers, it will be easy for migration
  3. The tokens are added in such a way that custom tokens are given high priority and this can also be modified by altering the unshift to push in jsLang keys


As monaco editor some how stores the language configuration inside monaco instance, We have to overwrite the language configuration before creation of any model (or) editor in that desired language

Take Away

This method is possible only because of monaco editor’s lazy loading feature (thanks to monaco editor team), where it loads the language configuration only when a model (or) editor instance is created for that language

So if we can change the configuration of the language before monaco uses it we can achieve the desired customization

Github Link

Profile Link : PranomVignesh

Repository Link : Extend Language Configuration in Monaco Editor

Top comments (2)

vikyd profile image
VikydZhang • Edited

languages json, plaintext do not have loader function, how to add custom tokenizer to json ?

pranomvignesh profile image
Pranom Vignesh

This example is done for the monaco-editor@0.20.0. In the latest version of monaco-editor@0.21.2 , They have added a new API 'registerDocumentSemanticTokensProvider'.
Please refer this link -