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Restrict Editable Area in Monaco Editor

pranomvignesh profile image Pranom Vignesh Updated on ・3 min read

This application shows how we can restrict editing of certain places in monaco editor.
To see the repository click here

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Table of Contents

Actual Code

    paths: {
        vs: "./monaco-editor/min/vs"
require(["vs/editor/editor.main"], initEditor)
const startPhrase = `// Start of editable area`
const endPhrase = `// End of editable area`
const editableArea = ((startPhrase,endPhrase) => {
    return {
        includes : function(changes,model){
            const fullRange = model.getFullModelRange();
            let { range : startRange } = model.findMatches(startPhrase,fullRange).shift() || {};
            let { range : endRange } = model.findMatches(endPhrase,fullRange).pop() || {};
            const {
            } = fullRange;
            const isEmpty = text => text === ''; // ? refers to backspace and delete
            const isEnter = text => /\n/.test(text);
            if(startRange && endRange){
                startRange = startRange.setStartPosition(startLineNumber,startColumn);
                endRange = endRange.setEndPosition(endLineNumber,endColumn);
                return changes
                        .every(({ text,range }) => {
                            const specialCases = () => {
                                 ? This is done for my use case
                                 ? This allows enter at the end of the start Range and 
                                 ? This allows the enter and backspace on the start of the end Range
                                 ? This is an optional case
                                return ( 
                                    ( isEnter(text) || range.startLineNumber > startRange.endLineNumber) &&
                                    ( isEnter(text) || isEmpty(text) || range.endLineNumber < endRange.startLineNumber)
                            return  !startRange.strictContainsRange(range) && 
                                    !endRange.strictContainsRange(range) && 

            return false;
function initEditor(){
    const sampleJs = `
 * This place cannot be edited
// Start of editable area
function editable(){
    console.log('This part can be edited and all stuff can be done here')
// End of editable area
 * This place cannot be edited
    const jsModel = monaco.editor.createModel(sampleJs,"javascript");
    const editorDiv = document.querySelector('.editorDiv');
    const jsContainer = monaco.editor.create(editorDiv);
    jsModel.onDidChangeContentFast(({ changes,isUndoing }) => {
                 * This Promise.resolve() sends the code to the micro task queue 
                 * And it gets called before the event queue ( micro task queue has more priority than event queue)
                 * Thus, Promise.resolve() will be better than a setTimeout(fn,0) here
                 * If we do it synchronously, it affects some of monaco editor's functions
                Promise.resolve().then(() => jsContainer.trigger('someIdString','undo'))
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Why this snippet is needed ?

Monaco editor is one of the best online javascript editors, But recently setEditableRange functionality was removed from it. Refer this.

This snippets tries to establish the restriction in editable area

How this is achieved ?

  • This snippet needs the start and end phrases, to which the editable restriction has to be implemented
  • The onDidChangeContentFast hooks is watched for the changes and if that change is not happening in the allowed area the undo will be triggered, which nullifies the content which is typed/pasted
  • Promise.resolve() - is used to move the undoing function to the microtask queue , by this monaco editor is allowed to do its stuff and once it gets completed, this undoing function gets triggered
  • The area above the start phrase and end phrase will be restricted

Note : Microtask queue is having more priority than Event queue, thus Promise.resolve becomes a better option than setTimeout(fn,0) in this scenario

So, What it does ?

By this, we can create an illusionlike, nothing is allowed to type in the restricted area, but what actually happens is all the typed values are getting undo once it is getting typed in the restricted area


  • Advantage of using this snippet is there will not be any UI lag while undoing, Previously when using setTimeout, the undoing operation will be visible to the user
  • Autocompletion suggestions from the restricted area will be available


  • This requires starting and ending phrase
  • I haven't tested this code with large amount of pasting of texts, So it may break at that stage. This will work fine for typing

Use Cases

  • If you are trying to design an online coding interview platform using monaco editor and you wish to allow the candidates to edit only certain places, This can be used

See Part 2

Might Solve this

This might become a solution to this github issue

Discussion (0)

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