Failed to start the frontend application

I have been working on a custom extension for Theia that was deploying sucessfully, but, suddenly, i got this error on browser console:

And i haven’t been able to deploy my extension again. This is my package.json for the extension:

This is my package.json for the browser-app:

And these are the imports used in my custom extension:

Things that i have tried

  1. Delete the node modules folder of both the extension and browser-app, and reinstalling using yarn.

  2. Install missing dependencies in the extension folder AND in the browser-app folde using yarn add .

  3. Erase all files and reimplement the extension from scratch, only editing the extension package.json and the code of the extension itself.

  4. Rebuild constantly using lerna with yarn prepare and yarn rebuild:browser.

  5. Watching code using yarn watch on both the extension folder and the browser-app folder (sometimes yarn cannot unbind libraries when trying to compile using yarn prepare on root folder and i am forced to restart my machine to build succesfully).

  6. Search on StackOverflow for similar errors without luck.

Things that i want to achieve

  1. Run a process just after Theia’s start.

  2. Recieve confirmation that the process is running as it should (from stdout or something like that).

  3. Deploy a mini-browser previewing a localhost address where the process is exposed.

  4. Make a http request from the extension.

Things that the extension was doing before the issue

  1. Done successfully

  2. On process.

  3. Done successfully.

  4. Not able to.

The last two sections were put because they could help to understand the problem.

Thank you for your help!.

[original thread by loladmebrah]

try to remove express from dependencies

@theia/core alredy brings the proper version, if you declare your own i’m not sure what can happen

if it does not help, could you make a reproducible repo?


@anton-kosyakov Thank you for your fast response, i did as you instructed. I regenerated the proyect using the yeoman generator for theia’s extensions, and used the following dependency list:

“dependencies”: {
@theia/callhierarchy”: “latest”,
@theia/console”: “latest”,
@theia/core”: “latest”,
@theia/debug”: “latest”,
@theia/debug-nodejs”: “latest”,
@theia/editor”: “latest”,
@theia/file-search”: “latest”,
@theia/git”: “latest”,
@theia/json”: “latest”,
@theia/markers”: “latest”,
@theia/merge-conflicts”: “latest”,
@theia/messages”: “latest”,
@theia/mini-browser”: “latest”,
@theia/navigator”: “latest”,
@theia/outline-view”: “latest”,
@theia/preferences”: “latest”,
@theia/preview”: “latest”,
@theia/process”: “latest”,
@theia/search-in-workspace”: “latest”,
@theia/terminal”: “latest”,
@theia/textmate-grammars”: “latest”,
@theia/typescript”: “latest”
“devDependencies”: {
“rimraf”: “latest”,
“typescript”: “latest”


After that i compiled the extension without problem, and then, after i updated the aforementioned extension with my code and proceeded to compile it, i got this problem:



So, in my ingenuity i decided to install ‘tls’ from yarn, and after the instalation and succesfull compilation i got this message again:



What i can get of this is:

  1. Yarn succesfully installs all dependencies from Theia.

  2. Something in my code breaks the compilation in yarn, and makes it ask for tls.

  3. Manually installing tls breaks the way webpack solves it’s dependency management internally.


This is the code i have been trying to compile, maybe you can see what i am not able to.






@anton-kosyakov, I’m so sorry! i was in a rush so i didn’t read completely, here is the public repo:


Found the issue, when i comment the following line:

import { TerminalProcessFactory, TerminalProcess, ProcessManager } from ‘@theia/process/lib/node’

and remove the respective injections, the proyect can compile and deploy normally, i don’t really know if i did a mistake calling the library.

But there it is. What can i do to correctly use the process library?

From what I can tell you cannot import backend code node from the frontend browser.

Here I can see a mix of browser and node imports which is not possible

Such general guidelines can be viewed in our code organization.