Catch an error on requiring module in node.js

Question

Can't seem to find any articles on this anywhere. I basically want to catch the "Cannot find module" error from within the program, and optionally ask to install it, but I can't seem to catch any errors even with a try/catch around my require statements. Is this even possible? I haven't seen it done anywhere.

For example:

try {
  var express = require('express');
} catch (err){
   console.log("Express is not installed.");
   //proceed to ask if they would like to install, or quit.
   //command to run npm install
}

I suppose this could be done with a seperate .js startup file without any 3rd party requires, and simply uses fs to check for node_modules, and then optionally runs npm install from child process, then runs node app with another child. But it feels like it would be easier to do this from within a single app.js file

require itself suggests that this module is essential for the app to run. Cannot find module is a self-explanatory fault message if you ask me. As a developer it is your responsibility to include all dependencies to your package if you want others to use your app. — Amberlamps
– Amberlamps, Commented Jul 9, 2013 at 16:35
@Amberlamps It's usually better to not provide dependencies to an app as part of the package, but have them downloaded at the time of usage. So as not to provide stale/outdated modules, and not to waste space. That's the purpose of package.json and npm install. My hunt for this solution is for the sake of making my app as user friendly and automatic as possible. — krb686
– krb686, Commented Jul 9, 2013 at 16:46
I am not able to get this working with esm syntax (import) — C RICH
– C RICH, Commented Mar 27, 2024 at 2:06

Mariusz Nowak · Accepted Answer · 2013-07-10 09:00:11Z

24

To make it right, make sure to catch only Module not found error for given module:

try {
    var express = require('express');
} catch (e) {
    if (e.code !== 'MODULE_NOT_FOUND') {
        // Re-throw not "Module not found" errors 
        throw e;
    }
    if (e.message.indexOf('\'express\'') === -1) {
        // Re-throw not found errors for other modules
        throw e;
    }

}

answered Jul 10, 2013 at 9:00

Mariusz Nowak

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Comments

Peter Lyons · Accepted Answer · 2013-07-09 16:35:04Z

13

That works fine for me as you have it. Are you sure there's no node_modules/express folder somewhere above you in the filesystem that require is finding? Try doing this to be clear about what's happening:

try {
  var express = require('express');
  console.log("Express required with no problems", express);
} catch (err){
   console.log("Express is not installed.");
   //proceed to ask if they would like to install, or quit.
   //command to run npm install
}

answered Jul 9, 2013 at 16:35

Peter Lyons

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1 Comment

krb686 Over a year ago

I don't know how I messed that up, but I deleted the node_modules folder and it does seem to be working just fine now. Thanks. I'll accept your answer for prompting me to retry it and getting it fixed.

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