Bundling expressjs with esbuild gives missing view.js warning

Question

I'm trying to bundle an "express.js" script with esbuild using

esbuild index.js --bundle --platform=node --outfile=server.js

to run as a netlify/aws lambda function and seem always to get this warning:

> node_modules/express/lib/view.js: warning: This call to "require" will not be bundled because the argument is not a string literal
    81 │     var fn = require(mod).__express
       ╵              ~~~~~~~

The function seems to run but I would like to find out what could be wrong and I couldn't find any hints online anywhere?

Answer 1

The bundling process tries to produce a single file containing all of your code, but the warning is telling you that express contains some code that cannot be bundled into a single file. I'm not familiar with express but apparently this is also a problem with express and Webpack (see this question for an example).

Assuming you still keep the node_modules/express folder around next to your bundle when you run the code, you could solve this problem with esbuild the same way the Webpack solution works: by marking the express package as external. That would look like this:

esbuild index.js --bundle --platform=node --outfile=server.js --external:express