function used across several lambda functions in aws - node.js

Question

I am new to nodeJS, npm, and aws lambda. Suppose I have a custom sort algorithm that I want to use in several lambda functions. This should be something very simple, but the only way I have found is to create a layer with a node module published as an npm package. I do not want any of the code be uploaded to an npm.
I tried to download the layer I am currently using, create a folder in node_modules along with other packages that are published in npm with
npm init
fill all the info for the package.json
created a function code in a index.js
'use strict'
exports.myfunc= myfunc;
function myfunc() {
console.log("This is a message from the demo package");
}
zip all the layer again and upload it in a version 2 of the layer
pick the new version in a lambda function and call it as i would do with any other node_module form a third party, like this:
const mypack= require('mypack');
mypack.myfunc();
but it tells me:
"errorMessage": "Error: Cannot find module ... \nRequire stack:\n- /var/task/index.js\n- /var/runtime/UserFunction.js\n- /var/runtime/index.js",
I think it maybe because in the layer in the nodejs folder, there is package-lock.json and my module is not there. I tried to put it there without the "resolved" and "integrity" that the packages published on npm have but did not work.
Is there a way to simply upload the code I want in a nodejs layer having nothing to do with npm?
I would like to try something such as this in the accepted answer,
var moduleName = require("path/to/example.js")
How to install a node.js module without using npm?
but I don't know where is the path of the modules of a layer, the path of lambda that shows __dirname is /var/task, but it looks like any lambda has same path. I am lost...

Answer 1

I was able to import js in a folder in the following manner:
download the zip of the layer uncompress it, put in node_modules any additional folder with your custom js and upload it as a new version of the layer.
In the lambda function, reference it with
moduleName = require("path/to/example.js")
You can find that path, which is the trick here, using a known library you already have in your layer and is working, in my case I used base64-js, then I returned the path of that library like this:
require.resolve('base64-js')
That returned
'/opt/nodejs/node_modules/base64-js/index.js'
So I used
moduleName = require("/opt/nodejs/node_modules/MYCUSTOMFOLDER/index.js")
and that was it...