Drop in replacement for source-map-loader but this loader understands file:// and http[s]:// prefixes in source map files like the kind you find in scala.js generated source maps. It is often recommended that scala.js resources use the -P:scalajs:mapSourceURI option to set their source maps to github resources since there is no stable mechanism to include them in jars.

Including scala sources increases the map size so use bundleHttp: false to not bundle them. They are bundled by default. Warnings that "file://" resources cannot be found can be turned off using skipFileURLWarnings which is true by default. If you depend on other scala.js libraries, they may have file:// links embedded that cause warnings. If you work offline and do not have access to http resources, then set your bundleHttp option using a nodejs environment variable: bundleHttp: process.env.OFFLINE and set the environment variable OFFLINE="true" before you run webpack. Note that "true" is in quotes to make at a string environment variable.

Fetched resources are cached. The default cache directory is <cwd>/.scala-js-sources. Webpack already caches source maps so this cache mostly helps with cold starts.

As a drop-in replacement:

...
            {
                test: /\.js$/,
                // use loader defaults
                use: ["scalajs-friendly-source-map-loader"],
                // set options explicitly
                use: [
                { 
                    loader: "scalajs-friendly-source-map-loader",
                    options: {
                        skipFileURLWarnings: true, // or false, default is true
                        bundleHttp: true // or false, default is true,
			cachePath: ".scala-js-sources", // cache dir name, exclude in .gitignore
			noisyCache: false, // whether http cache changes are output
                        useCache: true, // false => remove any http cache processing
                    }
                }],
                enforce: "pre",
            },

To use in your loaders only for your scalajs output:

...
            {
                test: /\.js$/,
                use: [
                  { 
                      loader: "scalajs-friendly-source-map-loader"
                      options: {
                        bundleHttp: false // or use the short version above
                       },
                  }],
                enforce: "pre",
                include: [scalapath],
            },
            {
                test: /\.js$/,
                use: ["source-map-loader"],
                enforce: "pre",
                // does not handle scala.js issued https: remote resources
                exclude: [/node_modules/, scalapath],
            },
...

Loaders are used right to left so the normal source-map-loader will run first but ignore the scala.js output leaving the scala js file to the friendly loader. scalapath is a resolved path to your scala.js compiler output file.