Hot reloading breaks the app. Final loader didn't return a Buffer or String

Question

I have a custom loader that load the following file data.js

  const data = {
    a: ()=> 8885555,
    b: ()=> 55555
  }

  module.exports = name => {
    return data[name] && data[name]()
  }

when I change the above file like changing b value the app breaks

ERROR in ../data.js?name=a
Module build failed: Error: Final loader (../data-loader.js) didn't return a Buffer or String
    at runLoaders (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\webpack\lib\NormalModule.js:319:18)
    at C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:373:3
    at iterateNormalLoaders (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:214:10)
    at iterateNormalLoaders (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:221:10)
    at C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:236:3
    at runSyncOrAsync (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:124:12)
    at iterateNormalLoaders (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:232:2)
    at Array.<anonymous> (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\loader-runner\lib\LoaderRunner.js:205:4)
    at Storage.finished (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\enhanced-resolve\lib\CachedInputFileSystem.js:43:16)
    at provider (C:\Users\010\Saber.js\webpack-hmr-3-ways\middleware\node_modules\enhanced-resolve\lib\CachedInputFileSystem.js:79:9)
 @ ./index.js 1:10-40
 @ multi webpack-hot-middleware/client?path=/__webpack_hmr&timeout=20000 ./index.js

this is the custom webpack loader

const devalue = require('devalue')
const vm = require('vm');

module.exports = async function(source, map) {
    const callback = this.async()
    this.addDependency(this.resourcePath);
    const sandbox = {
        require,
        module
    }
    vm.createContext(sandbox);
    const mod = vm.runInContext(source, sandbox)
    const result = await mod(this.resourceQuery.replace('?name=', ''))
    return callback(null, `export default ${devalue(result)}`, map);
}

Here is a reproduction repo

Answer 1

So this is what should be done

const devalue = require('devalue')
const vm = require('vm');

module.exports = async function(source, map) {
    const callback = this.async()
    this.addDependency(this.resourcePath);
    const sandbox = {
        require,
        module: {exports: {}}
    }
    vm.createContext(sandbox);
    const mod = vm.runInContext(source, sandbox)
    const result = await mod(this.resourceQuery.replace('?name=', ''))
    return callback(null, `export default ${devalue(result)}`, map);
}

The problem is that I was overwriting the loader module.exports with the data.js file module.exports because I was passing the same module to the sandbox so when hot reloading the exported function in the loader would have been replaced with the data.js function which doesn't return a string (in this case because data[name] would be undefined) and that's the cause of the error