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build.js.map is the source map of build.js. Source maps are files that help to debug bundled code and are supported by all major browsers. Your build.js file is probably a concatenated file, maybe minified, of all your source files, so it won’t be easy if you try to debug it from the browser, for example, when you click a stack trace.
Your browser will load the source map so in your debugger you can see the original code, although it is actually executing the bundled file.
Generating source maps, especially with big code bases, slows down the compilation process. Probably you have it disabled in Webpack and that is why it is not generating a source map file.This is not a breaking error, and generally, source maps are only generated for development environments.
If you want to enable the generation of source maps in your Webpack configuration, you should change the devtool configuration option. You have several to choose from, with more accuracy but less performance. If your app is not very big and you don’t have many big dependencies, you should not notice a significant difference changing it to, e.g. eval-source-map.