I might be daft but I honestly don't even understand what "build" is in the context of a language that isn't compiled. Is it copying files to an output location together with dependencies and possibly including some transformations (minify etc)?
"Is it copying files to an output location together with dependencies and possibly including some transformations (minify etc)?".
That, and also other things, like compiling ES7+ JavaScript code to ES5 to run in most browsers, or compiling a typed JavaScript file to non-typed (e.g. TypeScript to JavaScript). You can also compile files to .css and .html from different languages like pug and scss. There's a large amount of various transformations that can be applied to web files.
Some of the other build steps we run: checking to ensure all dependencies are installed, concatenation, minification, obfuscation, code splitting, template compilation, copying files, deleting files, resolving require/bundling whatever that is called.
