A website's JavaScript is often minified: shortened names, comments removed. For debugging, build tools produce a source map (.map file) that often contains the full original code, comments included. Give the address of a Lab script (the tool looks for its sourceMappingURL line) or of a .map file, or choose a file.
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In real life: the Sources tab of the browser's developer tools, sourcemapper, unwebpack-sourcemap. Source maps left in production reveal code, comments, internal API addresses or features not launched yet.