A source map allows a JavaScript debugger to map line and symbol information from a transformed (eg. minified, optimized) JavaScript file back to the original input file used for the transformation. The original file can be any kind of text file which in effect enables in-browser debugging of any language compiling to JavaScript.

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At its core, a source map allows a browser to map the source of an asset to the final product. If the JS file had a source map, it would allow the browser to translate the location of the error to the original unmodified file on disk. Pretty cool.