How to use sourcemaps to restore the original file?

Question

I have a file that has been transpiled from ES6 to ES5 using Babel. I have sourcemap. I am assuming I can restore the original file (the way it looked when written in ES6) using these resources.

How is it done?

Is there a CLI tool to do this?

Answer 1

Open up the source map in a text editor, and you’ll see that it’s mostly just a simple JSON object. The “sources” field contains an array of URLs/paths to all source files, which can help you to find them. There is also an optional “sourcesContent” field, which also is an array, where each item contains the contents of the file at the same index in the “sources” array; if so, you could find the original code right in the source map.

A CLI tool? Well, there’s source-map-visualize which tries to find all original sources and pre-loads them into this online source map visualization.

Answer 2

Very simple NodeJS implementation which I wrote for my needs.

const fs = require('fs');
const { SourceMapConsumer } = require("source-map");

fs.readFile('./example.js', 'utf8' , (err, data) => {
  if (err) return console.error(err);

  const sourceMapData = data.split('//# sourceMappingURL=data:application/json;base64,')[1];
  let buff = new Buffer.from(sourceMapData, 'base64');
  let rawSourceMap = buff.toString('ascii');

  const parsed = SourceMapConsumer(rawSourceMap);

  fs.writeFile('example.ts', parsed.sourcesContent, function (err) {
    if (err) return console.log(err);
  });
});