halfnelson / remapping

Remap sequential sourcemaps through transformations to point at the original source code

@ampproject/remapping

Remap sequential sourcemaps through transformations to point at the original source code

Remapping allows you to take the sourcemaps generated through transforming your code and "remap" them to the original source locations. Think "my minified code, transformed with babel and bundled with webpack", all pointing to the correct location in your original source code.

With remapping, none of your source code transformations need to be aware of the input's sourcemap, they only need to generate an output sourcemap. This greatly simplifies building custom transformations (think a find-and-replace).

Installation

npm install @ampproject/remapping

Usage

function remapping(
  map: SourceMap | SourceMap[],
  loader: (file: string) => (SourceMap | null | undefined),
  excludeContent?: boolean
): SourceMap;

remapping takes the final output sourcemap, and a loader function. For every source file pointer in the sourcemap, the loader will be called with the resolved path. If the path itself represents a transformed file (it has a sourcmap associated with it), then the loader should return that sourcemap. If not, the path will be treated as an original, untransformed source code.

// Babel transformed "helloworld.js" into "transformed.js"
const transformedMap = JSON.stringify({
  file: 'transformed.js',
  // 1st column of 2nd line of output file translates into the 1st source
  // file, line 3, column 2
  mappings: ';CAEE',
  sources: ['helloworld.js'],
  version: 3,
});

// Uglify minified "transformed.js" into "transformed.min.js"
const minifiedTransformedMap = JSON.stringify({
  file: 'transformed.min.js',
  // 0th column of 1st line of output file translates into the 1st source
  // file, line 2, column 1.
  mappings: 'AACC',
  names: [],
  sources: ['transformed.js'],
  version: 3,
});

const remapped = remapping(
  minifiedTransformedMap,
  (file) => {

    // The "transformed.js" file is an transformed file.
    if (file === 'transformed.js') {
      return transformedMap;
    }

    // Loader will be called to load transformedMap's source file pointers as well.
    console.assert(file === 'helloworld.js');
    return null;
  }
);

console.log(remapped);
// {
//   file: 'transpiled.min.js',
//   mappings: 'AAEE',
//   sources: ['helloworld.js'],
//   version: 3,
// };

In this example, loader will be called twice:

1. "transformed.js", the first source file pointer in the minifiedTransformedMap. We return the associated sourcemap for it (its a transformed file, after all) so that sourcemap locations can be traced through it into the source files it represents.
2. "helloworld.js", our original, unmodified source code. This file does not have a sourcemap, so we return null.

The remapped sourcemap now points from transformed.min.js into locations in helloworld.js. If you were to read the mappings, it says "0th column of the first line output line points to the 1st column of the 2nd line of the file helloworld.js".

Multiple transformations of a file

As a convenience, if you have multiple single-source transformations of a file, you may pass an array of sourcemap files in the order of most-recent transformation sourcemap first. So our above example could have been writen as:

const remapped = remapping(
  [minifiedTransformedMap, transformedMap],
  () => null
);

console.log(remapped);
// {
//   file: 'transpiled.min.js',
//   mappings: 'AAEE',
//   sources: ['helloworld.js'],
//   version: 3,
// };