# @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

```sh
npm install @ampproject/remapping
```

## Usage

```typescript
function remapping(
  map: SourceMap | SourceMap[],
  loader: (file: string) => (SourceMap | null | undefined),
  options?: { excludeContent: boolean, decodedMappings: 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.

```js
// 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:

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

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

### Options

#### excludeContent

By default, `excludeContent` is `false`. Passing `{ excludeContent: true }`
will exclude the `sourcesContent` field from the returned sourcemap. This is
mainly useful when you want to reduce the size out the sourcemap.

#### decodedMappings

By default, `decodedMappings` is `false`. Passing `{ decodedMappings: true }`
will leave the `mappings` field in a [decoded
state](https://github.com/rich-harris/sourcemap-codec) instead of encoding
into a VLQ string.