Source maps allow JavaScript runtimes to match running JavaScript back to the original source code that corresponds to it. This can be minified JavaScript, but in our case, we’re concerned with mapping pretty-printed JavaScript back to CoffeeScript.

In order to produce maps, we must keep track of positions (line number, column number) that originated every node in the syntax tree, and be able to generate a map file — which is a compact, VLQ-encoded representation of the JSON serialization of this information — to write out alongside the generated JavaScript.

A LineMap object keeps track of information about original line and column positions for a single line of output JavaScript code. SourceMaps are implemented in terms of LineMaps.

class LineMap
  constructor: (@line) ->
    @columns = []

  add: (column, [sourceLine, sourceColumn], options={}) ->
    return if @columns[column] and options.noReplace
    @columns[column] = {line: @line, column, sourceLine, sourceColumn}

  sourceLocation: (column) ->
    column-- until (mapping = @columns[column]) or (column <= 0)
    mapping and [mapping.sourceLine, mapping.sourceColumn]

Maps locations in a single generated JavaScript file back to locations in the original CoffeeScript source file.

This is intentionally agnostic towards how a source map might be represented on disk. Once the compiler is ready to produce a “v3”-style source map, we can walk through the arrays of line and column buffer to produce it.

class SourceMap
  constructor: ->
    @lines = []

Adds a mapping to this SourceMap. sourceLocation and generatedLocation are both [line, column] arrays. If options.noReplace is true, then if there is already a mapping for the specified line and column, this will have no effect.

  add: (sourceLocation, generatedLocation, options = {}) ->
    [line, column] = generatedLocation
    lineMap = (@lines[line] or= new LineMap(line))
    lineMap.add column, sourceLocation, options

Look up the original position of a given line and column in the generated code.

  sourceLocation: ([line, column]) ->
    line-- until (lineMap = @lines[line]) or (line <= 0)
    lineMap and lineMap.sourceLocation column

A static source maps cache filename: map. These are used for transforming stack traces and are currently set in CoffeeScript.compile for all files compiled with the source maps option.

  @sourceMaps: Object.create null

  @registerCompiled: (filename, source, sourcemap) =>
    if sourcemap?
      @sourceMaps[filename] = sourcemap

  @getSourceMap: (filename) =>
    @sourceMaps[filename]

Builds up a V3 source map, returning the generated JSON as a string. options.sourceRoot may be used to specify the sourceRoot written to the source map. Also, options.sourceFiles and options.generatedFile may be passed to set “sources” and “file”, respectively.

  generate: (options = {}, code = null) ->
    writingline       = 0
    lastColumn        = 0
    lastSourceLine    = 0
    lastSourceColumn  = 0
    needComma         = no
    buffer            = ""

    for lineMap, lineNumber in @lines when lineMap
      for mapping in lineMap.columns when mapping
        while writingline < mapping.line
          lastColumn = 0
          needComma = no
          buffer += ";"
          writingline++

Write a comma if we’ve already written a segment on this line.

        if needComma
          buffer += ","
          needComma = no

Write the next segment. Segments can be 1, 4, or 5 values. If just one, then it is a generated column which doesn’t match anything in the source code.

The starting column in the generated source, relative to any previous recorded column for the current line:

        buffer += @encodeVlq mapping.column - lastColumn
        lastColumn = mapping.column

The index into the list of sources:

The starting line in the original source, relative to the previous source line.

        buffer += @encodeVlq mapping.sourceLine - lastSourceLine
        lastSourceLine = mapping.sourceLine

The starting column in the original source, relative to the previous column.

        buffer += @encodeVlq mapping.sourceColumn - lastSourceColumn
        lastSourceColumn = mapping.sourceColumn
        needComma = yes

Produce the canonical JSON object format for a “v3” source map.

    sources = if options.sourceFiles
      options.sourceFiles
    else if options.filename
      [options.filename]
    else
      ['<anonymous>']

    v3 =
      version:    3
      file:       options.generatedFile or ''
      sourceRoot: options.sourceRoot or ''
      sources:    sources
      names:      []
      mappings:   buffer

    v3.sourcesContent = [code] if options.sourceMap or options.inlineMap

    v3

Note that SourceMap VLQ encoding is “backwards”. MIDI-style VLQ encoding puts the most-significant-bit (MSB) from the original value into the MSB of the VLQ encoded value (see Wikipedia). SourceMap VLQ does things the other way around, with the least significat four bits of the original value encoded into the first byte of the VLQ encoded value.

  VLQ_SHIFT            = 5
  VLQ_CONTINUATION_BIT = 1 << VLQ_SHIFT             
  VLQ_VALUE_MASK       = VLQ_CONTINUATION_BIT - 1   

  encodeVlq: (value) ->
    answer = ''

Least significant bit represents the sign.

    signBit = if value < 0 then 1 else 0

The next bits are the actual value.

    valueToEncode = (Math.abs(value) << 1) + signBit

Make sure we encode at least one character, even if valueToEncode is 0.

    while valueToEncode or not answer
      nextChunk = valueToEncode & VLQ_VALUE_MASK
      valueToEncode = valueToEncode >> VLQ_SHIFT
      nextChunk |= VLQ_CONTINUATION_BIT if valueToEncode
      answer += @encodeBase64 nextChunk

    answer

  BASE64_CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

  encodeBase64: (value) ->
    BASE64_CHARS[value] or throw new Error "Cannot Base64 encode value: #{value}"

Our API for source maps is just the SourceMap class.

module.exports = SourceMap