next.js/packages/next/src/server/dev/get-source-map-from-file.ts

import fs from 'fs/promises'
import path from 'path'
import url from 'url'
import type { RawSourceMap } from 'next/dist/compiled/source-map08'
import dataUriToBuffer from 'next/dist/compiled/data-uri-to-buffer'

function getSourceMapUrl(fileContents: string): string | null {
  const regex = /\/\/[#@] ?sourceMappingURL=([^\s'"]+)\s*$/gm
  let match = null
  for (;;) {
    let next = regex.exec(fileContents)
    if (next == null) {
      break
    }
    match = next
  }
  if (!(match && match[1])) {
    return null
  }
  return match[1].toString()
}

export async function getSourceMapFromFile(
  filename: string
): Promise<RawSourceMap | undefined> {
  filename = filename.startsWith('file://')
    ? url.fileURLToPath(filename)
    : filename

  let fileContents: string

  try {
    fileContents = await fs.readFile(filename, 'utf-8')
  } catch (error) {
    throw new Error(`Failed to read file contents of ${filename}.`, {
      cause: error,
    })
  }

  const sourceUrl = getSourceMapUrl(fileContents)

  if (!sourceUrl) {
    return undefined
  }

  if (sourceUrl.startsWith('data:')) {
    let buffer: dataUriToBuffer.MimeBuffer

    try {
      buffer = dataUriToBuffer(sourceUrl)
    } catch (error) {
      throw new Error(`Failed to parse source map URL for ${filename}.`, {
        cause: error,
      })
    }

    if (buffer.type !== 'application/json') {
      throw new Error(
        `Unknown source map type for ${filename}: ${buffer.typeFull}.`
      )
    }

    try {
      return JSON.parse(buffer.toString())
    } catch (error) {
      throw new Error(`Failed to parse source map for ${filename}.`, {
        cause: error,
      })
    }
  }

  const sourceMapFilename = path.resolve(
    path.dirname(filename),
    decodeURIComponent(sourceUrl)
  )

  try {
    const sourceMapContents = await fs.readFile(sourceMapFilename, 'utf-8')

    return JSON.parse(sourceMapContents.toString())
  } catch (error) {
    throw new Error(`Failed to parse source map ${sourceMapFilename}.`, {
      cause: error,
    })
  }
}