Magic source

magic_source.js

import { urlToFileSystemPath } from "@jsenv/filesystem"

import { require } from "@jsenv/core/src/internal/require.js"

export const createMagicSource = ({ url, content, sourcemap }) => {
  if (content === undefined) {
    throw new Error("content missing")
  }
  const filename = urlToFileSystemPath(url)
  const {
    OriginalSource,
    SourceMapSource,
    ConcatSource,
    ReplaceSource,
  } = require("webpack-sources")

  let firstSource
  if (sourcemap) {
    firstSource = new SourceMapSource(content, filename, sourcemap)
  } else {
    firstSource = new OriginalSource(content, filename)
  }

  const mutations = []
  return {
    prepend: (string) => {
      mutations.push((source) => {
        const concatSource = new ConcatSource(string, source)
        return concatSource
      })
    },
    append: (string) => {
      mutations.push((source) => {
        const concatSource = new ConcatSource(source, string)
        return concatSource
      })
    },
    replace: ({ start, end, replacement }) => {
      mutations.push((source) => {
        const replaceSource = new ReplaceSource(source)
        replaceSource.replace(start, end, replacement)
        return replaceSource
      })
    },
    insert: ({ position, insertion }) => {
      mutations.push((source) => {
        const replaceSource = new ReplaceSource(source)
        replaceSource.insert(position, insertion)
        return replaceSource
      })
    },
    toContentAndSourcemap: (options) => {
      const lastSource = mutations.reduce((previous, mutation) => {
        return mutation(previous)
      }, firstSource)
      const { source, map } = lastSource.sourceAndMap(options)
      return {
        content: source,
        sourcemap: map,
      }
    },
  }
}

magic_source.test.js

import { assert } from "@jsenv/assert"

import { createMagicSource } from "./magic_source.js"

{
  const magicSource = createMagicSource({
    url: "file:///file.js",
    content: "console.log(42)",
    map: null,
  })
  magicSource.prepend("foo")
  magicSource.append("bar")
  const { content } = magicSource.toContentAndSourcemap()
  const actual = content
  const expected = `fooconsole.log(42)bar`
  assert({ actual, expected })
}

{
  const magicSource = createMagicSource({
    url: "file:///file.js",
    content: "ZZaaZZ",
    map: null,
  })
  magicSource.replace({
    start: 2,
    end: 3,
    replacement: "b",
  })
  magicSource.replace({
    start: 2,
    end: 2,
    replacement: "ccc",
  })
  const { content } = magicSource.toContentAndSourcemap()
  const actual = content
  const expected = `ZZcccZZ`
  assert({ actual, expected })
}

Unfortunately webpack-sources fails to override (it erases the end of the line).
Moreover by using @ampproject/remapping we can compose an original sourcemap with a sourcemap from magic-string