index.js

'use strict'

// based on https://github.com/gotwarlost/istanbul/blob/master/lib/hook.js

/*
 Copyright (c) 2012, Yahoo! Inc.  All rights reserved.
 Copyrights licensed under the New BSD License.
 See the accompanying LICENSE file for terms.
*/

const fs = require('fs')
let Module = require('module')

// dummy definition in case module is not available:
if (!Module) Module = {}
if (!Module._extensions) { console.log('dummy module def'); Module._extensions = [] }

const originalLoaders = {}
const nestedTransforms = {} // allow nested transforms

function warn (message) {
  console.log('🔥 ', message)
}

const verify = {
  extension: function (str) {
    if (typeof str !== 'string') {
      throw new Error('expected string extension, have ' + str)
    }
    if (str[0] !== '.') {
      throw new Error('Extension should start with dot, for example .js, have ' + str)
    }
  },
  transform: function (fn) {
    if (typeof fn !== 'function') {
      throw new Error('Transform should be a function, have ' + fn)
    }
  }
}

function hook (extension, transform, options) {
  options = options || {}
  if (typeof extension === 'function' &&
    typeof transform === 'undefined') {
    transform = extension
    extension = '.js'
  }
  if (options.verbose) {
    console.log('hooking transform', transform.name, 'for', extension)
  }

  verify.extension(extension)
  verify.transform(transform)

  if (!nestedTransforms[extension]) {
    nestedTransforms[extension] = []
  }
  if (!nestedTransforms[extension].length) {
    // only store the first one -DJ
    originalLoaders[extension] = Module._extensions[extension]
  }
  nestedTransforms[extension].push(transform) // allow nested transforms -DJ

  Module._extensions[extension] = function (module, filename) {
    if (options.verbose) {
      console.log('transforming', filename)
    }
    var source = fs.readFileSync(filename, 'utf8')
    var ret = null // transform(source, filename);
    nestedTransforms[extension].every(function (nested) {
      // nesting order performs earlier first, later last
      ret = nested(source, filename)
      if (ret === undefined) {
        warn('source transform returned undefined for file ' + filename)
      }
      source = ret + '' // convert to string and keep going
      return true // continue
    })
    if (typeof ret === 'string') {
      module._compile(ret, filename)
    } else if (options.verbose) {
      console.error('transforming source from', filename, 'has not returned a string')
    }
  }
  if (options.verbose) {
    console.log('hooked function')
  }
}

function unhook (extension) {
  console.log('unhooking require hook for', extension)
  if (typeof extension === 'undefined') {
    extension = '.js'
  }
  verify.extension(extension)
  if (nestedTransforms[extension] && nestedTransforms[extension].length) {
    nestedTransforms[extension].pop()
    if (!nestedTransforms[extension].length) {
      // restore original only once
      Module._extensions[extension] = originalLoaders[extension]
    }
  } else {
    warn('Hmm, trying to unhook extension for ' + extension +
      ' it has not been registered')
  }
}

module.exports = {
  hook: hook,
  unhook: unhook
}