Defines a function that allows you to overwrite/extend an existing accessor in JavaScript. Particularly useful for extending properties of Request/Response objects based on optional headers..

extendGetter.js

/*! extendGetter.js | Samuel Jones 2017 | MIT License | gist.github.com/samthecodingman */
/**
 * @file Extends built-in getter functions
 * @author Samuel Jones 2017 (github.com/samthecodingman)
 */

// Example usage: Overwrite `req.protocol` when behind a Google App Engine CDN/proxy 
// extendGetter(req, 'protocol', function() {
//   let xHttps = this.get('x-appengine-https'); // eslint-disable-line
//   if (!xHttps) return; // header not present - fallback to internal getter
//   return xHttps == 'on' ? 'https' : 'http';
// });

/**
 * Injects the given getter in front of the built-in one.
 * @param  {Object} obj - The object on which to (re)define the property.
 * @param  {String} name - The name of the property to be defined or modified.
 * @param  {Function} getter - The new getter for the property being defined
 *                             or modified.
 * @return {Object} - The object that was passed to the function.
 * @public
 */
function extendGetter (obj, name, getter) {
  if (typeof getter !== 'function') {
    throw new TypeError('getter must be a function')
  }
  if (Object.hasOwnProperty(obj, name)) {
    return extendGetter_(obj, name, getter)
  }
  let oldGetter = null
  let proto = Object.getPrototypeOf(obj)
  while (proto != null) {
    let propDesc = Object.getOwnPropertyDescriptor(proto, name)
    if (propDesc) {
      if (propDesc.get) {
        oldGetter = propDesc.get
      } else if (typeof propDesc.value !== 'undefined') {
        let v = propDesc.value
        oldGetter = () => v
      }
      break
    }
    proto = Object.getPrototypeOf(proto) // recurse
  }
  if (oldGetter) {
    return defineGetter_(obj, name, function () {
      let v = getter.call(this)
      return typeof v !== 'undefined' ? v : oldGetter.call(this)
    })
  } else {
    return defineGetter_(obj, name, getter)
  }
}
module.exports = extendGetter

/**
 * Defines a getter for the named property on the given object.
 * @param  {Object} obj - The object on which to define the property.
 * @param  {String} name - The name of the property to be defined or modified.
 * @param  {Function} getter - The new getter for the property being defined
 *                             or modified.
 * @return {Object} - The object that was passed to the function.
 * @private
 */
function defineGetter_ (obj, name, getter) {
  return Object.defineProperty(obj, name, {
    configurable: true,
    enumerable: true,
    get: getter
  })
}

/**
 * Injects the given getter in front of the built-in one.
 * @param  {Object} obj - The object on which to (re)define the property.
 * @param  {String} name - The name of the property to be defined or modified.
 * @param  {Function} getter - The new getter for the property being defined
 *                             or modified.
 * @return {Object} - The object that was passed to the function.
 * @private
 */
function extendGetter_ (obj, name, getter) {
  let originalGetter
  let propDesc = Object.getOwnPropertyDescriptor(obj, name)
  if (!propDesc) return defineGetter_(obj, name, getter)
  if (typeof propDesc.value !== 'undefined') {
    let val = propDesc.value
    originalGetter = () => val
  } else {
    originalGetter = propDesc.get
  }
  // If no original getter is present, assign getter, otherwise inject.
  propDesc.get = !originalGetter ? getter : function () {
    let v = getter.call(this)
    // return if getter returns something, otherwise call the original getter.
    return typeof v !== 'undefined' ? v : originalGetter.call(this)
  }
  delete propDesc.value // remove value descriptor if present
  return Object.defineProperty(obj, name, propDesc)
}