Object.defineProperties modeled in JS

Object.defineProperties.js

Object.defineProperties = function(O, Properties) {
  // 1. If Type(O) is not Object throw a TypeError exception.
  if (typeof(O) !== "object" || O == null) {
    throw TypeError("Object.defineProperties called on non-object");
  }
  
  // 2. Let props be ToObject(Properties)
  var props = Object(Properties); // not *exactly* the same, but similar enough
  
  // 3. Let names be an internal list containing the names of each enumerable own property of props.
  var names = Object.getOwnPropertyNames(props);
  
  // 4. Let descriptors be an empty internal List.
  var descriptors = [];
  
  // 5. For each element P of names in list order,
  names.forEach(function(P) {
    // 5.a Let descObj be the result of calling the [[Get]] internal method of props with P as the argument.
    var descObj = props[P];
    
    // 5.b Let desc be the result of calling ToPropertyDescriptor with descObj as the argument.
    // the following is literally what ToPropertyDescriptor does. It's *really* simple.
    var desc = {};
    if (descObj.hasOwnProperty("enumerable")) desc.enumerable = Boolean(descObj.enumerable);
    if (descObj.hasOwnProperty("configurable")) desc.configurable = Boolean(descObj.configurable);
    if (descObj.hasOwnProperty("value")) desc.value = descObj.value;
    if (descObj.hasOwnProperty("writable")) desc.writable = Boolean(descObj.writable);
    if (descObj.hasOwnProperty("get")) {
      if (typeof(descObj.get) !== "function") {
        throw TypeError("Getter must be a function");
      }
      desc.get = descObj.get;
    }
    if (descObj.hasOwnProperty("set")) {
      if (typeof(descObj.set) !== "function") {
        throw TypeError("Setter must be a function");
      }
      desc.set = descObj.set;
    }
    if ((desc.get || desc.set) && (desc.hasOwnProperty("value") || desc.hasOwnProperty("writable"))) {
      throw TypeError("value or writable cannot be specified if a getter or setter is specified")
    }
    
    // 5.c Append the pair (a two element List) consisting of P and desc to the end of descriptors.
    descriptors.push([P, desc]);
  });
  
  // 6. For each pair from descriptors in list order,
  descriptors.forEach(function(pair) {
    // 6.a Let P be the first element of pair.
    var P = pair[0];
    
    // 6.b Let desc be the second element of pair.
    var desc = pair[1];
    
    // 6.c Call the [[DefineOwnProperty]] internal method of O with arguments P, desc, and true.
    // DefineOwnProperty is an internal method not accessible from JS, but we'll pretend it exists here
    // this is a lot like: Object.defineProperty(O, P, desc);
    O.DefineOwnProperty(P, desc, true);
  });
  
  // 7. Return O.
  return O;
};