27.implement-completeAssign.md

27. implement completeAssign()

Problem

https://bigfrontend.dev/problem/implement-completeAssign

Problem Description

This is a follow-up on 26. implement Object.assign().

Object.assign() assigns the enumerable properties, so getters are not copied, non-enumerable properties are ignored.

Suppose we have following source object.

const source = Object.create(
  {
    a: 3, // prototype
  },
  {
    b: {
      value: 4,
      enumerable: true, // enumerable data descriptor
    },
    c: {
      value: 5, // non-enumerable data descriptor
    },
    d: {
      // non-enumerable accessor descriptor
      get: function () {
        return this.\_d;
      },
      set: function (value) {
        this.\_d = value;
      },
    },
    e: {
      // enumerable accessor descriptor
      get: function () {
        return this.\_e;
      },
      set: function (value) {
        this.\_e = value;
      },
      enumerable: true,
    },
  }
);

If we call Object.assign() with source of above, we get:

Object.assign({}, source);

// {b: 4, e: undefined}
// e is undefined because `this._e` is undefined

Rather than above result, could you implement a completeAssign() which have the same parameters as Object.assign() but fully copies the data descriptors and accessor descriptors?

In case you are not familiar with the descriptors, this page from MDN might help.

This problem is solely checking your understanding of how property descriptors work.

Solution

function completeAssign(target, ...sources) {
  if (!target) {
    throw new Error();
  }

  if (typeof target !== 'object') {
    const constructor = Object.getPrototypeOf(target).constructor;
    target = new constructor(target);
  }

  for (const source of sources) {
    if (!source) {
      continue;
    }

    const descriptors = Object.getOwnPropertyDescriptors(source);
    const keys = [
      ...Object.getOwnPropertyNames(source),
      ...Object.getOwnPropertySymbols(source),
    ];
    for (const key of keys) {
      Object.defineProperty(target, key, descriptors[key]);
    }
  }

  return target;
}