Shallow object difference

diff.js

// Create a difference of entries in an object when that object
// does not equal the new value shallowly.
//
// Example:
//   a0 = {a: {b: {c: 3}, d: 4}}; a1 = {a: {b: a0.a.b, d: 5}};
//   diff(a0, a1) = {a: {d: 5}};

const diff = (a, b) => {
  let o;
  if (a !== b) {
    if (typeof b === 'object' && !Array.isArray(b)) {
      o = {};
      for (let key in b) {
        const result = diff(a[key], b[key]);
        if (typeof result !== 'undefined') {
          o[key] = result;
        }
      }
      for (let key in a) {
        if (!(key in b)) {
          o[key] = undefined;
        }
      }
    }
    else {
      o = b;
    }
  }
  return o;
}

patch.js

const patch = (a, b) => {
  if (typeof b === 'object' && !Array.isArray(b)) {
    const o = {};
    for (let key in a) {
      if (!(key in b)) {
        o[key] = a[key];
      }
    }
    for (let key in b) {
      if (typeof b[key] !== 'undefined') {
        o[key] = patch(a[key], b[key]);
      }
    }
    return o;
  }
  return b;
}

test.js

let a0 = {a: 0};
let a1 = {};
diff(a0, a1);
patch(a0, diff(a0, a1));

let b0 = {a: {b: {c: 3}, d: 4}};
let b1 = {a: {b: b0.a.b, d: 5}};
diff(b0, b1);
patch(b0, diff(b0, b1));