diff.js

const diff = (() => {
const toString = Function.prototype.toString;
const objectString = toString.call(Object);

return function diff(a, b, opts) {
  opts = {
    printShallow: false,
    ...opts,
  };

  return diffValues(a, b, '$', opts);
}

function diffValues(a, b, path, opts) {
  if (a === b) {
    return [];
  }

  let childResults;

  if (Array.isArray(a) && Array.isArray(b)) {
    childResults = diffArrays(a, b, path, opts);
  } else if (isPlainObject(a) && isPlainObject(b)) {
    childResults = diffObjects(a, b, path, opts);
  }

  const results = [];

  if (!childResults || opts.printShallow) {
    results.push({
      path,
      a,
      b
    });
  }

  if (childResults) {
    results.push(...childResults);
  }

  return results;
}

function diffObjects(a, b, path, opts) {
  const results = [];
  const keys = [...new Set([...Object.keys(a), ...Object.keys(b)])];
  const n = keys.length;
  let i = -1;

  while (++i < n) {
    const k = keys[i];
    results.push(...diffValues(a[k], b[k], addToPath(path, k), opts));
  }

  return results;
}

function diffArrays(a, b, path, opts) {
  const aLen = a.length;
  const bLen = b.length;
  let n = aLen

  if (aLen < bLen) {
    [a, b] = [b, a];
    n = bLen
  }

  let i = -1;
  const results = [];

  while (++i < n) {
    results.push(...diffValues(a[i], b[i], addToPath(path, i), opts));
  }

  return results;
}

function addToPath(path, key) {
  return path + '.' + key;
}

function isPlainObject(v) {
  if (!v || Array.isArray(v) || typeof v !== 'object') {
    return false;
  }

  const proto = Object.getPrototypeOf(v);
  return !proto || toString.call(proto.constructor) === objectString;
}
})();