discourse/app/assets/javascripts/discourse-common/addon/lib/object.js

function isObject(obj) {
  return obj && typeof obj === "object";
}

// a fairly simple deep merge based on: https://gist.github.com/ahtcx/0cd94e62691f539160b32ecda18af3d6
// note: this approach might reference the original object. If you mutate an object once you've deep
// cloned it, say in a test, it might remain modified. Consider `cloneJSON` instead.
export function deepMerge(...objects) {
  function deepMergeInner(target, source) {
    Object.keys(source).forEach((key) => {
      const targetValue = target[key];
      const sourceValue = source[key];

      if (Array.isArray(targetValue) && Array.isArray(sourceValue)) {
        target[key] = targetValue.concat(sourceValue);
      } else if (isObject(targetValue) && isObject(sourceValue)) {
        target[key] = deepMergeInner(
          Object.assign({}, targetValue),
          sourceValue
        );
      } else {
        target[key] = sourceValue;
      }
    });

    return target;
  }

  if (objects.some((object) => object && !isObject(object))) {
    throw new Error('deepMerge: all values should be of type "object"');
  }

  const target = objects.shift();
  let source;

  while ((source = objects.shift())) {
    deepMergeInner(target, source || {});
  }

  return target;
}

export function deepEqual(obj1, obj2) {
  if (obj1 === obj2) {
    return true;
  } else if (isObject(obj1) && isObject(obj2)) {
    if (Object.keys(obj1).length !== Object.keys(obj2).length) {
      return false;
    }
    for (let prop in obj1) {
      if (!deepEqual(obj1[prop], obj2[prop])) {
        return false;
      }
    }
    return true;
  }
}

export function cloneJSON(obj) {
  return JSON.parse(JSON.stringify(obj));
}