@ou-imdt/utils/lib/mergeObject.js

Utility library for interactive media development

export const skip = Symbol('skip merge value');

export function isMergeable(target) {
  return typeof target === 'object' && target !== null;
}

/**
 * recursively merges enumerable own properties (string/symbol keyed) from one or more source objects to a target object.
 * - doesn't throw on unsupported source types (non-object)
 * - creates nested objects on target where needed
 * - doesn't override defined target values with undefined source values
 * @param {object} target - the target object
 * @param {object|function} params - source object(s), optional callback function
 * @returns {object} - the modified target object
 */
export default function mergeObject(target, ...params) {
  if (typeof target !== 'object' || target === null) return target;

  const depth = (typeof params[params.length - 1] === 'number') ? params.pop() : 0;
  const stack = (depth === 0) ? target : params.pop();
  const callback = (typeof params[params.length - 1] === 'function') ? params.pop() : null;
  const [source, ...sourceList] = params.filter(value => value !== null);

  if (typeof source === 'object' && source !== null) {
    const keys = Reflect.ownKeys(source).filter(key => Object.prototype.propertyIsEnumerable.call(source, key));
    
    keys.forEach((key) => {
      const targetValue = target[key];
      const sourceValue = source[key];

      if (typeof targetValue !== 'undefined' && typeof sourceValue === 'undefined') return;

      const customValue = callback?.({ sourceValue, targetValue, key, target, source, stack, depth });

      if (typeof customValue !== 'undefined') {
        if (customValue !== skip) {
          target[key] = customValue;
        }
        return;
      }
      if (isMergeable(sourceValue)) {
        if (!isMergeable(target[key])) {
          target[key] = Array.isArray(sourceValue) ? [] : {};
        }
        target[key] = mergeObject(target[key], sourceValue, callback, stack, depth + 1);
      } else {
        target[key] = sourceValue;
      }
    });
  }
  if (sourceList.length) mergeObject(target, ...sourceList, callback);
  return target;
}