Opinionated deepmerge

deepMerge.js

/**
 * deepMerge takes two objects and overwrites the values with
 * incoming values, if they are empty, undefined, or missing,
 * the default values will be used.
 * 
 * In the case of arrays, the first value in the object set
 * will be used as the template and applied to all values
 * in the incoming set.
 */
function deepMerge(defaultSet, incomingSet) {
  const isObject = obj => obj && typeof obj === 'object'

  if (!isObject(defaultSet) || !isObject(incomingSet)) {
    return incomingSet
  }

    let keys = Object.keys(incomingSet)
    for (let i = 0; i < keys.length; i++) {
      let key = keys[i]
      const targetValue = defaultSet[key]
      const sourceValue = incomingSet[key]

      if (Array.isArray(targetValue) && sourceValue === undefined || Array.isArray(sourceValue)) {
        // if the source is not set or empty, use whatever the default is
        if (sourceValue == undefined || sourceValue.length < 1) {
          // if sourceValue is empty, use the default array
          continue;
        }
        
        // we expect the first value in the default object array to be
        // the template, no other values will be considered.
        let templatedArrayDefault = targetValue[0]
        
        // create a buffer array
        let filledInArray = []
        
        sourceValue.forEach(v => {
          // Do a deep clone if possible, so we don't overwrite
          // any reference type variables.
          let clonedTemplateValue = templatedArrayDefault
          if (
            templatedArrayDefault instanceof Object &&
            !(templatedArrayDefault instanceof Date) &&
            !(templatedArrayDefault instanceof String)
          ) {
            clonedTemplateValue = deepClone(templatedArrayDefault) // deepClone(targetValue[0])
          } 
          filledInArray.push(deepMerge(clonedTemplateValue, v))
        })
        defaultSet[key] = filledInArray

      } else if (isObject(targetValue) && isObject(sourceValue)) {
        defaultSet[key] = deepMerge(Object.assign({}, targetValue), sourceValue)

      } else {
        defaultSet[key] = sourceValue
      }
  }

  return defaultSet
}