gistfile1.txt

this.fieldMappings = {
  "firstName": {
    "value": "AnnualRevenue",
    "option": "never"
  },
}

this.config = {
  "contact": {
    "firstName": {
      "label": "First Name", //where this field is displayed, this is the name we call it.
      "defaults": {
        "salesforce": "First_Name" 
      }, //this is also the default value for this provider (compare with DB)
      "getter": "name", //this is our local accessor for this field (on ES object)
      "specialFunction": "parseFirstName", //is this a special function like tech stack? if this exists check it first...
      "remoteReadOnly": false, //use this for fields like ID that should be no overwritten.
    }
  }
}

this.provider = "salesforce"


function buildIntermediaryObject(localObject, remoteObject) { 
  //FOR EACH FIELD IN DB,
  return Object.entries(this.config).reduce( (newObject, [internalKey, { defaults, remoteReadOnly }]) => {
    const { value, option } = this.fieldMappings[internalKey];
    const remoteGetter = value || defaults[this.provider];

    if (option === "never") return newObject; //bail
    if (option === "empty" && _.get(remoteObject, remoteGetter, null) !== null) return newObject; //if empty, and empty set, bail.

    if (remoteReadOnly) { //if we are preserving the remote value, set it.
      newObject[internalKey] = _.get(remoteObject, remoteGetter, null);
    } else { 
      newObject[internalKey] = _.get(localObject, internalKey, _.get(remoteObject, remoteGetter, null)); //get remote values, then put local values on top.
    }
    return newObject;
  }, {});
}

function mergeLocalAndRemote(localObject, remoteObject) { 
  const newLocalObject = buildIntermediaryObject(localObject, remoteObject);
  const newRemoteObject = Object.entries(this.config).reduce((newRemoteObject, [internalKey]) => {
    const { value } = this.fieldMappings[internalKey];
    const remoteGetter = value || defaults[this.provider];
    newRemoteObject[remoteGetter] = newLocalObject[internalKey];
    return newRemoteObject;
  }, {});
  const diff = Object.entries(newLocalObject).reduce(diff, ([key, values]) => { 
    if (values !== remoteObject[key]) { 
      diff[key] = values;
    }
    return diff;
  }, {});
  return {
    original: remoteObject,
    merge: newRemoteObject
    diff,
  }
}

//usage...
//it assumes fieldMappings, config, and provider are set.
const newObject = buildIntermediaryObject(localObject, remoteObject);
const {original, merge, diff} = mergeLocalAndRemote(localObject, remoteObject);