left-merge

const result = leftMerge(left, right);

Recursively compare two object, merge right into left, but maintain the structure of left, ignoring redundant fields from right. Inspired by "left join" in SQL.

install:

npm i left-merge

when can it be useful:

Imagine left is the default preference settings of your app, and right is the user's customized settings, which is stored on the client side. But the two have different shapes because the user hasn't logged on for a long time. You don't know what shape or what old version he has, but you want to update him to the latest version, meanwhile preserve his custom settings.

example:

import { leftMerge } from 'left-merge';
// user's existing preferences:
const userPreferences = {
  language: 'French',
  itemsPerPage: 50,
};
// now you evolved the default settings into a new structure:
const defaultSettings = {
  language: 'Enligsh',
  useDarkMode: false,
};
// on app start, update user preference to the new structure:
let updatedUserPreferences = leftMerge(defaultSettings, userPreferences); /* ->
{
  language: 'French',
  useDarkMode: false,
} */

another example:

const template = {
  species: 'cat',
  color: ['yellow', 'black'],
  stats: { speed: 10 },
};
const modification = {
  species: 'tiger',
  stats: { speed: 25, canFly: true },
};

let result = leftMerge(template, modification); /* ->
{
  species: 'tiger',
  color: ['yellow', 'black'],
  stats: { speed: 25},
} */

Immutability:

Both arguments and their nested children are never mutated, but may be shallow copied when no change is needed.

null and undefined:

If a field is null or undefined on the left, it'll be overwritten by the counterpart on the right. If null or undefined is on the right, it won't overwrite left.

(when these happen, you'll see more detailed reports in the verbose loggings in development)

type conflicts:

Except null and undefind, when a field has different types on left and right, right will be ignored.

(when this happens, you'll see more detailed reports in the verbose loggings in development)

if you're using commonjs require:

const { leftMerge } = require('left-merge');