This lesson will demonstrate how to recreate a simplified version of the popular lodash.merge method from scratch.

First we'll create a test file with two objects that we want to merge together and demonstrate the output after running them through lodash.merge , then we'll go about creating our own version of the function.

Our initial version will support only one source object to merge in, and will support only the subset of JavaScript syntax supported by the JSON spec, including the following types:

• boolean, number, string
• object literal
• array

After getting basic types to merge we'll show off two methods for merging arrays, either using concat or recursively going through the children and merging them.

Finally we'll build our own isObject function to check for object literals and recursively apply our mergeJSON function to any object literals in our source object.

And last we'll add support for multiple source objects to be merged in using a for...of loop.

Note

The mergeJSON method we create approximates the lodash.merge functionality but does not seek to replace it. It may be useful for some use cases, but primarily it's meant to be a tool for understanding how to merge objects with recursion.

function mergeJSON(obj, ...sources) {
  for (let source of sources) {
    for (let key in source) {
      if (source[key] == null) {
        continue;
      }
      obj[key] = replaceValue(obj[key], source[key]);
    }
  }

  return obj;
}

function replaceValue(value, newValue) {
  if (Array.isArray(value) && Array.isArray(newValue)) {
    return newValue.map((val, i) => {
      return replaceValue(value[i], val)
    });
  }
  if (isObject(value) && isObject(newValue)) {
    return mergeJSON(value, newValue);
  }

  return newValue;
}

function isObject(obj) {
  return obj && obj.constructor === Object;
}

module.exports = { mergeJSON };

index.js

import { mergeJSON } from "./merge";

const user1 = {
  name: "Joe",
  age: 33,
  links: {
    blog: "blog link",
    facebook: "facebook link"
  },
  interests: ["gaming"]
};
const user2 = {
  name: "Joe",
  age: 22,
  links: {
    twitter: "twitter link",
    blog: "another blog",
    facebook: null
  },
  interests: ["code"],
  extra: false
};
const user3 = {
  name: "Joe",
  age: 32,
  interests: ["gaming", "travling"]
};
const res = mergeJSON(user1, user2, user3);
console.log("mergeJSON", res);