Lodash - Difference Between .Extend()/.Assign() and .Merge()

Lodash - difference between .extend() / .assign() and .merge()

Here's how extend/assign works: For each property in source, copy its value as-is to destination. if property values themselves are objects, there is no recursive traversal of their properties. Entire object would be taken from source and set in to destination.

Here's how merge works: For each property in source, check if that property is object itself. If it is then go down recursively and try to map child object properties from source to destination. So essentially we merge object hierarchy from source to destination. While for extend/assign, it's simple one level copy of properties from source to destination.

Here's simple JSBin that would make this crystal clear:
http://jsbin.com/uXaqIMa/2/edit?js,console

Here's more elaborate version that includes array in the example as well:
http://jsbin.com/uXaqIMa/1/edit?js,console

Which is better to extend a Javascript Object with lodash? Whats the difference?

Adding properties to an object's prototype property is pretty much useless unless that object happens to be a function that is going to be used as a constructor. If it's any object that is not also a function, prototype is just an ordinary property:

var obj = {};
try {
  obj.prototype.someProp = 5; // Throws error, obj.prototype is undefined
} catch ( e ) { 
  obj.prototype = {}; // Nothing special about this
  obj.prototype.someProp = 5; // or this
  obj.someProp === undefined; // true
}

Adding properties to the actual prototype of an object (not the prototype property) affects all objects with that same prototype:

var obj = {};
Object.getPrototypeOf( obj ).someProp = 5;
var obj2 = {};
obj2.someProp === 5; // true

That doesn't seem to be what you want.

_.assign and Object.assign extend the actual object:

var obj = {};
Object.assign( obj, { someProp: 5 } );
obj.someProp === 5; // true

How to extend and merge a list of objects with another list using lodash

The easy way is to use the _.flatten method of lodash to achieve this:

const _ = require('lodash');

const data = _.map(api_response.environment_list, e => 
    _.map(api_response.member_list, m => ({
        id: m.id, 
        name: m.name, 
        role: m.role, 
        environment: e
    })
); // This step achieves the exact same thing that you have achieved up until now in your question.

const flattenedData = _.flatten(data); // This is the method you need to make the array "flat".

console.log(flattenedData);

You could also attempt to do this without using lodash in this, a bit more efficient, way:

const output = [];
for (const e of api_response.environment_list) {
    for (const m of api_response.member_list) {
        const obj = {
            id: m.id, 
            name: m.name, 
            role: m.role, 
            environment: e
        };
        output.push(obj);
    }
}

Object.assign vs $.extend

The two key differences are the optional boolean for deep merge which is recursive on the jQuery $.extend method (where false is not supported?!) ...

let object1 = {
  id: 1,
  name: {
    forename: 'John',
    surname: 'McClane'
  },
};

let object2 = {
  id: 2,
  name: {
  }
};

// merge objects
let objExtend = $.extend(true, {}, object1, object2);
let objAssign = Object.assign({}, object1, object2);

// diff
console.log(objExtend.name.forename); // "John"
console.log(objAssign.name.forename); //  undefined

Object.assign() copies property values. If the source value is a reference to an object, it only copies that reference value.

Example: JsFiddle

The second is the $.extend method ignores undefined ...

let object1 = {
  id: 1,
  name: 'hello world'
};

let object2 = {
  id: 2,
  name: undefined
};

// merge objects
let objExtend = $.extend({}, object1, object2);
let objAssign = Object.assign({}, object1, object2);

// diff
console.log(objExtend.name); // "hello world"
console.log(objAssign.name); //  undefined

Example: JsFiddle

Docs

MDN: Object.assign(target, ...sources)

jQuery: jQuery.extend([deep], target, object1 [, objectN])

Additionally:

If you are looking for a way to deep merge objects without jQuery, this answer is excellent:

How to deep merge instead of shallow merge?

Example: JsFiddle

How to deep merge using Object.assign with ES6:

function isObject(item) {
  return (item && typeof item === 'object' && !Array.isArray(item));
}

function mergeDeep(target, ...sources) {
  if (!sources.length) return target;
  const source = sources.shift();

  if (isObject(target) && isObject(source)) {
    for (const key in source) {
      if (isObject(source[key])) {
        if (!target[key]) Object.assign(target, { [key]: {} });
        mergeDeep(target[key], source[key]);
      } else {
        Object.assign(target, { [key]: source[key] });
      }
    }
  }
  return mergeDeep(target, ...sources);
}

Lo-dash extend/assign default behavior

This has been answered in the comments already, but here's an attempt on a more qualified answer and to close this topic:

The effect you're getting is due to combined effect of node.js module loading and lodash _.extend():

• Included modules are just objects and they are cached, so every subsequent call to 'require()' relating to the same physical file system path will return that object
• Lodash's extend/assign is annoyingly destructive, it's manipulates the passed object directly instead of returning a new object (like e.g. Rubys 'Hash#merge()' would do)

So you have one object representing all.js and _.assign() manipulates that. Any further require()'s will just return the same, modified object.

If this is undesired (and in many situations it is), only workaround I know of so far is this:

var someOptions = {/*...*/};
var extended = _.extend(_.clone(someOptions))

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