Javascript Merge And Assign

With the new es2015 Object.assign functionality you can easily merge objects together. However there is also a merge function available in different popular libraries.

The difference between merge and assign was source of confusion for me and I suspect It might be for others. Hopefully this article will clarify things.

All code is editable with live results.

Object.Assign

Let’s start with what we have available in the language: Object.assign. It is shallow. It means it won’t merge recursively Object properties. It only copies from the top level.

So in the following example, the foo prop from Obj1 replaces the one from Obj2.

Obj1 = { val: { foo: [1, 2, 3] }};
Obj2 = { val: { foo: [4], }};
testObject = {};
Object.assign(testObject, Obj1, Obj2);
testObject;

Additionally, It copies only direct properties and not ones accessible through the prototype link.

prototype = { val: { hi: 'hello' } };
Object.setPrototypeOf(Obj1, prototype);
testObject = {};
Object.assign(testObject, Obj1, Obj2);
testObject;

It copies Symbol properties;

testObject = {};
Obj1['not a symbol'] = 'cool';
Obj1[Symbol.for('newProp')] = 'cool';
Object.assign(testObject, Obj1, Obj2);
testObject[Symbol.for('newProp')];

However you may think those symbol properties are not copied since they are not enumerable

testObject;

It does not copy non-enumerable properties other than symbols

testObject = {};
Object.defineProperty(Obj1, 'notEmurable', {
  value: 'not emurable',
  enumerable: false
});

Object.assign(testObject, Obj1, Obj2);
testObject;

If your property was not writable it becomes writable!

testObject = {};
Object.defineProperty(Obj1, 'notWritable', {
  value: 'not writable',
  writable: false
});

Object.assign(testObject, Obj1, Obj2);
testObject.notWritable = 'changed';
testObject;

If your property was a function, assign copies the function but be aware it may not be visible in some consoles (not showing on my current version of Google Chrome for example) .

testObject = {};
Obj2.myFunction = function myFunction() {
  return 42
}
Object.assign(testObject, Obj1, Obj2);
testObject;

testObject.myFunction();

lodash Assign

_.assign is like Object.assign but will work in non-es6 environments.

testObject = {};
_.assign(testObject, Obj1, Obj2);
testObject;

lodash Merge

This is where it gets really interesting. _.merge will recursively copy object properties.

Obj1 = { val: { foo: [1, 2, 3] }};
Obj2 = { val: { foo: [4] }};
prototype = { val: { hi: 'hello' } };
Object.setPrototypeOf(Obj1, prototype);

testObject = {};
_.merge(testObject, Obj1, Obj2);
testObject;

Since the copy was recursive, foo was not replaced but instead the foo array first element replaces the one from Obj2 foo.

It can be good or bad. Maybe you wanted to add 4 to the end of the array? Of course I could replace it with an Object instead and that will work

Obj1 = { val: { foo: {'1': 1, '2': 2, '3': 3} }};
Obj2 = { val: { foo: { '4': 4} }};
prototype = { val: { hi: 'hello' } };
Object.setPrototypeOf(Obj1, prototype);

testObject = {};
_.merge(testObject, Obj1, Obj2);
testObject;

If we forgot the extra typing,that solves our particular problem but maybe that’s not what you wanted to do. Maybe replacing the first element of the array was what you wanted. In general you should be careful when you merge deep objects. This is not something trivial to do.

ImmutableJS

Immutable merge returns a new object but is shallow like Object.Assign! I find this a big source of confusion since Immutable uses merge to assign…

Obj1 = { val: { foo: ['f', 'o', 'o'] }};
Obj2 = { val: { foo: ['c'], bar:['b', 'a', 'r'] }};
iObj1 = Immutable.fromJS(Obj1);
iObj2 = Immutable.fromJS(Obj2);

iObj1.merge(iObj2);

Immutable mergeDeep is ImmutableJS equivalent to _.merge except that it does not mutate the object of course.

 
 iObj1.mergeDeep(iObj2);
 
 

Review

assign can catch you off guard as some of its behaviour might seem unexpected. assign will copy properties at the top level, won’t copy non-enumerable properties, won’t copy prototype properties, all copied properties become writable.

assign copies even thought they are not enumerable with many default js functions.

merge and assign have different behaviours regarding deep properties. merge recursively merges deep properties. It might seem to be what most programmers would expect by default.

However be aware of what you are trying to do, especially merging responses from an API. merge can be the right tool for the job in some occasions, assign in others.