Four reasons to use append() instead of Object.assign() and Object spread

Charles Lowell

September 18, 2018

Warning! This post was manufactured in a facility that also processes Monoids and may contain traces of functional programming.

I'd like to share with you what is easily one of my top 5 favorite discoveries in programing during the last year: The append operation for JavaScript Objects. Before you go looking in the API docs: no, this is not part of the official ECMA Script specification. It's a pattern that I found on the internet and implemented in funcadelic.js. You can use my implementation, or you can write your own, but either way, once you experience how much better append tastes than its analogues in vanilla JavaScript, you'll have a really hard time going back.

The TL;DR is that you can use append anywhere that you would use Object.assign or the object spread operator except it's more awesome. How? Well I'm glad you asked.

append is immutable.

It never changes any of its arguments, only ever creates new objects that are derived from the objects that you give it.

let ingredients = append({ eggs: 2}, { mushrooms: 10 })
//=> { eggs: 2, mushrooms: 10 }

I'll not try to convince you of the benefits of immutability here, only say that as programmers, we truck in that most precious of resources: information. Why willingly destroy it unless absolutely necessary?

When you use append instead of Object.assign, you'll never have to pay the overhead of that decision, or worse, accidentally mutate some object you never intended to.

append preserves type.

Every JavaScript object has a runtime type. This lets us define custom properties and operations on it. Unfortunately, if we're making an immutable transformation using Object.assign or Object spread, that type information is erased and no matter what we started out with we revert to a plain old JavaScript object.

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;

  get fullName() {
    return `${firstName} ${lastName}`;

let misspelled = new Person("Tony", "Stork");

let corrected = {...misspelled, { lastName: "Stark" } }
//=> Object { firstName: "Tony", lastName: "Stark" }

//=> undefined

Notice how the fact that we're work working with a Person object is just thrown into the dumpster? By contrast, append always preserves type of object that we were working with before.

let misspelled = new Person("Tony", "Stork");
let corrected = append(misspelled, { lastName: "Stark" });
//=> Person { firstName: "Tony", lastName: "Stark" }
corrected.fullName //=> "Tony Stark"

I mean you could throw away the type by default, but why?

append is lazy

JavaScript has a really nice feature called getters or "computed properties" that lets you defer evaluating a property's value until it's actually needed. This can be a great way to avoid computing a property if the dependencies required won't be available until later, or you'd just rather not pay the cost of evaluation until you absolutely need to.

Unfortunately, whatever your reasons, both Object.assign and Object spread will interfere with them because they eagerly evaluate each property of the objects being copied over into the result. For example, let's look at these two objects that have computed properties:

let left = {
  get thousand() {
    console.log('calculating 1000 in a really expensive way');
    let i = 0;
    for (; i < 1000; i++);
    return i;
let right = {
  get hundred() {
    console.log('calculating 100 in a really expensive way');
    let i = 0;
    for (; i < 100; i++);
    return i;

Let's see what happens when we combine them with Object spread:

let both = {...left, ...right};
// [LOG] calculating 1000 in a really expensive way
// [LOG] calculating 100 in a really expensive way

Or how about Object.assign?

let both = Object.assign({}, left, right));
// [LOG] calculating 1000 in a really expensive way
// [LOG] calculating 100 in a really expensive way

But we made those properties lazy for a reason! Just because I'm deriving a new object based off of those properties, doesn't mean I should have to evaluate them! Well with append you don't. It will follow the guidelines laid down by each object's original creator and keep normal properties normal, and computed properties computed.

let both = append(left, right);
// nothing printed, because nothing evaluated!
// only when we access the properties does the computation run.

both.thousand //=> 1000
// [LOG] calculating 1000 in a really expensive way

both.hundred //=> 100
// [LOG] calculatingc 100 in a really expensive way

append is a universal interface.

Remember when I said that I found append on the internet? That was true, but it also wasn't the full story. What I actually found on the internet was something far more powerful: the concept of a semigroup. A semigroup is (roughly) a type; any two of whose values can be combined with each other to produce a new value of the same type. For example, JavaScript Array is a semigroup. I can combine or "append" any two Array objects together to produce a single new Array:

[1,2] + [3, 4] //=> [1, 2, 3, 4]

JavaScript String is a semigroup too!

'Hello' + ' World!' //=> Hello World

This got me to thinking: what would a semigroup for object look like? What would it mean to take two Objects and combine them to produce another Object, and the append() function that I've been talking about was the inevitable consequence:

  1. Immutable because this is functional programming, and that's really just a given.
  2. Type-preserving: because the append operation of Semigroup must return the same type as its arguments.
  3. Lazy: This isn't strictly a requriment, but as long as we're building an amped up version of Object.assign we might as well respect object property semantics, no?

This is all to say that using append normalizes this operation to a shared understanding of what a Semigroup is. And in fact, in funcadelic.js, the append operation will work on all these types.

append([1,2], [3, 4]) //=> [1, 2, 3, 4]
append('Hello', ' World!') //=> 'Hello World!'
append({ eggs: 2 }, { bacon: 2 }); //=> { eggs: 2, bacon: 2 }

One concept, one function, lots and lots of different scenarios. And that's why I started using append and haven't looked back since.

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