Cover image for Human Readable JavaScript

Human Readable JavaScript

laurieontech profile image Laurie Originally published at tenmilesquare.com ・4 min read

For a long time, humans needed to "speak" like machines in order to communicate with them. And that's still true, we still have the need for people who work in assembly and other low-level languages. But for many of us, these complexities are abstracted away. Our job, is to focus on what is readable for humans and let the machines interpret our code.

This consideration is never more apparent than a situation in which identical code can be written in numerous ways. So today, I want to talk less about how something works, and more about how it reads. There is another post in here somewhere about functional JavaScript, but let's assume we're talking about map.

map is a function available for arrays in JavaScript. Think of it as for each. It takes a function as an argument and runs each element in the array through that function. The difference is that it doesn't alter the original array at all. The result is a new array.


const arr = [1,2,3]
let multipliedByTwo = arr.map(el => el*2)
// multipledByTwo is [2,4,6]

Ok, so we know what map does. But look at the code snippet above. An incredibly terse function that multiplies a variable by two.

So let's take a look at all the different ways we could write that same logic.

Optional Parentheses

The first optional addition we can make is to add parentheses to the parameter definition of the internal function. This makes that piece of code start to look more like a typical function definition.

const arr = [1,2,3]
let multipliedByTwo = arr.map((el) => el*2)

What's interesting about this is that the only reason we don't need them is because we're only passing one argument.

const arr = [1,2,3]
let multipliedByTwo = arr.map((el, index) => el*2)

In cases where we pass more than one argument, the parens are not optional. Our example is map, if it were reduce we would always use the parentheses.

So let's take stock for a moment. Do we lose anything by adding the parentheses? Do we gain anything? We're adding two characters, what information does that convey? These are the things we need to ask ourselves as we develop code for our teammates and future selves to maintain and read.

Curly braces and return

We can go a step further with making that internal function adhere to official function syntax. Doing so requires curly braces and the return keyword.

const arr = [1,2,3]
let multipliedByTwo = arr.map((el) => { return el*2})

How do we feel about this code now? It certainly reads more clearly as a function. Do the braces and return add more bulk? Does our view of this change depending on the logic being returned?

As it turns out, this is again non-optional if our function logic is more than one line.

const arr = [1,2,3]
let multipliedByTwo = arr.map(
(el) => { 
  if(el%2 === 0) {
      return el*2
  } else {
      return el+1

Interesting. Does our opinion of the extra characters change based on the use case? What does that mean for consistency throughout our code?

Use a separate function

As we know and have seen, map takes a function as an argument and passes each element in our array into it. Perhaps we could, or should, define our internal logic outside of the map. As it stands, it looks a bit like pyramid code.

const arr = [1,2,3]

const timesTwo = (el) => el*2

let multipliedByTwo = arr.map((el) => timesTwo(el))

What do we think? Realistically it's almost the same number of characters as the original version. But what about our example from above with more complex logic?

const arr = [1,2,3]

const timesTwoOrPlusOne = (el) => { 
  if(el%2 === 0) {
      return el*2
  } else {
      return el+1

let multipliedByTwo = arr.map((el) => timesTwoOrPlusOne(el))

Did this change your view? Or does it look cluttered and repetitive?

Just a function

Functional programming is an interesting paradigm. In part because of the way it allows us to write code. Again we're reminded that map takes a function as an argument. So why not give it a function.

const arr = [1,2,3]

const timesTwo = (el) => el*2

let multipliedByTwo = arr.map(timesTwo)

Yes, this is valid. map knows to pass the element it gets to the function and use the result. We can get even more in the weeds by determining what form our timesTwo function could take. Right now it's a terse one-liner.

And note that map is really smart. We can pass the same function even if that function now uses both the element and the index to arrive at a return value!

const arr = [1,2,3]

const timesTwoPlusIndex = (el, index) => (el*2) + index

let multipliedByTwo = arr.map(timesTwoPlusIndex)

Does this seem readable? multipledByTwo is certainly pleasant to read, but where is timesTwoPlusIndex located in our codebase? Is it hard to track down? If someone is looking at this for the first time do they know it's a function? Or do they assume it's an object or array variable?

Functions are objects in JavaScript, but ignore that duplication for the moment.

How do we determine what is readable

There is no one size fits all syntax. Who is your audience? Polyglots or JavaScript experts? Who is maintaining your code? How many people work in this codebase? All of these things matter.

It entirely depends on the use case, and consistency is important. However, seeing all the different representations of the same functionality is eye-opening. All of these examples will be built into the same minified code. So the decision for us, as developers, is based on human readability. It's completely absent of machine performance and functionality considerations.

I've posed a lot of questions and not a lot of answers. I have my own opinions but would love to hear yours. Which of these are the most readable? Are there versions you prefer to write? Let's discuss it below!

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Software dev at Gatsby | DC techie | Conference speaker | egghead Instructor | TC39 Educators Committee | Girls Who Code Facilitator | Board game geek | @laurieontech on twitter


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Love the point free (tacit programming) put in there. I am getting more into functional programming but have to consider my peers and juniors coming in when writing my code... Usually, I ask myself "could I have understood or figured this out when I first finished my coding Bootcamp...? If no I refactor."


That's a really good principle to work by.

Refactor only if:

  • It has bugs
  • A junior dev would be stressed out trying to read it

I think you should fix it, not refactor if it has bugs.
By definition, refactoring does not change behavior.

True, but when we encounter bugs, don't we ask what caused the bug? Too many moving parts? Unclear data-flow? In the postmortem of a bug-fix, the topic of refactoring doesn't come up?

Well said Ezell, I agree that refactoring during a bugfix rewrite is a good idea for sure!


What a great barometer!

Do you think you're losing sight of that level at all? That's always my fear. That I overestimate my past self.


Sometimes I do. I generally will have mentees or people I know learning code take look. If they have trouble understanding it ill have them describe why to be sure it's not just a knowledge issue but an over-complicated issue. 😆 So far it seems somewhat successful. I still write my own esoteric code on personal stuff lmao 🤣😅


The refactor from

const arr = [1,2,3]

const timesTwo = (el) => el*2

let multipliedByTwo = arr.map((el) => timesTwo(el))


const arr = [1,2,3]

const timesTwoPlusIndex = (el, index) => (el*2) + index

let multipliedByTwo = arr.map(timesTwoPlusIndex)

is one of my favourites.

Interestingly (well, interesting to me anyway), it's an example of η-conversion (eta-conversion), one of the three basic reduction steps in the Lambda calculus.


The most common pitfall though with this approach is when, for example, converting an array of strings into integers.

["1", "2", "3"].map(parseInt) = [1, NaN, NaN]

["1", "2", "3"].map(x => parseInt(x)) = [1, 2, 3]

The reason for this is that parseInt actually takes 2 arguments (string and radix). Map accepts methods that take up to 3 arguments (value, index, and array). So, when passed to map directly, string is getting value (what we expect!), but index is being passed as the radix. Meaning, you try to parse the 0th element in a natural way (base 10), then you try to parse the 1st element as base one (which it's not a valid base 1 number, so NaN), parse the 2nd element as base two (again, 3 is not valid base 2, so NaN), and so on...

I've been bitten by this bug quite a few times. When using map, bypassing the anonymous function and passing a named function should generally only be used if the function takes a single argument.


The default behaviour of Array.map is unintuitive given that it returns the index and the original array as the second and third arguments to the callback function respectively.

I would approach the parseInt problem by writing a map function that takes two args, supplied one at a time (to facilitate partial application).

  • the first arg, a function f that will be supplied only one value at a time, that is, the current iterated value
  • the second arg, a 1-dimensional array of values to apply the function f on

It may appear complicated seeing it for the first time, but come back to the example and mull it over and it will start to click.

// alternative map implementation
const mapAlt = f => xs => Array.prototype.map.call(xs, currentValue => f (currentValue));

// unterse
function map(f) {
  return function(xs) {
    return xs.map(function(currentValue, index, originalArray) {
      return f(currentValue);

const map = f => xs => xs.map(function(currentValue, index, originalArray) {
  return f (currentValue);

const xs = [1, 2, 3, 4];
const multiplier = x => x * 6;
const multiplied = map (multiplier) (xs);

const ys = ["1", "2", "3", "4"];
const parser = x => parseInt(x);
const parsed = map (parser) (ys);

console.log("xs:", xs); //=> [1, 2, 3, 4]
console.log("xs multiplied:", multiplied); //=> [6, 12, 18, 24]
console.log("ys:", ys); //=> ["1", "2", "3", "4"]
console.log("ys parsed:", parsed); //=> [1, 2, 3, 4]

But why include index and originalArray parameters when you don't use them?

For that matter, why not make this point-free?

const map = (f) => (xs) => xs .map ((x) => f(x))

(... and around in circles we go!)

That's true, they are superfluous - I left those other args there to make it clearer how the args are moving around.


Though it might be a distraction from the example... You could put Number there in place of parseInt.


That's a good one! Thanks for pointing it out.


Haha it is! I don't think I'd made that connection before.


I know! I think it's maybe the only place I've ever found a practical use for lambda calculus!


Love this Laurie!

I'm a .NET developer, but always make it my aim to make code read like a story. Things like

var settings = new Settings(this.Configuration)


var settings = Settings.LoadFrom(this.Configuration)

Simple example, but making your code read like a story makes it so much easier to pick up coming back to it.

I think short functions and readable code are fundamentals for code that others can pick up.


I think it's important not to simplify the code at a point where you avoid using functionalities which are useful but less readable for a newbie.

For example instead of the following:

    let multipliedByTwo = arr.map(
        (el) => { 
            if(el%2 === 0) {
            return el*2
        } else {
            return el+1

I would prefer using ternary, and no optional syntax characters such as parenthesis or curly braces:

let multipliedByTwo = arr.map(el => el % 2 === 0 ? el * 2 : el + 1)

It may seem less approachable for someone who is not used to ternay, arrow functions and the absence of unnecessary syntax characters, but when used to it, it's actually better to have such a one line operation rather than 8 line of basic if...else.

In other words, I prefer to write a code that requires the maintainer to raise his level of JavaScript understanding but enables code to be shorter, rather than making super simple but verbose JavaScript (I hate to search for bits of code in hundred-lines-long files).


Yeah - I wouldn't want to have to figure out what goes wrong in the middle of the night when they call with production issues.

And it is not unnecessary syntax characters except for the computer - for humans, it adds readability and understanding.

And your stuff will break down when the new guy is covering everything while you are on vacation.


Interesting. I tend to think of one liners as less readable in most cases. But it’s an interesting perspective!


Developers in general agree with you. Most one liners beyond a standard if/else ternary are caught by default linting configs. If someone has to translate the code you write, it's meant for a machine. Given that this already happens once it hits an engine, it is generally not wise to write code in this manner.

“Code is for humans to read and machines to interpret”! Still don’t know who said it first, but it’s a great quote.


How about splitting things up?

timesTwo = e => e * 2;
add = (a, b) => a + b;

const result = arr
.map(add) // add index

putting maps on separate lines helps people see it broken in to steps

adding in a comment on the "add" helps because most maps don't usually use index.

you could do .map(addIndex) but I don't like this as the original function can add any two things, not just index

or .map((a, i) => add(a, i)) but that creates another function


Hi Laurie, great question.

I don't have a 'one-shoe-fits-all' approach to writing the most readable code all the time. At one point I wished for that, but I'm understanding writing code is better off not that way.

The last pattern described particularly useful when the associated callback function for array.map is so long, I forget that it's a callback. In that case I may have something like this:

// Either import it or placed elsewhere in the same file if not used anywhere else
import {crazyLongCallback} from './crazyCallbacks'

const bigDataList = [...]
const parsedDataList = bigDataList.map(crazyLongCallback) 

So I find it useful when discovering different design patterns and ways to handle problems. I tend to gravitate toward the solution in which I hope me I and my colleagues can read after a day, week, or month's time.

Thanks for the read!


That's awesome. Do you find yourself staying consistent in the same codebase?


Yep, but I just thought of another question. Does the length of a code-block have a big impact on readability? I'd imagine this isn't the case for everyone.

I don't know. Does it? I'd argue that it absolutely does. But you can shorten a code block and make it less readable at the same time. It matters how you do these things.


As it turns out, this is again non-optional if our function logic is more than one line.

I know this isn't at all what you meant, but I thought I'd throw this in the comments for fun.

You can many times write code that would normally need curly braces without them by using parentheses expressions. You can't use statements (like creating new variables or if, else, for, etc), but you can call a few functions and return the last value.

const newArr = myArr.map(x => (console.log(x), changeX(x));

Just a little tidbit I thought I'd throw in. Excellent article nevertheless!


What a lovely article you've written! Thank you for putting it down in such easy to read and understand text and idea.

I have noticed the same pattern in my coding style over the last 10 years which you describe here, i.e. I truly tend to write for the next human who will read it (which is mostly myself again) later. And again, it might be someone else! Which means somehow this should be like a moral and professional imperative. I. Kant "the programmer" would like that I guess. :-) Or, as Ezell puts it in the comments before, nicely; "A junior dev would be stressed out trying to read it". It is the thing that we are always the junior dev when you meet some new codebase or logic. Isn't it?

I guess it is more up to the "wits" or "IQ" who gets the idea behind some code faster, and on the contrary the common to all of us is "the codebase" and its style. Why not agree on human readable code first then? So from that point it would be lovely if all tend to write human readable code first. Firstly to help ourselves, to be able to even optimize it to next performance level if needed later. A good test for anyone who doubts this is just open any to your grade complex (non-functional language) codebase on github and try figure out the logic in next 5 minutes. You won't in its totality. How ever hard you try there are hidden non-functional style friendly outside mutations and side-effects that you will not grasp from that single e.g. method or function. I am not praising the functional style here as the holy savior, but trying to figure it out a bit e.g. made a better OOP C# developer in terms of sharing my code with others. Again, never take this (or other forum discussion :p) literally as I'd always go with team compromise and policy and in this case with a named function in place of an anonymous one (an that is in cases where the logic spans further then an operation between two operands like a + b).

I always suggest devs I work with where there is smoke in code there will be fire later, let's rewrite or refactor, step by step of course.


Really enjoyed reading this. Thanks for your thoughts.


Honestly and acording to my experience:
Better less lines and one comentary line than a lot of curly braces and separates functions.
And second rule:
If a function is more than 10 lines, you should do two.
That is my opinion. Like an ass, everybody has one.


Everyone can have an opinion, it's good to talk about these things. I don't go by function lines, but by functionality. A function should do one thing. That normally keeps it short.


I like to talk about this when people is like you, giving examples. I was trying to make a point about the developers that likes to code in one way and push everybody on that way...only because the guy/girl like it.
Not even with coding reasons, performance or teamworking timing examples on a project.
Maybe now its more clear the point. I hope so. No offense about the ass, its just a funny sentence for me.


Very nice article and I love the conclusion "There is no one size fits all syntax". I probably would have written the first now version because I like the "functional programming"-mindset it applies.

For people who struggle with arrow functions I would have included the plain "function" version, too:

const arr = [1,2,3]
let multipliedByTwo = arr.map(function(el) {
    return el*2;

I had a pretty hard time in grasping lambdas when they were introduced in Java 8 and it was really irking me that most of the tutorials about this topic did not include a 1:1 non-lambda "translation" :/


excellent article!
brb I'm going to change my prettier config:
"arrowParens": "always"


That's some stuff to think about. If you think about books, they are written to a specific audience and their reading level. We should consider who we write code for. I think in most cases that are our colleagues. What level are they at / should they be at? I like Ezell Frazier's metric here in the comments "Would a junior dev be stressed out trying to read it".

In the cases where the code gets complex and can't be any more simplified, you should add comments describing what happens.


Awesome post! I have to make the subtle point of the performance difference when you use certain approaches though.

let multipliedByTwo = arr.map((el) => timesTwo(el));


let multipliedByTwo = arr.map((el) => timesTwo);
creates extra unnecessary function around a function. It can be more of a hit when you nest function definitions inside of a callback, but that's a subject for a different post altogether! Great work here!


Absolutely true. And given that I was trying to show identical code I struggled over whether to include that one. Thanks for noting it. I likely should have of the original post!


my team decided to adopt typescript. Bye-bye readability 😭


WOW, It is a nice and helpful article.

Thank you @Laurie


The book "The Art of Readable Code" g.co/kgs/ZWgHAB is not too thick and easy to read (the opposite would have been a terrible defect for a book on this theme, thinking of it...).
At the beginning I thought "Do I really need to read this?", but I ended up reading it all. Authors are really open-minded (not always the case when people talk about code style). It was even fun at times.


Loved your examples, makes it very simple to compare and think about the different choices.