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Stop using Array.map() everywhere 🥵

Suprabha on December 29, 2021

Most of the time I used to see the snippet like this 👇 const fruits = ["apple", "orange", "cherry"]; let text = ""; document.getElementById("ma...

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Amin • Dec 29 '21 • Edited

Hi @suprabhasupi and thanks for your article.

I wanted to point out that you could also have used map in a different way and still have the expected result.

const root = document.getElementById("root");

if (!root) {
  throw new Error("Root not found");
}

const fruits = ["banana", "apple", "pear"];

const indexed = fruits.map((fruit, index) => {
  return `#${index + 1}: ${fruit}`;
});

const html = indexed.join("<br>");

root.innerHTML = html;

Although I agree with you, I would change the title from Stop using Array.prototype.map everywhere to Start learning when to use Array.prototype.map.

A do/don't kind of article would have been great for beginners to quickly catch the idea, and maybe extend the article to some of the most popular array methods as well.

Stas Klymenko • Dec 30 '21

It's better to use Reduce for this task.

Mehul Jariwala • Dec 31 '21

const fruits = ["banana", "apple", "pear"];
fruits.reduce((initalArr, value, currentIndex) => {
initalArr.push(${currentIndex} --- ${value});
return initalArr;
}, []);

Amin • Dec 31 '21

Well, if you are going with Array.prototype.reduce, you better be not using any side-effects.

const root = document.getElementById("root");

if (!root) {
  throw new Error("Root not found");
}

const fruits = ["banana", "apple", "pear"];

const innerHTML = fruits.reduce((html, fruit, index) => {
  return `${html}#${index + 1}: ${fruit}<br>`;
}, "");

root.innerHTML = innerHTML;

Calin Baenen • Dec 30 '21

I should start making comprehensive guides, so I can gain popularity, since my YT career's dead.

Suprabha • Dec 30 '21

It's a common mistake that devs do sometimes, and the above example suits here, Thanks for sharing this ☺️

webbureaucrat • Dec 30 '21

Hard disagree. As others have said, the string concatenation example should have been reduce instead of map, but always favor immutable, functional operations over procedural ones because it is so much easier to introduce bugs in procedural operations like for loops.

Memory consumption is rarely a factor in most JavaScript apps, and garbage collectors get better every year. This is a classic example of a premature optimization. It's better to use map (and reduce!) for pretty much everything, and then go back and optimize into loops where you know there are bottlenecks, after your codebase is more mature.

Corentin Girard • Jan 1 '22

You missed the point of the article I think. The whole point of the article is to say that semantically, a map() should be stored in a variable because it creates a new array. And that if you don't care about the result and just want to do side-effects in a "functional" way (here, I use the word "functional" as opposited to with an actual for-loop statement), just use forEach() instead.

Yes, sometimes reduce() will be better and you should refactor to use it, but that's not the point here.

And yes, the argument of memory consumption was a bad way to try giving a reason not to not store the result of a map().

Suprabha • Dec 30 '21

Totally make sense, I also used to point out this in code reviews.
So first complete the task and then refactor it accordingly.

Jake Carpenter • Dec 30 '21

Getting a new array is very much the point. We do this to avoid side effects that are difficult to track. It’s the same reason functions should return a value, not modify a reference.

Immutability by default is a solid strategy that helps avoid countless bugs. Only modify the original array when you have proven map to be a performance bottleneck - and it’s rare.

Stas Klymenko • Dec 30 '21

map - if you expect an array as a result.
reduce - if you expect one value as a result.
forEach - if you don't need to return a result. Better for sync code.
for of - if you don't need to return a result and you might break the loop. Better for async code.

Corentin Girard • Jan 1 '22

Note that you can use functions like Promise.all() with map() too.

Alfredo Salzillo • Dec 29 '21

In your last example the forEach is the wrong choice. A reduce would have been better

Seven • Dec 29 '21

do agree

Suprabha • Dec 30 '21

Yeah, my main POC was to use map method wisely. Try to use other array methods.

Alfredo Salzillo • Jan 2 '22

Yeah, but a map used wisely would have been better than the for, wrong example

const text = array.map((e, i) => [e,i].join(':')).join('<br>')

Matthew Daly • Dec 29 '21

I disagree somewhat. While the correct use case for map() is for when you want to transform the items in an array, and you shouldn't use it if you're just carrying out an action on each item, these days loops (as opposed to the forEach() method) are best avoided for most use cases, particularly when using a UI library like React.

Using chainable array methods like map() and forEach() has numerous advantages over a loop:

It breaks your code into a series of steps, making it easier to understand and reason about than nested loops
Because the callback for each is a function, you can add type hints for the parameters and return value when using Flow or Typescript to help document what each item represents
It mitigates the need for multiple levels of indentation

Yes, if you're not changing the data, you should use forEach() rather than map(), but in code reviews I would actively discourage using a loop if you can use forEach() instead.

Narasimha Prasanna HN • Dec 30 '21

But yeah it is immediately garbage collected by V8s GC. But still, using map is not recommended for larger arrays, because you may need lot of heap memory at that point in time before it is garbage collected.

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Templar++ • Dec 30 '21 • Edited

So basically: "don't be stupid, use the right tool for the job". Duh... really?!

I have a question - aren't you doing code reviews in your company? Why do you need to write an article just to point out to newbies, that besides Array.map they have to learn what other methods exist and use them properly?

Oh and give a ridiculous example that is accessing a variable outside the function AND the foreach statement, when all you need in this case is Array.reduce:

const fruits = ["apple", "orange", "cherry"];
let text = fruits.reduce((result,item,index) => result + index + ": " + item + "\n", '');

console.log(text);

Please take some time to study what other options you have before writing articles.

PNS11 • Dec 30 '21

As an alternative to the first example I would propose Array.prototype.join(), i.e. remove the text variable and set the HTML to fruits.join(''). In well structured code scope and GC will clean up the fruits array when appropriate.

It's common to have use for both the old array and the new one, e.g. for time travel through app state or when both sets might get rendered depending on user input. When building complex applications one also tends to prefer to avoid data mutations to make code more readable and easier to debug, functional mapping tends to be appropriate for this purpose.

Jean • Jan 5 '22

This is cool, I worked on a project that i could see map() everywhere for no reason! As you said, when its working with small data, its fine to use it. But when have a massive amount of data, its complicated. Even more complicated when use micro services and the communication between the services is async. Take aaaages to load.

Daniël van den Berg • Dec 29 '21

Reduce would be best for the first example.

deathshadow60 • Dec 30 '21

I think your first example has the map and innerHTML backwards.

Both map and foreach far too often waste memory and execution time. I've dealt with so many codebases lately where people are making copies of copies of copies of arrays wasting so much memory and time it drags performance down noticeably for users. It also doesn't help that since these are all callback driven that means the overhead of functions for each and every iteration.

I'm not THAT familiar with JavaScript internals, but if this were a compiled language the assembly used for a function call -- inc SP to make room for vars, pusha, push args, , move copy of stack pointer into base pointer, do your function stuff. then popa and retf de-allocating the locals and arguments... I can only imagine the horror-show that must be in an interpreted language.

And for what? "Wah wah eye dunz wunna yuz teh four loupe?". I look at the crazy hoops a lot of people will go through just to avoid it, and it's utterly nutters.


const fruits = ["apple", "orange", "cherry"];
let text = "";
for (let fruit of fruits) text += fruie;
document.getElementById("main").textContent = text;

Not that one would do that in real life, given the power of Array.join.

Laugh is, I suggested using for/of on another site, and someone said "stop being stuck in the past, for loops are ancient"... IT's like "hey bozo, for/of is newer than Array.foreach" I mean For/of seems like it was created for the explicit purpose of being a more efficient method than Array.foreach!

But don't try to argue with the "every line needs to be its own function, for/if/swtich/case are teh evilz" folks.

But what do I know, I'd have coded your final example as:


// assuming main is empty

const
    fruits = ["apple", "orange", "cherry"],
    main = document.getElementByID("main");
    
for (let i = 0, fruit; fruit = fruits[i]; i++) main.append(
    i, ": ", fruit, document.createElement("br")
);

Since assignment as the comparison results in the fastest loop, given you have no -- and likely wouldn't have -- loose false values in the array. Also, append is awesome. Be even more awesome if it included references to the nodes it created.

And I trust innerHTML with data about as far as I could throw the USS Iowa. YMMV.

Adam Crockett 🌀 • Dec 29 '21 • Edited

GC good at cleaning up orphaned things in memory, perhaps this is my head cannon talking, but I trust that in memory non memory critical applications (99% of cases, not running on a Saga Saturn), map is just fine. I don't have a problem using array.map because the old array will likely get cleared up sooner or later

sadiul hakim • Dec 29 '21

Yeah,we should not use map method everywhere. There are many array methods available in Javascript like filter,find,foreach,reduce etc. We should use them then need.