Don’t pay the for-loop tax

Ok, I'm gonna be the old fart here.

1. If anything has a "tax," it's using map/reduce/filter, because these require defining a function (frequently an anonymous one) to perform the unit work, then that function is called for each iteration. So for N loops, you get at least N+1 additional function calls, with all of the associated overhead, compared to just having some code in a loop.

2. Calling map/reduce/filter doesn't eliminate the for loop, it merely moves it. The implementation of these methods has... you guessed it... a for loop.

3. Loops can break or return early. While findIndex() helps with some situations you'd need that for, other use cases don't have an equivalent.

4. Loops can modify their iterator variable, allowing them to skip over values (or even go back). Granted, this is an edge case.

I suppose the idea is that loops somehow require a mental tax? I've been using "for" and its siblings for over 30 years in just about every programming language, so they're pretty natural for me.

I do use these abstractions, and enjoy the syntactic sugar where it makes sense. But avoiding a loop structure should be a choice made with a full understanding of the pros and cons, it shouldn't be some sort of default because "indentation is a code smell" or similar hair-brained rules.

This expands the array and may cause it to reallocate in memory being slow

That's surely correct, but if you're interested in performance you should definitely use a for loop, as any solution based on callbacks, while more expressive, is one or two orders of magnitude slower, for small and large arrays:

console.time("Array.push");
var array = [];
for (var i = 0; i < 1e7; i++) array[i] = i;
console.timeEnd("Array.push");
// Array.push: 220ms

console.time("new Array.push");
var array = new Array(1e7);
for (var i = 0; i < 1e7; i++) array[i] = i;
console.timeEnd("new Array.push");
// new Array.push: 47ms

console.time("Array.map");
var array = [ ...Array(1e7).keys() ].map((_, i) => i);
console.timeEnd("Array.map");
// Array.map: 1209ms

But let's not forget that we also have for...in and, above all, for...of in JavaScript, which bring much nicer semantics.

These are my main cases of why I use for loops in JavaScript:

1. I'm releasing an open source package which could be used in performance-intensive tasks;
2. my code needs to be transpiled and things like for...of are polyfilled with an unbearable amount of code (in the case Babel can't infer if the collection is actually an array or any other kind of iterable);
3. because - ah, screw it - it's clear enough since it's a pattern everyone knows, while building an array with [ ...Array(n).keys() ].map(...) is quite obscure at the first glance.

The rest of the article is pretty much spot-on. Concepts that need to be reiterated.

The for(;;) construct is error-prone but I feel "going functional" in JavaScript is hardly a good solution.

Take for example the sum() function from the post. In Haskell, it would be defined this way:

sum' l = foldl (+) 0 l

Or here it is in Scheme:

(use-modules (srfi srfi-1))
(define (sum l)
    (fold + 0 l))

The sum' function is quite concise because the (+) operator is already a function. Even Python (where such a function would be frowned upon) could make it clearer:

import operator
from functools import reduce

def sum(l):
    return reduce(operator.add, l)

So, when we go to JavaScript, we got this very weird line (total, current) => total + current, where there is a total, and a current, and they are added up. Now I have a bit more to understand here. The for(;;) loop is brittle, but why not this?

const sum = (array) => {
    let result = 0;
    for (let i in array) {
        result += array[i];
    }
    return result;
}

Or, better, this?

const sum = (array) => {
    let result = 0;
    for (let v of array) {
        result += v;
    }
    return result;
}

There is a mutable variable but it is clearer what is going on. The JavaScript syntax is helping me here. Do you disagree?

I find trying to optimize for sum() functions problematic. sum() is a solved problem, whatever the solution you choose. But get another code where one calls, let us say, map() inside the function given to reduce(). It is surprising confusing. Given the trend to avoid naming functions in JS, it will become a mess.

Also, many functional patterns are helpful in functional languages, or specific situations, but not in all places. Many JS programmers write apply(compose(fn1, fn2.bind(arg)), value) because they think it is as good as f1 . (f2 arg) $ value. But is it? I don't think so. JavaScript didn't make this construction clear, as Haskell did.

Functional patterns make sense a lot of times. If you have an algorithm which can be reused except for one part, it will be great to pass the function (instead of, let us say, a Strategy pattern house-of-cards). Functions as values are a cornerstone for asynchronous code. I even like the each()/forEach() methods, so I do not have to discover if it uses a[0], a.get(0), a.item(0).

However, in general, a "more functional" JavaScript code will not be better than one that uses some loops. Trying to get better code by being "more functional" is very much like improving by being "more OO." And we know the result: how many Command pattern instances we found that should have been a function?

Now I wonder the same about lambdas that could be a for(of) block.

I think the only argument for using a a for-loop is to break out of the loop. I think pretty much everything else can be done using the functional approach.

Let's say you have to build a nightly batch process to process milions of entities (orders, invoices, bank transactions ... whatever). A typical approach would be to open a result set (or the equivalent structure in JS) to process the entities one by one and do partial commits. With a functional approach, will you read the milions of entities into an array just to use map/filter/reduce ? I hope your server has some Gigabytes of contiguous memory free, otherwise it wil fail miserably. The optimal solution will use a loop.

My argument has always been that writing a for loop that takes each element in an array, transforms it, and adds the output to another array, is writing your own implementation of map.

map is a well understood function these days that everyone can read so don't keep re-implementing it. It's just more code for you to own and read, and there's a chance of bugs in your implementation. To put it another way, you should use the map function for the same reason you use any functions at all: to avoid repeating code.

The same goes for filter and reduce.

RE performance: Measure it in the context of your application. I doubt there is a difference in the vast majority of cases given engine optimisations, but you can always fall back to a for loop where you can prove it's worth doing.

for-loop-tax is accompanied with an array-method-tax.

I agree the first go-to should be array-methods, even with objects "for in" usually sucks compared to Object.keys(foo).map((propName,i)...) as you get i included.

The exceptions IMO are performance, break, continue and return.

It requires discretion because in some circumstances you could sometimes be writing spaghetti code when trying to be too functional where break, continue & return "sometimes" result it far less code and better maintainability (sometimes). But obviously vice versa as you've demonstrated.

I 100% agree with mantra here, but I'd like to add to the examples. For me, the functional style/indirection in these examples IS the value, and once you're doing things in a functional way you've already captured most of the technique's purpose. For me, whether you continue to use a declarative iterator or a for loop inside your abstraction is less important.

Here's an admittedly naive example of the price discounting that I think very clearly shows the difference between a for loop and an Array.forEach.

let prices = [5, 25, 8, 18];
let discount = 1 - 0.2;

// more imperative
for (let i = 0; i < prices.length; i++) {
    console.log(prices[i] * discount)
};

// more declarative
prices.forEach((price) => {
  console.log(price * discount);
});

Just reading these aloud to yourself shows the difference in clarity:

"For when i equals zero and i is less than the length of prices while incrementing i, console log prices at position i times discount."

vs.

"Prices: for each price, console log price times discount".

When moving from an imperative to declarative style, the code turns from near gibberish to an honestly comprehensible English sentence. I think that's a win for the whole team at every skill level, and the value this style is attempting to capture.

A for loop has one great advantage over every other approach: it's common. I know how a for loop works and I can rely on it working in different languages and projects. It also has a common meaning with a single concept.

While I'm strongly in favour of higher level functions, it's difficult to remember which language has which, and precisely what they do. Is there a map, fold or join? What types does it support? What if a need a small variation, like fold_right instead?

I know how to do all of this in a for loop, so it doesn't bother me to use it if I'm unaware of another solution. If somebody points out an alternate syntax, I'll gladly switch the loop.

I could not agree more with this article. A map, filter, or reduce tells me what is going on without looking too deeply at the code. A for loop requires a careful eye to make sure you don't miss a side effect when you refactor something. The performance difference for most cases hardly compares to the maintenance benefits.