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Array manipulation in JavaScript 🧐

thomasaudo on February 12, 2019

Introduction When we start programming, we tend do create our own function with our own loop to manipulate arrays. In reality, almost al...
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designalchemy profile image
Luke • Edited

Your brackets in your arrays make me feel nauseous

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l2aelba profile image
l2aelba

and the white texts :D

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qm3ster profile image
Mihail Malo

Horrible. Truly horrible.
How did these make it into the language?
And it was already after people started caring about JS performance, not before :/

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Alfonso

Can you explain your answer plis?

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Mihail Malo

These garbage Array -> Array methods made it into the runtime, instead of something good like iterator adapters.
Now numbers.filter(Number.isInteger).map(x=>x*2).reduce((a,b)=>a+b) does

const filterArr = []
for (const num of numbers) {
  if (Number.isInteger(num)) filterArr.push(num)
}
const mapArr = new Array(filterArr.length)
for (let i = 0; i < filterArr.length, i++) {
  mapArr[i] = filterArr[i] * 2
}
let acc = mapArr[0]
for (let i = 1; i < mapArr.length, i++) {
  acc += mapArr[i]
}

instead of numbers.filter(Number.isInteger).map(x=>x*2).reduce((a,b)=>a+b) doing

let acc
for (let i = 0; i < numbers.length; i++) {
  const num = numbers[i]
  if(!Number.isInteger(num)) continue
  acc = typeof acc === "undefined" ? num * 2 : acc + num * 2
}
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rokkoo profile image
Alfonso

I didn't know how works this functions at low levels, but yes, this was a really good answer.
Maybe I think when you are not looking optimization of code and only legibility of it, use reduce, filter, etc. can have sense, no?

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Mihail Malo

Legibility is extremely important.
I want everyone to write semantic, functional code without boilerplate.
But that depends on the language providing first-class support for it, instead of normalizing iterating 3 times instead of once and creating intermediate arrays.

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Darkø Tasevski • Edited

I find using map/reduce/filter much more easier to read than this mumbo jumbo:

if(!Number.isInteger(num)) continue
  acc = typeof acc === "undefined" ? num * 2 : acc + num * 2
}

This wouldn't pass any code review in my office... We're writing code for the machine but humans will need to read that code too, and as far as I know, performance gains are minimal, and in the end, you have @babel plugin which transforms map/reduce/find and similar methods into for loops.

Would like to add that you should use those "advanced" array methods only if you know what you're doing, in the end, it's really important for the developer to know what's going under the hood and to know, at least, to write some of those examples in regular for/while loops.

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Mihail Malo

I know exactly what they're doing, not just from practice but I've read the spec. One of the few ES specs I actually fully read.

These are just simplified versions of what is going on, what happens if we inline a crude implementation of the current spec vs what would be happening if it was done with transducers/iterator adapters, where all operations are applied to each item in order.
Also, the formatting in your quote is a bit strange.
It should look like this:

if(!Number.isInteger(num)) continue
acc = typeof acc === "undefined" ? num * 2 : acc + num * 2

But yeah, if it isn't clear by now, I didn't advocate that anyone actually write either of the two unrolled codes in a project, they were just desugared explanations.

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rokkoo profile image
Alfonso

Truly reasons, I'm a junior developer and I'm used to using these methods because they are easy to use and their implementation too.
In countable occasions, I need to work in low-level loops to search optimization of code.
Thanx for sharing your knowledge it has been helpful and this babel package.

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qm3ster profile image
Mihail Malo • Edited

Writing it by hand it would probably look something like this:

let sumOfDoubles = 0
for (let i = 0; i < numbers.length; i++) {
  const num = numbers[i]
  if(Number.isInteger(num))
    sumOfDoubles += num * 2
}

But if you are writing application code with a framework, and not making your own library, you will probably have better returns if you look at your algorithms and data structures. It's very rare to be limited by loop performance in a frontend application.
If your collection is like 10 items, and your filtering and mapping predicates are complicated, there's usually no shame in applying a filter and then a map separately.

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mdor profile image
Marco Antonio Dominguez • Edited

I agree depends on the case, some of the issues are related to maintainability other to performance issues, most of the cases is because some people is trying to overcomplicate things because some imaginary scalability issues.

Despite the fact the code is more performant, we should consider:
1 - Number of items
2 - Amount of operations
3 - Think in the different benefits coming from different worlds, FP, OOP, etc.

I saw more issues coming from extremely premature optimizations than performance issues, even with large datasets...

In addition, the code can be even readable

// example 1
const total = numbers.reduce((t, c) => t + (!Number.isInteger(c) ? 0 : (c*2)), 0);

// Example 2
let acc = 0;
for (let i = 0, size = numbers.length; i < size; i++) {
  const cur = numbers[i]
  acc += (!Number.isInteger(cur) ? 0 : (cur * 2));
}

// Example 3
const total = numbers
  .filter(cur => !!cur && Number.isInteger(cur))
  .reduce((t,c) => (t + c* 2), 0)
  // We clean up the falsy values (including 0), and we avoided another iteration

Again, it depends on the conditions,

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Raphaël Moreau

Thank you for these tips :)

One question :
console.log(arr.every(condition))
// Should output true

It should not be false instead ? (There is at least one human under 18 in the list).

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thomasaudo profile image
thomasaudo

Totally!
Thanks for noticing, I've fixed it :)

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Thamaraiselvam

Awesome dude

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Luke Davies

Thank you! this is a really handy page to bookmark

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Stephen Chiang

Just purely a question... But is using these convenience methods really more advanced than doing it manually without them? I can see arguments both ways...

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Cedric

Cool man,thanks. What about some() though?
developer.mozilla.org/en-US/docs/W...

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Mihail Malo • Edited

It tries the predicate on every item until it returns a truthy value.
If it reaches the end, returns false instead:

Array.prototype.some = function some(fn){
  for (const item of this) if (fn(item)) return true
  return false
}

The opposite is every():

Array.prototype.every = function every(fn){
  for (const item of this) if (!fn(item)) return false
  return true
}
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Matheus Luz

Finally some good examples of how this works, I've never really understood the image with food analogy

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Tomer

the every function was new to me . thanks

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MosALs

Thank u very much