React – replacing an asynchronous .forEach() with for…of (and why it’s better that way)

In this post: My investigation into why ESLint didn’t like my .forEach loop and why for...of is the better choice for an asynchronous loop.

Key words: React, TypeScript, loops, JavaScript array iterators, forEach, for let, asynchronous

So there I was, .forEach-ing through some post replies when I noticed the code was triggering an ES-Lint error, no-loop-func:

Line 73:28: Function declared in a loop contains unsafe references to variable(s) 'repliesTotal' no-loop-func

My code seemed to work fine as it was, though.

const processUnreadReplies = async (userPosts: Post[]) => {
  let repliesTotal = 0;
  for await (let post of userPosts) {
    const replyArray: Array<Reply> = await getRepliesToPost(post.pid);
      replyArray.forEach((reply: any) => {
        if (!reply.read) {
          post.unreadReplyCount = (post.unreadReplyCount ?? 0) + 1;
          repliesTotal++;
        }
      });
  };
  setUnreadRepliesTotal(repliesTotal);
}

In English, this is what this code is doing:

  1. It gets all of the user’s posts as userPosts (and they are of type Post)
  2. It loops through each post in userPosts, getting that post’s replies as replyArray
  3. For each reply in replyArray, it checks each one to see if it is read or not by waiting on an asychronous function call
  4. If the reply is not read (reply.read is false), then it increases the post’s unreadReplyCount
  5. When it’s done with the loop, it sets a state variable called unreadRepliesTotal to the total it tallied up during the loop

ES-Lint’s documentation for no-loop-func was informative, but the examples didn’t make it clear enough to me what I had done wrong. Their “don’t do this” examples were "let i = 0, i < n; i++" and do ... while style loops, and I had a .forEach. They also didn’t have anything on the topic of asynchronous loops.

(Could my code be so awful that they didn’t even consider including it as a “do not do this” example? :D )

I decided to investigate.

First, I had to rethink my assumption that .forEach was the preferred ES6 style – maybe it wasn’t always the case, and maybe my case was one of them.

Two helpful (ie: plain English) posts I read while researching this problem:

For...in iterates through the enumerable properties of an object or array, which means it steps through all the X in object[X] or array[X]. You can still use it to access the elements of an array like so:

for (const idx in cars) {
  console.log(cars[idx]);
}

But that a more roundabout way of accessing the array data, and I soon found a more direct approach:

For...of was the next thing I tried and it worked just as well as my .forEach, but the linter liked it better.

const processUnreadReplies = async (userPosts: Post[]) => {
  let repliesTotal = 0;
  for await (let post of userPosts) {
    const replyArray: Array<Reply> = await getRepliesToPost(post.pid);
      for (let reply of replyArray) {
        if (!reply.read) {
          post.unreadReplyCount = (post.unreadReplyCount ?? 0) + 1;
          repliesTotal++;
        }
      };
  };
  setUnreadRepliesTotal(repliesTotal);
}

But why? I went back to the MDN docs and discovered an important tidbit I overlooked earlier:

forEach expects a synchronous function

forEach does not wait for promises. Kindly make sure you are aware of the implications while using promises (or async functions) as forEach callback. 

from the MDN docs on forEach

As far as ESLint could tell from looking at the code, every loop run by .forEach was returning void. Furthermore, using await does not pause the .forEach (I didn’t expect it to pause it, and I don’t need the previous iteration’s result for the next one, but I wanted to make note of that important distinction anyway).

In any case, this seemed like one of those times where the thing appeared to be working to the user, but could be done better and for...of was the preferred approach here. (There are plenty of opinions and debates about this, though.)

In summary

  • Use for...of for asynchronous loops in JavaScript/TypeScript
  • let creates a block-scoped variable, so in the case of my code the let is creating a new “reply” instance for each iteration of the for...of and each “reply” will be an individual reply from the array
  • .forEach() does not wait for asynchronous code to complete, which may not be a problem in your specific use case but ESLint can’t tell that so it flags it as a potential problem
  • await suspends the current function until it is resolved
  • for...of creates an individual function call for each loop, whereas .forEach() counts as one function call for all iterations

So there we have it – a better, ESLint-approved, way of writing an asynchronous loop in TypeScript/JavaScript.

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