Don't mutate the iterator

Or better said, don't mutate the iterator you're iterating over. This is a common mistake that can lead to unexpected results. And yes, despite writing about mutating and not mutating before, I still got bitten by it. So let's take a look at what happened.

The problem

I needed to remove all empty headers before a request. For this, I created a function that mutates the request.headers object. Let's take a look:

function deleteEmptyHeaders(headers: Headers) {
  headers.forEach((value, key) => {
    if (value && value !== 'undefined' && value !== 'null') return;

This function iterates over the headers, and removes the ones that are "empty". Usage would be like:

const headers = new Headers();
headers.set('a', '');
headers.set('b', '');
headers.set('c', '');
headers.set('d', '');

Object.fromEntries(headers); // { b: '', d: '' } // ?!?

The problem is that we're mutating the iterator we're iterating over. By removing the first header, the second header shifts up to the first position. Like when you're pulling a book from a pile of books. As the iterator is now in the new second position, it never processes "header b" but instead processes "header c" as the new second header. And so on.

It's an old problem that we know of iterating over indexes using a for loop, like for (let i = 0; i < items.length; i++). When we remove an item from the array, the indexes shift, and skip items. The same thing happens here. It's just harder to spot.

The solution

So how do we fix this? Well, we can't mutate the iterator we're iterating over. So we need to create a new iterator. We can do this by using the Array.from method.

function deleteEmptyHeaders(headers: Headers) {
  const entries = Array.from(headers.entries());
  entries.forEach(([key, value]) => {
    if (value && value !== 'undefined' && value !== 'null') return;

Instead of Array.from we can also use the spread operator [...headers.entries()], and calling the .entries() method is optional as it's the default method to be called on such actions. So let's apply this:

function deleteEmptyHeaders(headers: Headers) {
  [...headers].forEach(([key, value]) => {
    if (value && value !== 'undefined' && value !== 'null') return;

Now we don't modify the iterator we're iterating over, and we end up with the expected result. Or said differently, the index of the current header isn't affected by the delete action, as we read a different pile than we modify.

Please do notice that the key and value props got reversed when we moved from headers.forEach to array.forEach.


Twitter being Twitter, I got a lot of feedback on my snippet. From folks being unable to spot the bug, to folks saying that a return in a forEach is bad practice, to completely burning me down. So let's take a look at some of the feedback.

folks that write code like this, get what they deserve

Yeah, thanks for the constructive feedback. Now go away. I didn't want to quote this one at first, but it's important to highlight that this is not the way to give feedback. It's not constructive, and it's not helpful. It's just rude.

don't mutate

A couple of folks suggested that I shouldn't mutate. Well, I don't have another option. I need to modify the request headers before the request is made. And request.headers is a read-only property. It is what it is. Tho I agree that generally speaking, modifying the input arguments is not good practice. As always, there are exceptions to the rule.

The other option would be to not set the header in the first place. If you have that option, go with it. At other times, cleanup is the only option we have.

don't return in forEach

I don't get it. The callback is a function. We can use early returns to reduce indentation and keep code readable. I don't see the problem here. Some folks suggested using continue or break instead, but that's not going to work. It's a function; we can't use those keywords. Returning from a forEach to exit early is fine. Note that it won't stop the iteration, but it will stop the execution of the callback.

key and value are reversed

Yeah, I know. It is the function signature tho. Seriously, why did TC39 decide to put the value first? It's so confusing.

const headers = new headers();

// value, key
headers.forEach((value, key) => {});

// key, value
[...headers].forEach(([key, value]) => {});

you're checking 'null' and 'undefined' instead of null and undefined

Yep. headers.set coerces the value into a string. header.set('key', null) will result in headers.get('key') === 'null'.

use a common loop

Sure, possible. This is the direct translation, including the same bug. Using forEach or a traditional loop is about personal preference. Switching between them doesn't change a thing.

function deleteEmptyHeaders(headers: Headers) {
  for (const [key, value] of headers) {
    if (value && value !== 'undefined' && value !== 'null') continue;

Interestingly enough, this makes the bug easier to spot for me. It's hard to say if that's because I'm used to this pattern or because I now know it's there.


Just a couple of lines and so many opinions and lessons. The "common for loop" could have been suggested in a kinder tone, but seeing this implementation in a traditional loop does remind me why I so strongly prefer those over helper methods like map and forEach.

This is the final solution I came up with and the way I've implemented it in fetch-addons.

function deleteEmptyHeaders(headers: Headers) {
  for (const [key, value] of [...headers]) {
    if (value && value !== 'undefined' && value !== 'null') continue;

request.headers.set('x-api-key', apiKey);
request.headers.set('x-user-id', userId); // string | null | undefined

// somewhere in a middleware

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