If you intended isIterable to be a public function (i.e., mixed into the _ object), then you should list it in modules/index.js. There should also be a comment before the function to explain its purpose. Take a look at a few other public modules to get a taste of how we write these comments.

If you intended isIterable to be only an internal function instead, then you should start the name of the module with a leading underscore, i.e., modules/_isIterable.js. In this case it would still be desirable to have a comment that explains the purpose of the function, but I'll readily admit that there are currently more internal functions where this is missing.

Also, you forgot export default.

This is an unnecessary comment.

We can't safely assume that Symbol or Symbol.iterator is defined, because that will (needlessly!) break compatibility with older environments that only know ES3. Instead, we would use feature detection and then create an internal getIterator function that simply returns undefined in environments where these objects don't exist. You can see an example of this practice in modules/keys.js, which uses Object.keys but only if it exists.

A similar, but more subtle, consideration applies to typeof x === 'function'. In some environments, this expression doesn't work as intended. This is why you should use _.isFunction instead whenever possible. isFunction(x) can also be minified much more compactly than typeof x === 'function'.

So this line should have read like this:

Given my previous comment, the entire body of this function would actually fit into a single expression:

Though getIterator should probably check against null and undefined anyway, so you could actually leave out the null check entirely.

Like with isIterable, you should either add a descriptive comment and list this function in modules/index.js if you intended this as a public function, or start the module name with _ otherwise. However, since this function seems to be purely an implementation detail of _.toArray, it should probably be internal, and maybe it should even be defined inside modules/toArray.js.

This blank line should stay.

When adding a feature to Underscore, you always also have to add tests to verify that the feature works as intended. This is especially important because this enables us to notice when a later change breaks a pre-existing feature.

I'm mentioning this here because _.toArray is already a public function. If isIterable or iteratorToArray was public as well, though, you'd have to add tests for that, too.