问题
I recently learned about optional chaining in Javascript and have been making use of it in a React/NodeJS project. Works great.
I noticed I have been using it with arrays map, even without thinking about it much -- it seemed a natural use (here items is an array, or possibly undefined)
{items?.map(postListItem => ....
That is, it will map if items exists, but not if items is undefined, but would avoid any run-time errors if I were to call map on undefined
Nonetheless I don't know if this is an acceptable use or whether I'm mis-using optional chaining. I searched for an answer but as of yet haven't been able to find one, which makes me suspect I'm mis-using it. Any info much appreciated!
回答1:
If the chaining fails, the expression will evaluate to undefined.
When a child is evaluated to undefined, it just won't render.
Conditional rendering is already quite a common strategy. Previously, when you have something that may be an array or may be undefined, and you want to map if there's an array, you would have had to do something like:
{ items && items.map( ...
This works because, if items is undefined, the whole expression will be evaluated to undefined, and no rendering will happen, and no errors will be thrown.
Using optional chaining works exactly the same way, except that it's more concise. So yes, this is a perfectly valid thing to do.
Optional chaining is stage 4, so you can count on it working reliably.
Arrays are objects. But optional chaining isn't only for objects - it can work for anything which may have a property or method. Eg const bar = foo?.toFixed(2) will work fine if foo is null, undefined, or a number (numbers have a toFixed method).
回答2:
The optional chaining way to your problem is:
{items?.map?.(postListItem => ....)
Read more here: MDN Web Docs: Optional chaining
来源：https://stackoverflow.com/questions/60165042/is-it-possible-to-combine-optional-chaining-with-arrays-and-map-in-javascript