Help Me Solve an Alpine.js Mystery

[giscus[bot]]

Help Me Solve an Alpine.js Mystery

Help me figure out some interesting Alpine.js behavior in regards to loops and variables.

https://www.raymondcamden.com/2024/08/01/help-me-solve-an-alpinejs-mystery

[JimCollinsDC]

I asked chatGPT and it said
"The bug appears with user interaction but not with the delayed version because of how Alpine.js manages reactivity and scope. In the delayed scenario, the variables and their bindings are fully established before user interaction begins, so the conflict doesn't manifest. However, when the user triggers the display immediately, Alpine.js tries to handle the bindings dynamically, which exposes the naming conflict between the loop variable and the model binding, causing the unexpected behavior."

[cfjedimaster]

I don't think that makes sense. And heck, in the last version, if I wait 5 seconds and click, I get the same issue. :\

[bennadel]

I believe the issue has to do with what this is bound to at the time of the click. When the user clicks on the button inside the x-for loop, the this scope is bound to the implicit scope created by the x-for loop. You can verify this by changing the x-for to include the iteration index value:

x-for="( cat, i ) in cats"

... and then logging console.log( this.i ) inside the click-handler. You will see that it shows 1 and 2 for the two buttons, respectively.

So, when you attempt to assign this.cat inside that click handler, you're actually assigning it to the x-for implicit scope, not to the root scope. You're basically overwriting the row-level .cat property, which is why the td values change.

This is also why inside an event handler this.$el is a reference to the element that triggered the event (ex, click) as opposed to the element that hosts the x-data. I think this was a breaking change they made going from v2 -> v3 (but I'm not exactly sure).

[cfjedimaster]

But - and this is the fault of me having messy code trying to make a repo - the buttons in each row are not meant to be specific. Yes, I pass cat.id to showCat, but you can see I don't actually use it.

But... you are kinda right. Check out this mod:

https://codepen.io/cfjedimaster/pen/jOjZvLy

This gets rid of the buttons in the table row and just adds a button at the bottom. That does NOT show the bug. So even though both buttons (in the last version in my main blog post and the button I linked to in the paragragraph above) functionally do the same, ones broken and ones not.

[cfjedimaster]

For the hell of it, I made another mod: https://codepen.io/cfjedimaster/pen/abgqaLK

This keeps the buttons in the table row, but removes cat.id from the argument. It's literally just @click="showTheCat()" and it has the bug.

[bennadel]

Right, ultimately the issue is that this.cat inside the x-for is pointing to the most local cat in the scope-array. You'd have to jump over the x-for scope when updating the value I think:

async showTheCat() {
	var scopeTree = Alpine.closestDataStack( this.$el );
	var rootScope = scopeTree.at( -1 );

	// Jump OVER the x-for scope and update the root scope directly.
	rootScope.cat = await getCat();
	console.log('got cat');
	this.showCat = true;
}

[cfjedimaster]

Fascinating so.... when the method (showTheCat) is run inside the loop, this.cat is still the loop one. Holy smokes... that... that does kind of make sense and feels like something REALLY hard to debug. ;)

[bennadel]

Yeah, it's definitely funky. If this were vanilla JavaScript, your call to showTheCat() would walk up the native prototype chain, locate the method, and invoke it; and then, all references to this within that function would be the same scope. And your this.cat reference would be exactly what you expect it to be.

But, I think what's happening here is that all the @click expression is being evaluated in the x-for implicit scope. Then, the showTheCat() "reference" is being exposed via the Proxy array. It's confusing; you'd definitely have to put in a number of debugger statements to see it all in play :D

[bennadel]

Here's the $el breaking change -- https://alpinejs.dev/upgrade-guide#el-no-longer-root