Store cart ID in cookies

[scottdixon]

Hey @lordofthecactus! Let me know what you think of this approach.

WHY are these changes introduced?

When switching from Shopify's Online Store to Hydrogen, existing customer carts are lost unless the merchant updates their liquid theme to use the Storefront API.

Soon, liquid/Storefront API carts will be interoperable - carts created on a traditional liquid storefront can be managed via the Storefront API and vice versa.

This change leverages the new functionality and shows an example of how to switch between Online Store/Hydrogen without losing carts.

WHAT is this pull request doing?

Updates the Demo Store template to use the same cart cookie set by the traditional Online Store channel.

HOW to test your changes?

1. Enable cart_repository_online_store_path on your test store.
2. Create a cart using the traditional Online Store and copy the cart ID from the cart cookie.
3. Running Demo Store locally, create the same cart cookie.
4. Switch between Hydrogen/Online Store and the cart should remain in sync.

Feel free to use my test store:

PUBLIC_STORE_DOMAIN="test-shop-987.myshopify.com"
PUBLIC_STOREFRONT_API_TOKEN="b5f455ac1cea1522791da1ad0e053559"

Post-merge steps

Will remove most of these docs: https://shopify.dev/docs/custom-storefronts/oxygen/migrate/share-carts

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[lordofthecactus]

Hi @scottdixon 👋 PR works well! I have been 🎩 (tophatting) with your demo store and I could go back and forth the online storefront and hydrogen 🔥

One request before merging!

I also did a deep dive on sessions in remix and made sure:

• Using request.headers is alright
• The Set-Cookie works as intended and does not affect other parts of the app. Only if we have another Set-Cookie with the same cookie name would this get overwritten.

Also:

• fixed the types
• rebased from latests

[lordofthecactus]

This needs to be a patch version. We are using calver, for example 2023.1.0 for jan 2023. So we are using the minor part of the version for the month.

Also, could you describe the changes as @wizardlyhel did here?
https://github.com/Shopify/hydrogen/blob/d827cc2d66dc08c3ce7da15a0723b061dd8039f5/.changeset/angry-months-pay.md

[lordofthecactus]

@scottdixon cart was complaining here because when you parsed the cookie with getCartId the function would have string | undefined union.

Then some of the actions below required to have a string. So instead I just did a check with invariant if the cartId actually exists.

[lordofthecactus]

@scottdixon I'm also wondering if we should update to add some text here https://shopify.dev/docs/custom-storefronts/hydrogen/cart/add#persist-and-return-the-cart about this.
Maybe some sort of note, that if cart wants to be shared with online storefront, to setup a plan cookie, not within session

[scottdixon]

Thanks so much @lordofthecactus! I've updated changeset and added a note to the cart docs as part of this PR which will roll out once the feature is live. LMK if I've missed anything 🙏

[lordofthecactus]

LGTM ✅