Shopify Functions Starter: Field Kit

Fit the rule

Write the rule in one sentence, such as 10% off lines with 3 or moreA vague rule becomes vague code, and edge cases surface at checkout instead of in review.
Classify it as a discount, delivery, payment, or validation ruleFunctions cover only these extension points, so anything else needs a different tool.
Check the specific Function type is available on your Shopify planMany types work beyond Plus, but some checkout customizations do not, so confirm first.
Confirm the rule needs no external API call during checkoutFunctions cannot call out mid-checkout, so a rule that needs live outside data will not run.
Store any outside data ahead of time in metafields or cart attributesData the Function cannot read from its input is data your rule cannot use.

Write it

Scaffold the extension inside an app with the Shopify CLIFunctions are not pasted into the admin, so there is no other supported way to ship one.
Request only the fields the logic uses in the input queryExtra data slows the Function and pushes it toward its strict time and size limits.
Put the business rule in the run function and return operations onlySide effects have no place in a Function, and logic that relies on them will not behave.
Return an empty discounts list when no line qualifiesAn unhandled no-match case risks a failed run or an unintended discount at checkout.
Test edge cases: empty carts, boundary quantities, mixed linesA pricing bug in checkout applies to every customer until someone notices the margin.

Ship it

Deploy with shopify app deploy so the Function is versioned with the appVersioned deploys make changes deliberate and reviewable instead of live edits.
Create the discount in the admin or API pointing at the FunctionA deployed Function does nothing until a discount or customization points at it.
Place a real test order and confirm the operation applies as expectedOnly a real checkout proves the rule, the input and the discount wiring work together.
Cancel the app or Script the Function now replacesLeaving the old tool running keeps the fee and can stack two discounts on one order.

How to use this

Take one rule through all three sections before starting a second. A good result: a single versioned Function that expresses your exact rule, applies on a real order, and replaces a Script or paid app.

The rest of the kit is one step away.

Leave your email and the full kit opens right here, ready to work through or save as a PDF.

Kept private. Never shared or sold.