Functions

Run your own JavaScript inside Shopify checkout — validate carts, customize delivery options and payment methods, apply automatic discounts, reshape cart lines.

Functions let you run your own JavaScript inside Shopify checkout. Unlike scripts, which run on DataJet's infrastructure in response to events and schedules, functions are deployed to Shopify and executed by Shopify itself — synchronously, while the buyer is checking out. That makes them the right tool for rules that must be enforced before an order can be placed.

You manage functions in the Functions Console section of the app.

Not to be confused with the Liquid function tag, which calls a reusable Liquid script from another script. This section is about Shopify checkout functions.

There are five categories of functions:

• Cart and Checkout Validation: block checkout with custom error messages — order limits, address rules, customer restrictions and any other rule you can express in code.

• Delivery Customization: hide, rename or reorder the delivery options shown at checkout.

• Payment Customization: hide, rename or reorder payment methods.

• Discounts: apply automatic product and order discounts based on your own rules.

• Cart Transform: merge cart lines into bundles, expand bundle SKUs, update line presentation (one per store).

Functions vs. scripts

	Scripts	Functions
Runs on	DataJet's infrastructure	Shopify's checkout infrastructure
Trigger	Events, webhooks, schedules, HTTP	Every checkout, automatically
Language	Liquid	JavaScript
Can call APIs	Yes (graphql, http, rest…)	No — sandboxed, input data only
Purpose	Automation and integrations	Checkout rules and customization
Credits	Consume credits	Free — no credits used

Creating a function

In the Functions Console, open a category folder and click Add new. You choose:

• Title — up to 100 characters; letters, numbers, spaces and _-|[] are allowed.

• Family — decides which checkout data your code receives as input. See Families and Variables for the full list. The family can't be changed after creation, so pick the one that covers the data your rule needs.

• Run on (validations only) — the checkout steps where your code executes: Cart, Checkout interaction and/or Checkout completion. The default is checkout interaction + completion.

New functions are created turned off, with starter code that documents the expected return shape for the category. Turn a function on from its menu in the tree, or with the Active toggle in the editor.

The number of functions you can have active at the same time depends on your plan:

Development and sandbox stores get the full limit of 25 (Shopify's maximum). During the free trial you can activate 1 function. You can create and edit any number of functions — the limit only applies to turning them on.

Active discounts additionally count against Shopify's store-wide cap of 25 automatic discounts, shared with the discounts created outside DataJet.

Plan	Active functions
Basic Plan	—
Advanced Plan	1
Pro Plan	5
DataJet Plan	25

Permissions

Managing functions requires additional access scopes (checkout validations, delivery customizations, payment customizations, discounts and cart transforms). If they haven't been granted yet, the Functions Console shows an Additional permissions required banner — click Grant permissions and approve the request.

Writing code

Your code is a JavaScript function body. It receives two variables: input, containing the checkout data selected by the function's family, and vars, holding your code variables (merged global + function scope). It returns a result whose shape depends on the category — validation errors, or delivery/payment/discount/transform operations. The editor's autocomplete knows the exact shape of both — type input. or vars. to explore them.

var errors = [];

if (parseFloat(input.cart.cost.totalAmount.amount) > 5000) {
  errors.push({ message: 'Orders above 5000 are not allowed', target: '$.cart' });
}

return errors;

The sandbox is minimal by design:

• ES2020 JavaScript — no browser APIs, no fetch, no network access.

• The only data available is the input object and your vars.

• Execution is synchronous and must be fast — it runs on every checkout.

Functions fail open: if your code throws an error, checkout continues as if the function returned nothing. Broken code never blocks your buyers — but it also means a faulty rule silently stops being enforced, so test your changes on a real checkout after saving.

The editor

• Save (or Cmd/Ctrl+S) deploys your code — if the function is active, the new version is live at checkout immediately.

• Unsaved changes are kept as local drafts per function, surviving tab switches.

• Function / Global / Query open the three variable scopes — code variables (per function and shared) and the family's query variables. See Families and Variables.

• Last saved opens the version history (Cmd/Ctrl+Shift+H) — every save is committed to git, and you can view and restore previous versions.

Every function can also be opened directly by link: /functions/<id> (the ID shown under the tree).

Managing functions from the dashboard

The main dashboard's Functions tab lists all functions. Clicking one expands its Function configuration — the merchant-facing form driven by variable metadata, for adjusting values without touching code. The tab also supports bulk turn on/off, delete, and export/import: selected functions download as a .datajet file (code, settings and variables included) that can be imported on another store.

The Shopify admin panel

Functions appear in the Shopify admin too — validations under Settings → Checkout → Checkout rules, customizations under their settings pages, discounts on the Discounts page. Cart transforms are the exception: they have no admin surface and are managed only from the Functions Console. Opening a DataJet rule there shows a panel with an Open in DataJet button that jumps straight to the function in the Functions Console.

Runtime errors and logs

There is no in-app test run yet — functions execute live at checkout. If your code throws, the error is recorded in the function's execution logs on Shopify's side and checkout proceeds unaffected. A Logs panel in the Functions Console is coming soon.