# Write a function in code

FunctionFlow AI · Build functions · Updated October 3, 2026

> Write Shopify Functions in JavaScript when the visual builder isn’t enough — pick an input, start from a snippet, test against real function input and publish.

The code editor lets you write discount, shipping, payment and validation functions in JavaScript. Your code runs inside Shopify’s checkout, in the same sandboxed runtime as any Shopify Function. The code editor is part of the **Enterprise** plan.

![The code editor with the JavaScript editor, test data tab and function input options](https://help.fleekapps.com/media/functionflow-ai/code-editor.webp "A new discount function in the code editor, starting from a working snippet.")

## Start a code function

Open **Code editor** in the app menu and choose what to build:

- **Product & order discount** — your own discount logic for products or the order total.
- **Shipping discount** — free or discounted rates with any condition you can code.
- **Shipping methods** — hide, rename or reorder the rates at checkout.
- **Payment methods** — hide, rename or reorder payment methods.
- **Checkout validation** — block checkout with your own rules and messages.

Each one starts from a working snippet. Your code functions are listed under **Code functions** on the same page.

## How the code works

Your code is a **function body**:

- `input` is the function input Shopify sends — the cart, customer and so on.
- You `return` the result.

```js
// 20% off cart lines whose product has one of the tags set under Input variables.
const candidates = input.cart.lines
  .filter((line) => line.merchandise.product?.hasTags.some((t) => t.hasTag))
  .map((line) => ({
    message: '20% off sale items',
    targets: [{ cartLine: { id: line.id } }],
    value: { percentage: { value: 20 } },
  }));

if (candidates.length === 0) return { operations: [] };

return {
  operations: [{ productDiscountsAdd: { candidates, selectionStrategy: 'ALL' } }],
};
```

Shopify’s current format (`{ operations: [...] }`) works for every kind:

| Kind | Operations |
|---|---|
| Product & order discounts | `productDiscountsAdd`, `orderDiscountsAdd` |
| Shipping discounts | `deliveryDiscountsAdd` |
| Shipping methods | `deliveryOptionHide`, `deliveryOptionRename`, `deliveryOptionMove` |
| Payment methods | `paymentMethodHide`, `paymentMethodRename`, `paymentMethodMove` |
| Checkout validation | `validationAdd` with `errors: [{ message, target }]` |

Older formats from other function apps — `{ discounts }`, `{ errors }` and `hide`/`rename`/`move` operations — are converted automatically, so scripts written for other apps usually work as they are.

## Choose the function input

Only fields in the function input reach your code — anything else is `undefined` at checkout. In the **Function input** card, pick the input your code needs:

- **Product & order discounts:** Standard, Customer, Products or Line properties input.
- **Shipping discounts:** Standard, Delivery or Address input.
- **Shipping methods, payment methods and validation:** Standard input.

Click **View available fields** to see exactly which fields the chosen input contains.

### Input variables

Some fields depend on values you set in **Input variables** — for example which tags `hasTags` checks:

- **Cart attribute key** and **Line property key** — which attribute or property to read.
- **Customer tags** and **Product tags** — checked by `customer.hasTags` and `product.hasTags`.
- **Collection IDs** — checked by `product.inCollections`, e.g. `gid://shopify/Collection/123`.

After changing tags, use **Reset to sample** on the **Test data** tab to refresh the sample input.

## Snippets

The **Snippets** menu has working examples for each kind, such as *VIP customers: 15% off the order*, *Free shipping over $100*, *Hide Cash on Delivery above $300* or *Minimum order of $50*. Choosing one replaces your code (you’ll be asked first if you’ve edited it).

## Test your code

1. Open the **Test data** tab. It holds a sample function input for your chosen input — edit it to try other carts.
2. Click **Run test**.

The **Test result** card runs your code in Shopify’s function runner and shows:

- **What happens at checkout** — the changes your code makes, or *Nothing changes for this cart*;
- the **instructions** used, out of Shopify’s 11 million per run;
- the raw **Function output**;
- **Logs** — anything you `console.log`;
- errors with a **Go to line** link.

## Discount settings

For discounts, choose:

- **Discount type** — product and order, or shipping.
- **Discount classes** — product discounts, order discounts or both. Operations for a class you haven’t ticked are ignored.
- **Method** — automatic or discount code.
- **Can combine with** — order, product and shipping discounts.

## Save and publish

**Save draft** checks your code for syntax errors and saves it. **Publish** puts it live in your store (on a live function the button reads **Save and update**). Code functions get [version history](https://help.fleekapps.com/functionflow-ai/version-history/) like any other function, and appear on the Functions page.

Code functions don’t have test mode or active dates — use them when you need full control.

## Limits

- Code can be about **9 KB**, and code plus settings at most **10 KB**.
- At most **11 million instructions** per run.
- Up to 100 values per tag or collection list.

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Source: https://help.fleekapps.com/functionflow-ai/code-editor/