Discount Functions

Overview

Discount Functions let you create custom discount logic beyond Shopify’s built-in discount types. Volume discounts, bundle pricing, tiered discounts, and customer-specific pricing.

How Discount Functions Work

Customer Action

Customer adds items to cart or enters checkout

Function Execution

Your function receives cart data and evaluates discount logic

Discount Applied

Function returns discount configuration

Price Updated

Shopify applies discount and updates cart/checkout

Discount Types

Volume Discounts

Discount based on quantity

Buy 2 get 10% off
Buy 5 get 20% off
Tiered pricing

Bundle Discounts

Discount on product combinations

Buy X + Y get Z free
Complete the set discount
Mix and match deals

Customer-Specific

Personalized pricing

VIP customer discounts
Loyalty tier pricing
Wholesale pricing

Conditional Discounts

Context-based pricing

First-time buyer discount
Time-based promotions
Location-based pricing

Example: Volume Discount

Generate a function that gives tiered discounts:

Describe Your Function

Create a discount function that gives 10% off when buying 
2-4 units of the same product, 15% off for 5-9 units, 
and 20% off for 10 or more units. Show the discount in 
the cart and checkout.

Review Generated Code

Synapse creates:

Rust or JavaScript function
Input query for cart data
Discount calculation logic
Output configuration
Extension UI (optional)

Deploy & Test

Automatic validation
Deployed to dev store
Test with different quantities

Activate Function

Go to Discounts in admin
Create new discount using your function
Configure settings
Set active dates

Generated Function Structure

extensions/volume-discount/
├── src/
│   ├── run.rs                 # Function logic (Rust)
│   │   OR
│   ├── run.js                 # Function logic (JS)
│   └── run.graphql           # Input query
├── shopify.extension.toml     # Config
├── Cargo.toml                 # Dependencies (Rust)
│   OR
└── package.json               # Dependencies (JS)

Function Code Examples

Rust (Recommended)
JavaScript
Input Query

Rust functions are faster and more efficient:

use shopify_function::prelude::*;
use shopify_function::Result;

#[shopify_function_target(query_path = "src/run.graphql", schema_path = "schema.graphql")]
fn run(input: input::ResponseData) -> Result<output::FunctionRunResult> {
    let discounts = input
        .cart
        .lines
        .iter()
        .filter_map(|line| {
            let quantity = line.quantity;
            
            let percentage = match quantity {
                2..=4 => 10.0,
                5..=9 => 15.0,
                10.. => 20.0,
                _ => return None,
            };
            
            Some(output::Discount {
                message: Some(format!("{}% volume discount", percentage)),
                targets: vec![output::Target {
                    product_variant: Some(output::ProductVariantTarget {
                        id: line.merchandise.id.clone(),
                        quantity: None,
                    }),
                }],
                value: output::Value::Percentage(output::Percentage {
                    value: Decimal::from(percentage),
                }),
            })
        })
        .collect();
    
    Ok(output::FunctionRunResult { discounts })
}

JavaScript functions are easier to modify:

export function run(input) {
  const discounts = input.cart.lines
    .map(line => {
      const quantity = line.quantity;
      
      let percentage;
      if (quantity >= 10) {
        percentage = 20;
      } else if (quantity >= 5) {
        percentage = 15;
      } else if (quantity >= 2) {
        percentage = 10;
      } else {
        return null;
      }
      
      return {
        message: `${percentage}% volume discount`,
        targets: [{
          productVariant: {
            id: line.merchandise.id
          }
        }],
        value: {
          percentage: {
            value: percentage.toString()
          }
        }
      };
    })
    .filter(discount => discount !== null);
  
  return { discounts };
}

GraphQL query defines function input:

query RunInput {
  cart {
    lines {
      id
      quantity
      merchandise {
        __typename
        ... on ProductVariant {
          id
          product {
            id
            title
            hasAnyTag(tags: ["volume-discount"])
          }
        }
      }
    }
  }
}

Common Discount Patterns

Buy X Get Y

Buy one product, get another free/discounted:

// Check if customer has product X
let has_product_x = input.cart.lines.iter()
    .any(|line| line.merchandise.product.id == "gid://shopify/Product/123");

if has_product_x {
    // Apply discount to product Y
    discounts.push(Discount {
        targets: vec![Target {
            product_variant: Some(ProductVariantTarget {
                id: "gid://shopify/ProductVariant/456".to_string(),
            }),
        }],
        value: Value::Percentage(Percentage { value: Decimal::from(100) }),
        message: Some("Free with purchase!".to_string()),
    });
}

Prompt Example:

"Create a discount function where if customers buy any t-shirt 
(tagged 'tshirt'), they get 50% off any socks (tagged 'socks')"

Spend Threshold

Discount when cart total reaches threshold:

use rust_decimal::Decimal;

let cart_total: Decimal = input.cart.lines.iter()
    .map(|line| {
        let price = Decimal::from_str(&line.merchandise.price).unwrap();
        price * Decimal::from(line.quantity)
    })
    .sum();

if cart_total >= Decimal::from(100) {
    // Apply 10% off entire cart
    discounts.push(Discount {
        targets: vec![Target::OrderSubtotal],
        value: Value::Percentage(Percentage { 
            value: Decimal::from(10) 
        }),
        message: Some("10% off orders over $100".to_string()),
    });
}

Prompt Example:

"Create a discount that gives 15% off the entire order when 
the subtotal is $150 or more"

Customer Segment

Discount for specific customer groups:

// Check customer tags
let is_vip = input.cart.buyer_identity
    .as_ref()
    .and_then(|identity| identity.customer.as_ref())
    .map(|customer| customer.has_any_tag(vec!["vip"]))
    .unwrap_or(false);

if is_vip {
    // 20% off for VIP customers
    discounts.push(Discount {
        targets: vec![Target::OrderSubtotal],
        value: Value::Percentage(Percentage { 
            value: Decimal::from(20) 
        }),
        message: Some("VIP member discount".to_string()),
    });
}

Prompt Example:

"Create a discount function that gives 25% off to customers 
tagged 'wholesale' on all products in the 'bulk' collection"

Bundle Builder

Discount for buying complete sets:

let required_products = vec![
    "gid://shopify/Product/1",
    "gid://shopify/Product/2",
    "gid://shopify/Product/3",
];

let has_all = required_products.iter().all(|product_id| {
    input.cart.lines.iter()
        .any(|line| line.merchandise.product.id == *product_id)
});

if has_all {
    // 30% off when buying complete bundle
    discounts.push(Discount {
        targets: required_products.iter().map(|id| {
            Target::ProductVariant(/* ... */)
        }).collect(),
        value: Value::Percentage(Percentage { 
            value: Decimal::from(30) 
        }),
        message: Some("Bundle discount!".to_string()),
    });
}

Prompt Example:

"Create a bundle discount where if customers buy a camera, 
lens, and memory card together, they get 25% off the total 
for those three items"

Function Configuration

Functions can have configurable settings:

# shopify.extension.toml
[[extensions.settings]]
name = "min_quantity"
type = "number_integer"
description = "Minimum quantity for discount"

[[extensions.settings]]
name = "discount_percentage"
type = "number_decimal"
description = "Discount percentage to apply"

[[extensions.settings]]
name = "eligible_products"
type = "product_reference"
description = "Products eligible for discount"

Access settings in your function:

let min_quantity = input.discount_node.metafield
    .as_ref()
    .and_then(|m| m.value.parse::<i64>().ok())
    .unwrap_or(2);

let discount_percentage = input.discount_node.metafield
    .as_ref()
    .and_then(|m| m.value.parse::<f64>().ok())
    .unwrap_or(10.0);

Testing Discount Functions

Create Discount in Admin

Go to Discounts
Create → Discount
Select your function
Configure settings

Test in Storefront

Add eligible products to cart
Verify discount appears
Check discount amount is correct
Test edge cases

Review Logs

Check function execution logs:

shopify app function logs

View input/output for each execution

Performance Testing

Test with many cart items
Measure execution time
Verify under 5ms threshold

Performance Best Practices

Discount functions must execute in under 5ms:

Minimize Loops

Avoid nested loops over cart items

// Good: Single pass
let total = lines.iter().map(|l| l.price).sum();

// Bad: Nested loops
for line in &lines {
    for other in &lines {
        // ...
    }
}

Use Rust

Rust functions are 10-100x faster than JavaScript

Compiled to WebAssembly
No garbage collection
Type-safe
Recommended for production

Cache Calculations

Don’t recalculate the same value

// Calculate once
let cart_total = calculate_total(&input.cart);

// Reuse
if cart_total > threshold {
    // ...
}

Limit API Surface

Only query data you need in run.graphql

# Good: Specific fields
query RunInput {
  cart {
    lines {
      quantity
      merchandise { id }
    }
  }
}

Debugging Functions

Local Testing
Live Logs
Error Handling

Test functions locally before deploying:

cd extensions/discount-function

# Test with sample input
shopify app function run

# Provide test input
cat test-input.json | shopify app function run

View function executions in real-time:

shopify app function logs --watch

Shows:

Input received
Output generated
Execution time
Errors

Functions should never panic:

// Good: Handle errors
let quantity = line.quantity.parse::<i64>()
    .unwrap_or(0);

// Bad: Panic on error
let quantity = line.quantity.parse::<i64>()
    .expect("quantity must be number");

Limitations

Discount Functions have certain restrictions:

Limitation	Details
Execution Time	Must complete in under 5ms
No Network Calls	Cannot fetch external data
Deterministic	Same input must give same output
Stateless	No access to previous executions
Cart Data Only	Limited to provided input query

Example Prompts

Tiered Spend Discount

Create a discount function with multiple spend tiers:
- $50-$99: 5% off
- $100-$199: 10% off
- $200+: 15% off
Apply to entire order. Show the tier achieved in the discount message.

Mix and Match

Create a discount where customers get 20% off when they buy 
at least 3 items from the 'summer-collection'. Items can be 
any combination. Show "Summer Mix & Match: 20% off" message.

First Order Discount

Give first-time customers (tagged 'first-order') 15% off 
their entire order. Check if customer is tagged, apply 
discount to order subtotal. Message: "Welcome! First order discount"

Loyalty Tier Pricing

Create tiered discounts based on customer tags:
- 'loyalty-bronze': 5% off
- 'loyalty-silver': 10% off
- 'loyalty-gold': 15% off
Apply to all products. Show tier name in message.

Next Steps

Payment Customization

Customize payment methods

Shipping Customization

Customize shipping rates

Functions Overview

Learn about all function types

Debugging

Debug and optimize functions

Get Started

Core Concepts

Building Extensions

Building Functions

Development

Deployment

Advanced

Discount Functions

Overview

How Discount Functions Work

Discount Types

Volume Discounts

Bundle Discounts

Customer-Specific

Conditional Discounts

Example: Volume Discount

Generated Function Structure

Function Code Examples

Common Discount Patterns

Function Configuration

Testing Discount Functions

Performance Best Practices

Minimize Loops

Use Rust

Cache Calculations

Limit API Surface

Debugging Functions

Limitations

Example Prompts

Next Steps

Payment Customization

Shipping Customization

Functions Overview

Debugging

Get Started

Core Concepts

Building Extensions

Building Functions

Development

Deployment

Advanced

Documentation Index

​Overview

​How Discount Functions Work

​Discount Types

Volume Discounts

Bundle Discounts

Customer-Specific

Conditional Discounts

​Example: Volume Discount

​Generated Function Structure

​Function Code Examples

​Common Discount Patterns

​Function Configuration

​Testing Discount Functions

​Performance Best Practices

Minimize Loops

Use Rust

Cache Calculations

Limit API Surface

​Debugging Functions

​Limitations

​Example Prompts

​Next Steps

Payment Customization

Shipping Customization

Functions Overview

Debugging

Overview

How Discount Functions Work

Discount Types

Example: Volume Discount

Generated Function Structure

Function Code Examples

Common Discount Patterns

Function Configuration

Testing Discount Functions

Performance Best Practices

Debugging Functions

Limitations

Example Prompts

Next Steps