How to Override a Tax Calculation Strategy
In this document, you’ll learn how to override a tax calculation strategy.
Overview
A tax calculation strategy is used to calculate taxes when calculating totals. The Medusa backend provides a tax calculation strategy that handles calculating the taxes accounting for defined tax rates, and settings such as whether tax-inclusive pricing is enabled.
You can override the tax calculation strategy to implement different calculation logic or to integrate a third-party service that handles the tax calculation. You can override it either in a Medusa backend setup or in a plugin.
Step 1: Create Strategy Class
A tax calculation strategy should be defined in a TypeScript or JavaScript file created under the src/strategies directory. The class must also implement the ITaxCalculationStrategy interface imported from the @medusajs/medusa package.
For example, you can create the file src/strategies/tax-calculation.ts with the following content:
import {
ITaxCalculationStrategy,
LineItem,
LineItemTaxLine,
ShippingMethodTaxLine,
TaxCalculationContext,
} from "@medusajs/medusa"
class TaxCalculationStrategy
implements ITaxCalculationStrategy {
async calculate(
items: LineItem[],
taxLines: (ShippingMethodTaxLine | LineItemTaxLine)[],
calculationContext: TaxCalculationContext
): Promise<number> {
throw new Error("Method not implemented.")
}
}
export default TaxCalculationStrategy
Note that you add a basic implementation of the calculate method because it’s required by the ITaxCalculationStrategy interface. You’ll learn about the method later in the guide.
Using a Constructor
You can use a constructor to access services and resources registered in the dependency container using dependency injection. For example:
// ...
import {
LineItemService,
} from "@medusajs/medusa"
type InjectedDependencies = {
lineItemService: LineItemService
}
class TaxCalculationStrategy
implements ITaxCalculationStrategy {
protected readonly lineItemService_: LineItemService
constructor({ lineItemService }: InjectedDependencies) {
this.lineItemService_ = lineItemService
}
// ...
}
Step 2: Implement the calculate Method
A tax calculation strategy is only required to implement the calculate method. This method is used whenever the totals are calculated.
If automatic tax calculation is disabled, then the tax calculation strategy will only be used when taxes are calculated manually as explain this guide.
The calculate method expects three parameters:
items: the first parameter is an array of line item objects.taxLines: the second parameter is an array of either shipping method tax line or line item tax line objects.calculationContext: an object holding the context of the tax calculation. The object has the following properties:shipping_address: an optional address object used for shipping.customer: an optional customer object.region: an optional region object.is_return: a boolean value that determines whether the taxes are being calculated for a return flow.shipping_methods: an array of shipping methods being used in the current context.allocation_map: an object that indicates the gift cards and discounts applied on line items. Each object key or property is an ID of a line item, and the value is an object having the following properties:gift_card: an optional object indicating the gift card applied on the line item.discount: an optional object indicating the discount applied on the line item.
The method returns a number, being the tax amount for the line items, tax lines, and context provided.
Step 3: Run Build Command
In the directory of the Medusa backend, run the build command to transpile the files in the src directory into the dist directory:
Test it Out
Run your backend to test it out:
To test it out, you can simulate a checkout flow and check the calculated taxes to see if it matches the logic you implemented in the calculate method.
As mentioned earlier, if automatic calculation of taxes is disabled in the region, you have to manually trigger tax calculation as explained in this guide.