medusa-store/docs/content/modules/taxes/backend/tax-calculation-strategy.md

description, addHowToData

description	addHowToData
Learn how to create a tax provider. You can create a tax provider in a Medusa backend or a plugin.	true

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.

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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.

:::tip

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.

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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:

npm run build

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Test it Out

Run your backend to test it out:

npx @medusajs/medusa-cli develop

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.

:::tip

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.

:::