133 lines
4.8 KiB
Markdown
133 lines
4.8 KiB
Markdown
---
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description: 'Learn how to create a tax provider. You can create a tax provider in a Medusa backend or a plugin.'
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addHowToData: true
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---
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# How to Override a Tax Calculation Strategy
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In this document, you’ll learn how to override a tax calculation strategy.
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## Overview
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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.
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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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---
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## Step 1: Create Strategy Class
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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.
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For example, you can create the file `src/strategies/tax-calculation.ts` with the following content:
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```ts title=src/strategies/tax-calculation.ts
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import {
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ITaxCalculationStrategy,
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LineItem,
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LineItemTaxLine,
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ShippingMethodTaxLine,
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TaxCalculationContext,
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} from "@medusajs/medusa"
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class TaxCalculationStrategy
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implements ITaxCalculationStrategy {
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async calculate(
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items: LineItem[],
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taxLines: (ShippingMethodTaxLine | LineItemTaxLine)[],
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calculationContext: TaxCalculationContext
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): Promise<number> {
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throw new Error("Method not implemented.")
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}
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}
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export default TaxCalculationStrategy
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```
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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.
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### Using a Constructor
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You can use a constructor to access services and resources registered in the dependency container using dependency injection. For example:
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```ts title=src/strategies/tax-calculation.ts
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// ...
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import {
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LineItemService,
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} from "@medusajs/medusa"
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type InjectedDependencies = {
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lineItemService: LineItemService
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}
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class TaxCalculationStrategy
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implements ITaxCalculationStrategy {
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protected readonly lineItemService_: LineItemService
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constructor({ lineItemService }: InjectedDependencies) {
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this.lineItemService_ = lineItemService
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}
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// ...
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}
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```
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---
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## Step 2: Implement the calculate Method
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A tax calculation strategy is only required to implement the `calculate` method. This method is used whenever the totals are calculated.
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:::tip
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If automatic tax calculation is disabled, then the tax calculation strategy will only be used when taxes are calculated manually as explain [this guide](../storefront/manual-calculation.md).
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:::
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The `calculate` method expects three parameters:
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- `items`: the first parameter is an array of [line item](../../../references/entities/classes/LineItem.md) objects.
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- `taxLines`: the second parameter is an array of either [shipping method tax line](../../../references/entities/classes/ShippingMethodTaxLine.md) or [line item tax line](../../../references/entities/classes/LineItemTaxLine.md) objects.
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- `calculationContext`: an object holding the context of the tax calculation. The object has the following properties:
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- `shipping_address`: an optional address object used for shipping.
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- `customer`: an optional customer object.
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- `region`: an optional region object.
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- `is_return`: a boolean value that determines whether the taxes are being calculated for a return flow.
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- `shipping_methods`: an array of shipping methods being used in the current context.
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- `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:
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- `gift_card`: an optional object indicating the gift card applied on the line item.
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- `discount`: an optional object indicating the discount applied on the line item.
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The method returns a number, being the tax amount for the line items, tax lines, and context provided.
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---
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## Step 3: Run Build Command
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In the directory of the Medusa backend, run the `build` command to transpile the files in the `src` directory into the `dist` directory:
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```bash npm2yarn
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npm run build
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```
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---
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## Test it Out
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Run your backend to test it out:
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```bash npm2yarn
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npx @medusajs/medusa-cli develop
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```
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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.
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:::tip
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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](../storefront/manual-calculation.md).
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:::
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