v4 code migration: Updating route middlewares

This guide is part of the v4 code migration guide designed to help you migrate the code of a Strapi application from v3.6.x to v4.0.x

:::strapi v3/v4 comparison In Strapi v3, policies are also used as route middlewares to make changes to the request or response payload or to wrap a controller with extra logic.

In Strapi v4, there is a clear distinction between policies, which are used for authorization and validation, and middlewares, which are used for extra logic, wrapping, and customization.

:::

A Strapi v3 policy acting as a middleware should be converted to a route middleware in Strapi v4 by creating a new middleware. Strapi v3 policies not acting as route middlewares should be migrated to v4 using the policies migration documentation.

The strapi generate interactive CLI is the recommended way to create a new middleware:

1. Run yarn strapi generate in a terminal.
2. Using the interactive CLI, choose where the middleware should apply and give it a name.
3. Add customizations to the generated file.

Example of a Strapi v3 policy converted to a Strapi v4 middleware

The following Strapi v3 policy acts as a middleware:

path: ./api/api-name/config/policies/my-policy.js

module.exports = async (ctx, next)  {
  const start = Date.now();

  await next();

  const delta = Math.ceil(Date.now() - start);
  ctx.set('X-Response-Time', delta + 'ms');
};

It should be converted to a Strapi v4 middleware using the following code:

path: ./src/api/api-name/middlewares/my-middleware.js

module.exports = (config, { strapi })  {
  return async (ctx, next)  {
    const start = Date.now();

    await next();

    const delta = Math.ceil(Date.now() - start);
    ctx.set('X-Response-Time', delta + 'ms');
  };
};