Migrating from Koa v1.x to v2.x

New middleware signature

Koa v2 introduces a new signature for middleware.

Old signature middleware (v1.x) support will be removed in v3

The new middleware signature is:

// uses async arrow functions
app.use(async (ctx, next) => {
  try {
    await next() // next is now a function
  } catch (err) {
    ctx.body = { message: err.message }
    ctx.status = err.status || 500
  }
})

app.use(async ctx => {
  const user = await User.getById(this.session.userid) // await instead of yield
  ctx.body = user // ctx instead of this
})

You don't have to use asynchronous functions - you just have to pass a function that returns a promise. A regular function that returns a promise works too!

The signature has changed to pass Context via an explicit parameter, ctx above, instead of via this. The context passing change makes koa more compatible with es6 arrow functions, which capture this.

Using v1.x Middleware in v2.x

Koa v2.x will try to convert legacy signature, generator middleware on app.use, using koa-convert. It is however recommended that you choose to migrate all v1.x middleware as soon as possible.

// Koa will convert
app.use(function *(next) {
  const start = new Date();
  yield next;
  const ms = new Date() - start;
  console.log(`${this.method} ${this.url} - ${ms}ms`);
});

You could do it manually as well, in which case Koa will not convert.

const convert = require('koa-convert');

app.use(convert(function *(next) {
  const start = new Date();
  yield next;
  const ms = new Date() - start;
  console.log(`${this.method} ${this.url} - ${ms}ms`);
}));

Upgrading middleware

You will have to convert your generators to async functions with the new middleware signature:

app.use(async (ctx, next) => {
  const user = await Users.getById(this.session.user_id);
  await next();
  ctx.body = { message: 'some message' };
})

Upgrading your middleware may require some work. One migration path is to update them one-by-one.

Wrap all your current middleware in koa-convert
Test
npm outdated to see which koa middleware is outdated
Update one outdated middleware, remove using koa-convert
Test
Repeat steps 3-5 until you're done

Updating your code

You should start refactoring your code now to ease migrating to Koa v2:

Return promises everywhere!
Do not use yield*
Do not use yield {} or yield [].
- Convert yield [] into yield Promise.all([])
- Convert yield {} into yield Bluebird.props({})

You could also refactor your logic outside of Koa middleware functions. Create functions like function* someLogic(ctx) {} and call it in your middleware as const result = yield someLogic(this). Not using this will help migrations to the new middleware signature, which does not use this.

Application object constructor requires new

In v1.x, the Application constructor function could be called directly, without new to instantiate an instance of an application. For example:

var koa = require('koa');
var app = module.exports = koa();

v2.x uses es6 classes which require the new keyword to be used.

var koa = require('koa');
var app = module.exports = new koa();

ENV specific logging behavior removed

An explicit check for the test environment was removed from error handling.

Dependency changes

co is no longer bundled with Koa. Require or import it directly.
composition is no longer used and deprecated.

v1.x support

The v1.x branch is still supported but should not receive feature updates. Except for this migration guide, documentation will target the latest version.

Help out

If you encounter migration related issues not covered by this migration guide, please consider submitting a documentation pull request.

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