Lambda Reproduce middleware with decorators

.babelrc

{
    "presets": ["es2015", "stage-0"],
    "plugins": ["transform-decorators-legacy"]
}

index.js

import { decorate } from 'core-decorators';

function checkJwt(target) {
    return (event, context) => {
        const { body: { jwt } } = event;
        if (!jwt) { return context.done('400: missing jwt'); } // if can't find the jwt return an error and don't call the next function
        // ... add logic to verify the token here ...
        // add the userId from the token to the data
        Object.assign(event.body, {
            authentificatedUser: 'the authentificated user entity'
        });
        return target(event, context); // call the next function
    };
}

function validateUsername(target) {
    return (event, context) => {
        setTimeout(() => {
            const { body: { username } } =  event;
            // do some kind of validation
            if (typeof username !== 'string') { return context.done('400 username must be a string'); }
            target(event, context);
        }, 1000);
    };
}

class GetUser {
    constructor() {
        this.handler = this.handler.bind(this);
    }
    @decorate(checkJwt) // same as a middleware to check the jwt on the body
    @decorate(validateUsername)
    handler(event, context) {
        context.done(null, event);
    }
}

class LambdaContext {
    done(error, success) {
        if (error) { return console.log('Error:', error); }
        console.log(success);
    }
}

const getUser = new GetUser();
[ { jwt: 'abc123', username: 'abc' }, { jwt: 'abc123',  username: 123 }, { random: 'hello' } ].forEach(body => {
    getUser.handler({ body }, new LambdaContext() );
});

// Returns
// 1: { body: { jwt: 'abc123', username: 'abc', authentificatedUser: 'the authentificated user entity' } }
// 2: Error: 400 username must be a string
// 3: Error: 400: missing jwt