Koa has a context object created during every request/response cycle. The context is referenced via this in the middleware. Much information are captured in the context, such as req and res. In order to avoid polluting the context and colliding with other middleware, the information from your application should be encapsulated into this.state property, a namespace for passing information through middleware during request/response cycle:

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require('koa')().use(function *(next) {
  var state = this.state;
  state.user = yield users.create(this.request.body);
  state.code = 201;
  state.body = {
    name: user.name,
    mail: user.mail
  };
  yield next;
}).listen(3000);

The this.state has been initialized into an empty object:

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require('koa')().use(function *(next) {
  this.body = this.state;
}).listen(3000);

Therefore, we can start using it right away without doing our own initialization:

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$ http :3000
HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 2
Content-Type: application/json; charset=utf-8
{}

LoDash has _.template function:

Creates a compiled template function that can interpolate data properties in “interpolate” delimiters, HTML-escape interpolated data properties in “escape” delimiters, and execute JavaScript in “evaluate” delimiters. Data properties may be accessed as free variables in the template. If a setting object is provided it takes precedence over _.templateSettings values.

The template settings affect the behavior of LoDash, and due to Node’s module caching, this will affect the entire application. This will likely cause unexpected error, especially you are writing a module or a library, and the application that requires the module will also use LoDash but with a different template setting:

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SyntaxError: Unexpected token =

The rule of thumb: Don’t do it!