how to detect invalid JSON and response some helpful message?

some guy may try to post some wired string and set header to be json, i can caught the error but don’t know how to response a custom message that tell them," dude, your json is invalid !"

i tried something like this:

app.use(function*(next){
    if(/get|delete|head/i.test(this.method)) return yield next;
    if( /json/i.test( this.get('Content-Type') ) ){
      try{
        require('co-body').json(this)()....... blah blah blah.....
      }catch(e){

      }
    }

    yield bodyParser({
      formLimit: '200kb',
      jsonLimit: '1mb',
    }).call(this,next)
  });

i looked the source code, there is no error handling;

[dead-horse]

https://github.com/koajs/bodyparser/commit/194181298fe3bffce6b5fcf3cfebc35b8cda6c89

[eisneim]

pretty efficient!