connect-validation is a middleware for connect.js (so you can use it with express applications) that helps handling 400 BAD REQUEST errors on JSON APIs, in a deterministic way.

Specifically, the top-level JSON object in the response will always contain a key named "status", which maps to a string identifying the cause of the error. Unexpected errors will have a status string of “error”; errors expected as part of the protocol flow will have a specific status.

Errors will have three different keys:

• location is the location of the error. It can be “querystring”, “header” , “url” or “body”;
• name is the eventual name of the value that caused problem;
• description is a description of the problem encountered.

For instance, that would be something like:

{
  "status": "errors",
  "errors": [{"location": "body",
              "name": "version",
              "description": "version should be an integer"
             }]
}

You have to install the middleware in your express application.

var express = require('express');
var validationMiddleware = require('connect-validation');

var app = express();
var app.use(validationMiddleware);

Once installed, addError and sendError methods will be available on all Response objects.

Once installed, you can use these two methods whenever you need to send back errors to the client. You can add as many errors as you want by using the req.addError(location, name, description) method.

At the end of the validation, use sendError() to build the 400 errors response.

app.get('/', function (req, res) {
  if (!req.query.hasOwnProperty('id')) {
    res.addError("querystring", "id", "missing; id");
    res.sendError();
    return;
  }
});

If you have only one error, you can use sendError directly.

app.get('/', function (req, res) {
  if (!req.query.hasOwnProperty('id')) {
    res.sendError("querystring", "id", "missing; id");
    return;
  }
});

That's all, folks!