Middleware.md

Middleware

Synchronous example

// .. appName/src/middleware/ContentTypeHeader.js

'use strict';

/**
 * Middleware to send Content-Type header.
 */
module.exports = (req, res, next) => {
  res.setHeader('Content-Type', 'text/html');

  next(); // Run subsequent handler.
};

Asynchronous example

// .. appName/src/middleware/SessionCheck.js

'use strict';

const {RouterError} = require('@lambda-lambda-lambda/router/src/router/Error.js');

/**
 * Middleware to define session state, if exists.
 */
module.exports = async (req, res, next) => {
  if (await checkSession()) {

    // Passed down the Router stack.
    req.plugin('session', true);

  } else {

    // Write to CloudWatch, continue..
    throw new Error('Output to console');
  }

  // next() should be omitted
};

See L³ middleware for more complex use cases.

Handling exceptions

Exception type	Description
throw new RouterError	When called will immediately exit the middleware stack and write the exception to CloudWatch logs.
throw new Error	When called will write the exception to CloudWatch and run the subsequent handler in middleware stack.
Promise.reject()	Same as throw new Error with no exception thrown.