$ npm install express-http-proxy --save

proxy(host, options);

var proxy = require('express-http-proxy');
var app = require('express')();

app.use('/proxy', proxy('www.google.com'));

  app.use(proxy('/reject-promise', {
    proxyReqOptDecorator: function() {
      return Promise.reject('An arbitrary rejection message.');
    }
  }));

next('An arbitrary rejection messasage');

app.use('/', proxy('http://google.com'))

function selectProxyHost() {
  return (new Date() % 2) ? 'http://google.com' : 'http://altavista.com';
}

app.use('/', proxy(selectProxyHost));

app.use('/proxy', 'http://foo.bar.com')

// Declare use of body-parser AFTER the use of proxy
app.use(bodyParser.foo(bar))
app.use('/api', ...)

  app.use(proxy('localhost:12345', {
    proxyReqPathResolver: function (req) {
      var parts = req.url.split('?');
      var queryString = parts[1];
      var updatedPath = parts[0].replace(/test/, 'tent');
      return updatedPath + (queryString ? '?' + queryString : '');
    }
  }));

app.use('/proxy', proxy('localhost:12345', {
  proxyReqPathResolver: function(req) {
    return new Promise(function (resolve, reject) {
      setTimeout(function () {   // simulate async
        var parts = req.url.split('?');
        var queryString = parts[1];
        var updatedPath = parts[0].replace(/test/, 'tent');
        var resolvedPathValue = updatedPath + (queryString ? '?' + queryString : '');
        resolve(resolvedPathValue);
      }, 200);
    });
  }
}));

app.use('/proxy', proxy('www.google.com', {
  filter: function(req, res) {
     return req.method == 'GET';
  }
}));

  app.use(proxy('localhost:12346', {
    filter: function (req, res) { 
      return new Promise(function (resolve) { 
        resolve(req.method === 'GET');
      }); 
    }
  }));

app.use('/proxy', proxy('www.google.com', {
  userResDecorator: function(proxyRes, proxyResData, userReq, userRes) {
    data = JSON.parse(proxyResData.toString('utf8'));
    data.newProperty = 'exciting data';
    return JSON.stringify(data);
  }
}));

app.use(proxy('httpbin.org', {
  userResDecorator: function(proxyRes, proxyResData) {
    return new Promise(function(resolve) {
      proxyResData.funkyMessage = 'oi io oo ii';
      setTimeout(function() {
        resolve(proxyResData);
      }, 200);
    });
  }
}));

app.use('/proxy', proxy('www.google.com', {
  limit: '5mb'
}));

  function coinToss() { return Math.random() > .5 }
  function getHost() { return coinToss() ? 'http://yahoo.com' : 'http://google.com' }

  app.use(proxy(getHost, {
    memoizeHost: false
  }))

app.use('/proxy', proxy('www.google.com', {
  userResHeaderDecorator(headers, userReq, userRes, proxyReq, proxyRes) {
    // recieves an Object of headers, returns an Object of headers.
    return headers;
  }
}));

app.use('/proxy', proxy('www.google.com', {
  skipToNextHandlerFilter: function(proxyRes) {
    return proxyRes.statusCode === 404;
  }
}));

// Example of skipping all error handling.

app.use(proxy('localhost:12346', {
  proxyErrorHandler: function(err, res, next) {
    next(err);
  }
}));


// Example of rolling your own

app.use(proxy('localhost:12346', {
  proxyErrorHandler: function(err, res, next) {
    switch (err && err.code) {
      case 'ECONNRESET':    { return res.status(405).send('504 became 405'); }
      case 'ECONNREFUSED':  { return res.status(200).send('gotcher back'); }
      default:              { next(err); }
    }
}}));

app.use('/proxy', proxy('www.google.com', {
  proxyReqOptDecorator: function(proxyReqOpts, srcReq) {
    // you can update headers
    proxyReqOpts.headers['Content-Type'] = 'text/html';
    // you can change the method
    proxyReqOpts.method = 'GET';
    return proxyReqOpts;
  }
}));

app.use('/proxy', proxy('www.google.com', {
  proxyReqOptDecorator: function(proxyReqOpts, srcReq) {
    return new Promise(function(resolve, reject) {
      proxyReqOpts.headers['Content-Type'] = 'text/html';
      resolve(proxyReqOpts);
    })
  }
}));

app.use('/proxy', proxy('www.google.com', {
  proxyReqBodyDecorator: function(bodyContent, srcReq) {
    return bodyContent.split('').reverse().join('');
  }
}));

app.use('/proxy', proxy('www.google.com', {
  proxyReqBodyDecorator: function(proxyReq, srcReq) {
    return new Promise(function(resolve, reject) {
      http.get('http://dev/null', function (err, res) {
        if (err) { reject(err); }
        resolve(res);
      });
    })
  }
}));

app.use('/proxy', proxy('www.google.com', {
  https: true
}));

app.use('/proxy', proxy('www.google.com', {
  preserveHostHdr: true
}));

app.use('/proxy', proxy('www.google.com', {
  parseReqBody: false
}));

app.use('/proxy', proxy('www.google.com', {
  reqAsBuffer: true
}));

app.use('/post', proxy('httpbin.org', {
  reqBodyEncoding: null
}));

app.use('/', proxy('httpbin.org', {
  timeout: 2000  // in milliseconds, two seconds
}));

DEBUG=express-http-proxy npm run YOUR_PROGRAM
DEBUG=express-http-proxy npm run YOUR_PROGRAM  | grep 'express-http-proxy'   # to filter down to just these messages

app.use('/proxy', proxy('www.google.com', {
  intercept: function(proxyRes, proxyResData, userReq, userRes, cb) {
    data = JSON.parse(proxyResData.toString('utf8'));
    data.newProperty = 'exciting data';
    cb(null,  JSON.stringify(data));
  }
}));

app.use('/proxy', proxy('www.google.com', {
  userResDecorator: function(proxyRes, proxyResData, userReq, userRes) {
    data = JSON.parse(proxyResData.toString('utf8'));
    data.newProperty = 'exciting data';
    return JSON.stringify(data);
  }
}));

app.use('/', proxy('internalhost.example.com', {
  proxyReqOptDecorator: function(proxyReqOpts, originalReq) {
    proxyReqOpts.ca =  [caCert, intermediaryCert]
    return proxyReqOpts;
  }
})

app.use('/', proxy('internalhost.example.com', {
  proxyReqOptDecorator: function(proxyReqOpts, originalReq) {
    proxyReqOpts.rejectUnauthorized = false
    return proxyReqOpts;
  }
}))