checker.js

'use strict';

function headerChecks(ctx) {
  return {
    expectHost: function (host) {
      return ctx.host == host;
    }
  }
  
}

function bodyChecks(ctx) {
  return {
    contains: (wat) => ctx.indexOf(wat) !== -1
  }
  
}

function createChecker(req) {
    const ctx = req;
    return {
        headers: headerChecks.bind(null, ctx.headers),
        body: bodyCheck.bind(null, ctx.body)
    };
}

var requestChecker = createChecker({ 
  headers: { 'foo': 'bar' }, 
  body: 'oh no' 
});

function report(val) {
  document.body.innerHtml += '<p>' + !!val + '</p>';
}

requestChecker
  .headers()
    .isHost('bar')
 
requestChecker
  .body()
    .contains('woot')