Cycle through data request strategies (ajax, jsonp) within a Promise.

ajax-jsonp-promise.js

  function get(url){
    return new Promise(function(resolve, reject){

      // request strategies: ajax, jsonp
      var types = ['json', 'jsonp'];

      function call(){

        // try data type strategies one-by-one; mutates the array
        var type = types.shift();
        
        // no more strategies left; give up and reject the promise
        if (!type) {
          reject();
          return;
        }
        
        $.ajax({
          url: url,
          dataType: type,
          method: 'GET',
          success: function(resp){ resolve(resp) },
          error: function(){ call() }
        });
      }

      // attempt the first strategy
      call();
    });
  }

  function show(data){
    console.log(data)
  }

  get('http://localhost:5000/v1/rates/eur')
    .then(show)
    .catch(function(){
      console.log('all request strategies have failed')
    })