Call an external JSON API via a local cross domain proxy

cross_domain_proxy.js

// See: http://api.jquery.com/jQuery.ajaxPrefilter/
$.ajaxPrefilter( function( options ) {
  if ( options.crossDomain ) {
    var newData = {};
    // Copy the options.data object to the newData.data property.
    // We need to do this because javascript doesn't deep-copy variables by default.
    newData.data = $.extend({}, options.data);
    newData.url = options.url;

    // Reset the options object - we'll re-populate in the following lines.
    options = {};

    // Set the proxy URL
    options.url = "http://mydomain.com/proxy";
    options.data = $.param(newData);
    options.crossDomain = false;
  }
});

// How to use the cross domain proxy
$.ajax({
  url: 'http://the-real-api-url.com/getdata',
  data: {
    username: 'myUsername',
    password: 'myPassword'
  },
  crossDomain: true, // set this to ensure our $.ajaxPrefilter hook fires
  processData: false // We want this to remain an object for  $.ajaxPrefilter, and for performance reasons
{).success(function(data) { // Use the new jQuery promises interface and assume our API call returns a JSON object
    var jsonData = JSON.parse(data); // Assume it return a JSON string
    console.log(jsonData); // Do whatever you want with the data
});