Having needed an pure javascript alternative to jQuery's $.getJSON() and $.parseJSON() recently I thought I would try and clear this up for future users. There is a lot of misinformation out there on the web. By far the main trouble maker is the belief that by including a .JSON file between the head tags of your HTML document you can access structured JSON.

The wrong way

 
<script type="text/javascript" src="mydata.json"></script>
 
Many examples will evidence that you can access the data with a simple function such as the one below. In fact, what this is not actually loading a JSON document but creating a Javascript object. This technique will not work for true JSON files.
 
  // 'JSON' data included as above
   data = '[{"blue" : "is ok", "red" : "is my fave color"}]';

  // Function to 'load JSON' data
   function load() {
    var someData_notJSON = JSON.parse(data);
    console.log(someData_notJSON[0].red); // Will log "is my fave color"
    }
 
If you're not fussy about using an actual JSON file then creating a Javascript object in a seperate .js file may be the way to go. If like me you do need to work with JSON in pure Javascript here's what you'll need to do.

The correct method - create a new XMLHttpRequest

The clue here is the jQuery method $.getJSON() which is shorthand for $.ajax(). It may seem an odd approach requesting a local file in this way but it offers the most flexibility with minimum fuss.
 
 function loadJSON(callback) {   

    var xobj = new XMLHttpRequest();
        xobj.overrideMimeType("application/json");
    xobj.open('GET', 'my_data.json', true); // Replace 'my_data' with the path to your file
    xobj.onreadystatechange = function () {
          if (xobj.readyState == 4 && xobj.status == "200") {
            // Required use of an anonymous callback as .open will NOT return a value but simply returns undefined in asynchronous mode
            callback(xobj.responseText);
          }
    };
    xobj.send(null);  
 }
 
The function above will create a new instance of a XMLHttpRequest and load asynchronously the contents of my_data.json. I have gone with asynchronous but you can change the argument to false if you want a synchronous load. Thankfully all modern browsers support the native JSON.parse method. Remember our anonymous callback? here's how you use it.

Usage

 
function init() {
 loadJSON(function(response) {
  // Parse JSON string into object
    var actual_JSON = JSON.parse(response);
 });
}
 

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