Web Workers

webWorkers.js

// Consider, for example, parsing a large JSON string. Suppose that the data is large 
// enough that parsing takes at least 500 milliseconds. That is clearly too long to allow 
// JavaScript to run on the client, as it will interfere with the user experience. This particular 
// task is difficult to break into small chunks with timers, so a worker is the ideal solution. The 
// following code illustrates usage from a web page:

var worker = new Worker("jsonparser.js");
//when the data is available, this event handler is called

worker.onmessage = function(event){
    //the JSON structure is passed back
    var jsonData = event.data;
    //the JSON structure is used
    evaluateData(jsonData);
};

// pass in the large JSON string to parse worker.postMessage(jsonText);

// The code for the worker responsible for JSON parsing is as follows:
// inside of jsonparser.js

// this event handler is called when JSON data is available

self.onmessage = function(event){
    //the JSON string comes in as event.data
    var jsonText = event.data;
    //parse the structure
    var jsonData = JSON.parse(jsonText);
    //send back to the results
    self.postMessage(jsonData);
};

// Note that even though JSON.parse() is likely to take 500 milliseconds or more, there is
// no need to write any additional code to split up the processing. This execution takes
// place on a separate thread, so you can let it run for as long as the parsing takes without
// interfering with the user experience.