onmessagen worker.js code example

Example 1: what is webworker

web worker
Web Workers are a mechanism to create a separate thread 
in parallel to your main JS program thread. 
can only communicate with each other through normal async events 
always abide by the event-loop one-at-a-time behavior 
required by run-to-completion.

Example 2: web worker multiple data

w.postMessage({user:username,url:server_url})