javascript - XMLHttpRequest syntax for photos -

according google's chrome getting started tutorial: https://developer.chrome.com/extensions/getstarted

i having difficulty understanding source code javascript function xhr request grab photos flickr

requestkittens: function() {     var req = new xmlhttprequest();     req.open("get", this.searchonflickr_, true);     req.onload = this.showphotos_.bind(this);     req.send(null);     }, 

i understand syntax of req.open, req.send,etc. why boolean true, placeholder, , null inserted? these necessary parameters request? can please explain or refer resources explain it?

thanks!

the best resource xmlhttprequest specification.

you can read there, that:

• open method has following syntax: open(method, url [, async = true [, username = null [, password = null]]]) can see: async argument optional , defaults true. in example can omitted.

• send method, send([data = null]), takes optional argument providing request entity body. defaults null , ignored when request method (so in case).