what is BOM? what is the use of it in js ??

Answer 1

Browser Objects Model. Example, window.alert("test");

Answer 2

will you please explain more??

Answer 3

when you see or write window.something, that browser/window/global objects. DOM is Document Objects Model. When you see or write document.something, that's DOM.
We use BOM, to get user browser history, create cookie, alert, prompt, confirmation box and etc.