javascript - What are Closures and Callbacks? -

what closures , callbacks in javascript? i've yet find explanation of either.

closures have been handled in stackoverflow here selection:-

how javascript closure work?
what “closure” refer in javascript?
can right example of javascript closure.. places need consider avoiding closures??
javascript scope , closure
javascript closures , ‘this’ context
javascript - how learn “closures” usage?

callbacks simpler concept. callback function accepts function parameter. @ point during execution called function execute function passed parameter, callback. quite callback happens asynchronous event, in case called function may return without having executed callback, may happen later. here common (browser based) example:-

 function fn() { alert("hello, world"); }  window.settimeout(fn, 5000); 

here function fn passed callback settimeout function. set timeout returns 5 seconds later function passed callback executed.

closures , callbacks

quite reason closures created (either incidentally, accidentally or deliberately) need create callback. example:-

 function alertthislater(message, timeout)  {      function fn() { alert(message); }      window.settimeout(fn, timeout);  }   alertthislater("hello, world!", 5000); 

(please read of linked posts grasp closures)

a closure created containing in part message parameter, fn executed quite time after call alertthislater has returned, yet fn still has access original content of message.