[jQuery] How to wait for getJSON to complete before continue?

can i want for getJSON to complete before excecuting code? cos i am
validating my input. and i need some values from server 1st. so apart
from putting all code that is to be executed after getJSON() inside

.getJSON("url", function(json) {
// *** here ***
});

can i have it outside?

and how can i access the json object outside the

.getJSON("url", function(json) {});

function?