How to use a return value in another function in Javascript?

Call the function and save the return value of that very call.

function firstFunction() {
  // do something
  return "testing 123";
}

var test = firstFunction();  // this will grab you the return value from firstFunction();
alert(test);

You can make this call from another function too, as long as both functions have same scope.

For example:

function testCase() {
  var test = firstFunction(); 
  alert(test);
}

Demo

You could call firstFunction from secondFunction :

function secondFunction() {
    alert(firstFunction());
}

Or use a global variable to host the result of firstFunction :

var v = firstFunction();
function secondFunction() { alert(v); }

Or pass the result of firstFunction as a parameter to secondFunction :

function secondFunction(v) { alert(v); }
secondFunction(firstFunction());

Or pass firstFunction as a parameter to secondFunction :

function secondFunction(fn) { alert(fn()); }
secondFunction(firstFunction);

Here is a demo : http://jsfiddle.net/wared/RK6X7/.