Learning JS from MDN Docs came across code in “functions section”. Not able to understand what does return; in the below code accomplish
function foo(i) {
if (i < 0)
return; // what does this do?
console.log('begin: ' + i);
foo(i - 1);
console.log('end: ' + i);
}
foo(3);Output
'begin: 3' 'begin: 2' 'begin: 1' 'begin: 0' 'end: 0' 'end: 1' // why are these printing 'end: 2' // this one 'end: 3' // this one
I understood first 5 lines of output, but not able to understand why end: 0,1,2,3 are coming?
Please help !
Answer
return terminates the current function, and returns control flow to the caller.
When foo(3); is called, the function is initialized with an i parameter of 3. It fails the if statement, prints begin: 3, and then calls foo(3 - 1);. At this point, the current function (the one with an i parameter of 3) is paused until the foo(3 - 1); call completes.
foo(2); prints begin: 2, and then pauses while calling foo(1).
foo(1) prints begin: 1, and then pauses while calling foo(0).
foo(0) prints begin: 0, and returns: it terminates, and yields control flow back to its caller, the function call of foo(1).
foo(1) resumes, and continues executing, printing end: 1. That’s the end of the function block, so the foo(1) function call ends, yielding control flow back to the foo(2) function call.
foo(2) resumes, printing end: 2, then terminates, yielding control flow back to foo(3). Then foo(3) prints end: 3, and terminates.
return only terminates the current function. The only way to terminate all calling functions (until a catch is encountered) would be to throw an error:
function foo(i) {
if (i < 0)
throw new Error();
console.log('begin: ' + i);
foo(i - 1);
console.log('end: ' + i);
}
foo(3);