I tried to search for good resources on empty statement, but it seem like nothing show up. Even on MDN, they don't have much to say about it.
i.e:
for(var i = 0; i < a.length; a[i++] = 0);
if((a==0) || (b == 0));
I would like to know what are some real examples that one should use empty statements on their project. What are the reason behind it?
The examples you've given don't make much sense. They should better be written
for (var i = 0; i < a.length;) a[i++] = 0;
for (var i = 0; i < a.length; i++) a[i] = 0;
; // the comparisons really don't do anything (assuming a and b are no objects)
(a==0) || (b = 0); // Oh wait, that's the one given by @Shomz
if (a != 0) b = 0;
However, there are real-world applications for the empty statement. I'll just list 3 that come to my mind:
function x() {
…
};
A semicolon where it doesn't belong (e.g. after the function declaration above) makes an empty statement.
;
…
A leading semicolon on your script files helps to guard against buggy inclusions or file concatenations.
while (!check_for_finish()); // do nothing
An empty loop body can be used for busy-waiting loops (not recommended) and similar.
none/lazyness. there is absolutely no difference to
for(var i = 0; i < a.length;) a[i++] = 0;
and just a minimal difference to
for(var i = 0; i < a.length; i++) a[i] = 0;
the first one is a few ms faster after a few billion iteration steps; aka. premature optimization
EDIT:
if((a==0) || (b == 0));
this makes no sense at all, since it does nothing.
but expresions like
a==0 || (b=0);
//or maybe sth like this:
//var noop = ()=>void 0; //FYI
typeof a === "function" || (a = noop);
are pretty useful to me, since they are short, and readable and an additional if-statement doesn't add any value to readability or understanding (at least once you know this pattern).
a[i++]=0 i++ (increment and getting the value of i) are processed as one instruction, whilst in a[i]=0; i++; they are two; but as i said, premature optimizations. Every function-call or deoptimized function, and so much more has a way higher impact on performance than this optimization.The first one obviously loops through the array and assigns all values to zero, without having the code specified in the statement.
The other one seems like a typo, because it is useless.
However, something like
if((a==0) || (b = 0));
would make sense, as it would assign b to zero in case a is not zero.
var a = 1, b = 1;
if((a == 0) || (b = 0));
alert("a: " + a + ", b: " + b);
make sense - if(a===0) b=0; or even b = (a===0 ? 0 : b) are both more readable IMO
I do not think that they are really useful, but I can be wrong. One can try to use side effects of the evaluation of the conditions in an if, but I do not see a good reason to do so.
My favorite use for it is to wait for a condition to become true.
while ( !condition );
// do what happens once your condition is met
This is nice to read, in my opinion, but the same can be done with { } instead of the empty statement.
The first example for(var i = 0; i < a.length; a[i++] = 0); is useful IMO, and the reasons would be:
The second one if((a==0) || (b == 0)); seems doesn't nothing.
Let us suppose you have two functions X and Y and let us suppose that Y must only be executed when X returns true, in such a situation you will write:
if( X() && Y() );
// Assign all array values to 0{}:for(a=0;a++<100;b=a,c=b+a);