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Why does setInterval initiate when I'm only assigning it?

I'm assigning to a variable, a function that uses setInterval, but I don't want the function to run until I call it. However, the function is running from just the assignment statement.
sessionClock = setInterval(function() {
console.log("Hi")
}, 1000)
I have also tried like this:
sayHi = function() {
console.log("Hi");
}
var sayHiStarter = setInterval(sayHi, 1000);
Both of these initiate the function and will log "Hi" to the console.
Why is it running on assignment? And what can do I do fix this?

If you only want to bind a function to setInterval, but call it later, you can use bind:
var sessionClock = setInterval.bind(null, function() {
console.log("Hi")
}, 1000);
//... later
var myInterval = sessionClock(); // start the timer
// ... later if you need to clear it
clearInterval(myInterval);
In principle, bind returns a new function that calls your original function (in this case, setInterval) with predefined arguments. So when you call sessionClock, that returned function is called. There a other aspects to bind, but they don't seem to apply in this context.

The call to setInterval does not return a function, but an identification for the created interval. This id is used to remove the interval when you don't want it to execute anymore:
sessionClock = setInterval(function() {
console.log("Hi")
}, 1000)
...
clearInterval(sessionclock);
What you want is something like this:
sessionClock = function () {
return setInterval(function() {
console.log("Hi")
},
1000);
}
//When needed
var intervalId=sessionClock();

Javascript: setInterval isn't removed when nullifying an object

The setInterval function keeps running even though the object is nullified, should I change the setInterval var to null first or should I do something else? Also, will the GC remove the object even if the setInterval is still running?
Test = function(){
function start(){
// setTimout for controllable FPS
var loop = setInterval(function(){
console.log("TEST");
}, 1000);
}
start();
};
var test = new Test();
setTimeout(function(){
console.log("DIE!!");
test = null;
}, 2000);
JsFiddle

the value returned by setInterval is just a number that used to identify the reference to the interval. you can't just null it - you need to call window.clearInterval on the reference.
there's a few other things that don't make sense in the code you posted. for example, you're declaring a function in a function then just calling it once. i think this is probably closer to what you want:
var Test = function(){
this.start();
}
Test.prototype = {
loop : null,
start : function(){
this.loop = window.setInterval(function(){
console.log('TEST');
}, 1000);
},
stop : function(){
window.clearInterval(this.loop);
}
}
var test = new Test();
window.setTimeout(function(){
test.stop();
}, 5000);
That'll run the interval 5 times.
FWIW, the GC isn't really involved here. As long as there's a reference to any variable, it won't be collected.
HTH

Referencing Non-Global Variable for Timer JS

I have this function.
function changeFrame(){
var time = setInterval(start, 250);
}
and I want to stop it from firing in another function, but haven't been able to figure out how to do it.

Do you mean this?
function changeFrame(){
var time = setInterval(function() {
// Do stuff
}, 250);
}
Think it's in the comments.
Ok amended the fiddle to do what you want. I made time a global var. Call clearInterval in stop with the global var http://jsfiddle.net/QNWF4/3/

In order to call clearInterval you need to have the handle returned by setInterval. That means something will either be global to the page or global to a containing function in which your script resides.
function Timer()
{
var handle = null;
this.start = function (fn,interval) {
handle = setInterval(fn,interval);
};
this.stop = function ()
{
if (handle) { clearInterval(handle); handle = null; }
};
return this;
}

Unable to Clear Timeout

I have the following function that performs a setTimeout back on itself, i.e.:
var doIt = function(){
$('#comment_report').trigger('apexrefresh');
setTimeout("doIt()", 5000);
}
My question is, I am trying to clear the setTimeout that is looping through every 5 seconds, by issuing:
clearTimeout(doIt);
Unfortunately this didn’t work.
How would I go about clearing the above setTimeout?

You don't pass a string to clearTimeout(), you pass the Number which identifies the setTimeout() (which is returned by said function).
var timeoutId = null;
var doIt = function() {
$('#comment_report').trigger('apexrefresh');
timeoutId = setTimeout(doIt, 5000);
};
clearTimeout(timeoutId);
Alternatively, inside of doIt(), check for the condition and if it's met, then return only before you issue another call to setTimeout() (you won't have to keep track of the id that way).

var doit = function(){
};
var my_timeout_name = setTimeout( doit, 5000 );
clearTimeout( my_timeout_name );

setTimeout returns you an id that you can then pass to clearTimeout function.
Also, instead of this:
setTimeout("doIt()", 5000);
You can just pass a function:
setTimeout(doIt, 5000);

var myTimeout = false;
var doIt = function(){
$('#comment_report').trigger('apexrefresh');
myTimeout = setTimeout(doIt, 5000);
}
Then you should use
clearTimeout(myTimeout);

You are doing it wrong clearTimeout(timerid) this function takes the id of Timeout.
you can achieve this as
var mytimerid;
var doIt = function()
{
$('#comment_report').trigger('apexrefresh');
mytimerid = setTimeout("doIt()", 5000);
}
and then when ever required stop the timeout by
clearTimeout(mytimerid);

You need to assign a variable to the setTimeout that holds the id.
But in your code you seem to be assigning doIt to a function that does not return anything.
var timer;
timer = setTimeout("doIt()", 5000);
Then use
if(timer)
clearTimeout(timer);

How to stop time interval in my function?

I have this function:
function timedFunction(functionString,timeoutPeriod) {
setTimeout(functionString+"timedFunction(\""+functionString+"\","+timeoutPeriod+");",timeoutPeriod);}
This function me call:
timedFunction("startStopky();",1000);
startStopky(); is a function that I want in a specified time interval repeatedly run. Everything works excellently, but if I want stop this interval, I have to stop as follows:
for (var i = 1; i < 99999; i++) {
window.clearInterval(i);
}
Unfortunately this will stop all intervals, and I want to stop just one particular. How can I do it?

Instead of doing recursive calls to timedFunction just do:
var intervalId = setInterval(startStopky, 1000);
and to clear it just do:
clearInterval(intervalId);

The setTimeout function returns a timeout ID that you use with clearTimeout to remove that timeout. The same goes for intervals but in that case it's a setInterval and clearInterval combo.
E.g.:
var t = setTimeout(yourFunction, 1000);
clearTimeout(t);
var i = setInterval(yourFunction2, 500);
clearInterval(i);

You have a Timeout, but you are clearing an Interval. clearInterval clears intervals, not timeouts.
You want window.clearTimeout(timeoutId)
If you want to stop a single one, you use the processId of that interval.
window.clearTimeout("13");

You really shouldn't be using strings to do this:
function timedFunction(fn, interval) {
var timerHandle;
function runIt() {
fn();
timerHandle.id = setTimeout(runIt, interval);
}
return timerHandle = { id: setTimeout(runIt, interval) };
}
Then you can call it like this:
var handle = timedFunction(startStopky, 1000);
To stop the process:
clearTimeout(handle.id);