Why does setTimeout trigger instantly? [closed]

Answer 1

The setTimeout() accepts a function as a first parameter and the time as the second parameter. I hope you have heard of functions returning a function as well. So for that cases, you can also call the function there.

The moment you add () to any function, it calls it immediately. So you call the function and return nothing to execute to the setTimeout.

Case 1

setTimeout(myFunc, 3000);

Here you are passing the function itself to get executed after 3 seconds.

Case 2

setTimeout(myFunc(), 3000);

Here you are passing the function's executed return value to get executed after 3 seconds.

Answer 2

myVar = setTimeout(alertFunc(), 3000); is the same as:
aF = alertFunc(); myVar = setTimeout(aF, 3000);

From this you can see that alertFunc is called outside of setTimeout.

Answer 3

shouldn´t it be the same?

No, not at all.

setTimeout(alertFunc, 3000) passes the value of alertFunc (a reference to a function) into setTimeout. setTimeout stores that function reference in order to call it three seconds later.

setTimeout(alertFunc(), 3000) calls alertFunc, immediately, and passes its return value into setTimeout. Exactly the way foo(bar()) calls bar and passes its return value into foo.