When do I use parenthesis and when do I not?

The setTimeout function expects a function reference^* as an argument: references are variables, and don’t have parentheses.

Function calls require parentheses (even if the function takes no parameters).

Nutshell: myFunction is a reference to the function. myFunction() executes the function, and in an expression, will “equal” the function’s return value (or undefined if nothing is returned).

Digging Deeper: There are circumstances where setTimeout(myFunction(), 1000) might make sense, like if myFunction() itself returns a function. For example:

var myFunction = function() {
    return function() {
        alert("ohai");
    };
};

• The anonymous function (containing a single return statement) is executed immediately.
• The return value of that function is itself a function containing an alert.

So:

• myFunction alone is a reference to a function (that happens to return a function).
• myFunction() will execute. It evaluates to a function reference, suitable for setTimeout().

Finally:

setTimeout(myFunction(), 1000);

This calls myFunction()‘s return value in one second. One second later, up pops the alert.

See also Why function statement requires a name?

^{* Or a string to be evaluated, but a reference is preferred.}