Question

I can’t help but notice there are two seemingly useless functions in the source code of jQuery (For v1.9.1, it’s line 2702 and line 2706):

function returnTrue() {
    return true;
}

function returnFalse() {
    return false;
}

Which both are called quite often within jQuery. Is there a reason why they don’t simply substitute the function call with a boolean true or false?

Answer 1

it was used like this:

stopImmediatePropagation: function() {
    this.isImmediatePropagationStopped = returnTrue;
    this.stopPropagation();
}

here isImmediatePropagationStopped is a query method. used like this event.isImmediatePropagationStopped()

of course, you can define a instance method, like:

event.prototyoe.isImmediatePropagationStopped = function() { return this._isImmediatePropagationStopped };

stopImmediatePropagation: function() {
    this._isImmediatePropagationStopped = true; //or false at other place.
    this.stopPropagation();
}

but you have to introduce a new instance property _isImmediatePropagationStopped to store the status.

with this trick, you can cut off bunch of instance properties for hold true/false status here, like _isImmediatePropagationStopped, _isDefaultPrevented etc.

so that, in my opinion, this is just a matter of code style, not right or wrong.

PS: the query methods on event, like isDefaultPrevented , isPropagationStopped, isImmediatePropagationStopped are defined in DOM event level 3 sepc.

spec: http://www.w3.org/TR/2003/NOTE-DOM-Level-3-Events-20031107/events.html#Events-Event-isImmediatePropagationStopped