The Observer Pattern - Test-Driven JavaScript Development - page 238

Listing 11.35 Emptying the constructor
function Observable() {
}
Running the tests shows that all but one is now failing, all with the same message:
“this.observers is not defined.” We will deal with one method at a time. Listing 11.36
shows the updated addObserver method.
Listing 11.36 Defining the array if it does not exist in addObserver
function addObserver(observer) {
if (!this.observers) {
this.observers = [];
}
/* ... */
}
Running the tests again reveals that the updated addObserver method fixes
all but the two tests that do not call it before calling other methods, such as
hasObserver and notifyObservers . Next up, Listing 11.37 makes sure to
return false directly from hasObserver if the array does not exist.
Listing 11.37 Aborting hasObserver when there are no observers
function hasObserver(observer) {
if (!this.observers) {
return false;
}
/* ... */
}
We can apply the exact same fix to notifyObservers , as seen in Listing
11.38.
Listing 11.38 Aborting notifyObservers when there are no observers
function notifyObservers(observer) {
if (!this.observers) {
return;
}
/* ... */
}