Yes, that is a good example of a useful closure. The call to warnUser creates the calledCount
variable in its scope and returns an anonymous function which is stored in the warnForTamper
variable. Because there is still a closure making use of the calledCount variable, it isn't deleted upon the function's exit, so each call to the warnForTamper()
will increase the scoped variable and alert the value.
The most common issue I see on Stack Overflow is where someone wants to "delay" use of a variable that is increased upon each loop, but because the variable is scoped then each reference to the variable would be after the loop has ended, resulting in the end state of the variable:
for (var i = 0; i < someVar.length; i++)
window.setTimeout(function () {
alert("Value of i was "+i+" when this timer was set" )
}, 10000);
This would result in every alert showing the same value of i
, the value it was increased to when the loop ended. The solution is to create a new closure, a separate scope for the variable. This can be done using an instantly executed anonymous function, which receives the variable and stores its state as an argument:
for (var i = 0; i < someVar.length; i++)
(function (i) {
window.setTimeout(function () {
alert("Value of i was " + i + " when this timer was set")
}, 10000);
})(i);