`this.some_property` becomes undefined inside anonymous callback function
The scope changes when you enter that anonymous closure and 'this' changes. You can hack around it by doing
var self = this;
And then using self in place of this (eg):
function Goal(name) {
var self = this;
/* ... */
this.gDiv.addEventListener('click', function(){
alert(self.tasks);
});
If you're using jQuery you could do something nicer:
this.gDiv.addEventListener('click', $.proxy(function() {
alert(this.tasks);
}, this));
Either way works just fine.
EDIT: In ES6, arrow functions can be used instead as they don't bind their own "this", so it becomes even simpler:
this.gDiv.addEventListener('click', () => {
alert(this.tasks);
});
Here is a comparison of some methods (including your problem), to give you a taster, and to try and explain things a little.
// This is the problem that you have,
// where `this` inside the anonymous function
// is a different scope to it's parent
function Test1(something) {
// `this` here refers to Test1's scope
this.something = something;
setTimeout(function() {
// `this` here refers to the anonymous function's scope
// `this.something` is `undefined` here
console.log(this.something);
}, 1000);
};
new Test1('Hello');// This solution captures the parent `this` as `test2This`,
// which can then be used inside the anonymous function
function Test2(something) {
var test2This = this;
this.something = something;
setTimeout(function() {
console.log(test2This.something);
}, 1000);
}
new Test2('World');// This solution captures `this` as `test3This` in an `IIFE closure`
// which can then be used in the anonymous function
// but is not available outside of the `IIFE closure` scope
function Test3(something) {
this.something = something;
(function(test3This) {
setTimeout(function() {
console.log(test3This.something);
}, 1000);
}(this));
}
new Test3('Goodbye');// This method requires that you load an external library: jQuery
// and then use it's `$.proxy` method to achieve the basics of
// Test3 but instead of being referred to as `test3This` the
// outer scope `this` becomes the inner scope `this`
// Ahh, that's much clearer?
function Test4(something) {
this.something = something;
setTimeout($.proxy(function() {
console.log(this.something);
}, this), 1000);
}
new Test4('Mum');<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>// This is approximately what jQuery's `$.proxy` does
// but without having to load the whole library
function Test5(something) {
this.something = something;
setTimeout((function(func, context) {
return function() {
func.call(context);
};
}(function() {
console.log(this.something);
}, this)), 1000);
}
new Test5('Dad');// Lets create the proxy method as a reuseable
function proxy(func, context) {
var args = Array.prototype.slice.call(arguments, 2);
return function() {
return func.apply(
context,
args.concat(Array.prototype.slice.call(arguments))
);
};
}
// and now using it
function Test6(something) {
this.something = something;
setTimeout(proxy(function() {
console.log(this.something);
}, this), 1000);
}
new Test6('Me want cookies');Then we have Function#bind
function Test7(something) {
this.something = something;
setTimeout(function() {
// `this` was bound to the parent's `this` using bind
console.log(this.something);
}.bind(this), 1000);
};
new Test7('Num num');<script src="https://cdnjs.cloudflare.com/ajax/libs/es5-shim/4.5.9/es5-shim.min.js"></script>And most recently ES2015 Arrow functions
function Test8(something) {
this.something = something;
setTimeout(() => console.log(this.something), 1000);
};
new Test8('Whoop');