Weird warning messages in TypeScript 0.9.1 that keep on changing

Weird warning messages in TypeScript 0.9.1 that keep on changing

document.getElementById("someElement").addEventListener('click', function
(event) {
this._output.innerHTML = this.add(
parseInt(this._x.value),
parseInt(this._y.value)
).toString();
If you take a look at the above statement, you can see that it is simply
some HTML that is used to attach an event-listener to a piece of DOM.
Below is some code that does the same thing:
document.getElementById("someElement").addEventListener('click', (event) => {
this._output.innerHTML = this.add(
parseInt(this._x.value),
parseInt(this._y.value)
).toString();
});
Here is the problem, in the first code sample, TypeScript does not give me
any warnings, meaning that all paremeters for addEventListener match.
However, when I use the second one, there seems to be a problem.
Typescript says, "Argument Types do not match paremeters".
What can I do to fix this. I am using Typescript 0.9.1 with Webstorm 7.0 EAP.
UPDATE 1
So, I've fixed my initial problem by changing the code to the following:
document.getElementById("Add").addEventListener('click',(event):void => {
this._output.innerHTML = this.add(
parseInt(this._x.value),
parseInt(this._y.value)
).toString();
If you take a look, I just added a return type. Now, I have a new problem:
Typescript complains that this._output is an unresolved variable. This is
my full typeScript class:
class Calculator {
private _x:HTMLInputElement;
private _y:HTMLInputElement;
private _output:HTMLSpanElement;
constructor (xID: string, yID: string, outputID: string) {
this._x = <HTMLInputElement> document.getElementById(xID);
this._y = <HTMLInputElement> document.getElementById(yID);
this._output = <HTMLInputElement> document.getElementById(outputID);
this.makeCalculator();
}
add (x: number, y: number) {
return x + y;
}
subtract (x: number, y: number) {
return x - y;
}
makeCalculator () {
document.getElementById("Add").addEventListener('click',(event):void
=> {
this._output.innerHTML = this.add(
parseInt(this._x.value),
parseInt(this._y.value)
).toString();
});
document.getElementById("Subtract").addEventListener('click',
(event):void => {
this._output.innerHTML = this.subtract(
parseInt(this._x.value),
parseInt(this._y.value)
).toString();
});
}
}
As you can see, I am trying to make a simple calculator in typescript.