The this keyword in typescript. Is it a bug?

Question

I have a member function render(). This function calls another member of the class add(any). This is the snippet.

render(){
    collection.each(this.add);
}

If I use the keyword "this" in add, the type is window. I would expect it to be the instance of the member class.

In a constructor, member function, or member accessor, this is of the class instance type of the containing class.

Answer 1

As JcFx points out, your this scope is different within the each callback.

However, if you use a 'fat arrow' anonymous function, that will use lexical scoping rules for this so that you get what you're looking for:

render(){
    collection.each((x) => this.add(x));
}

Compiles to the following JavaScript:

X.prototype.render = function () {
    var _this = this;
    collection.each(function (x) {
        return _this.add(x);
    });
}

Another option is to explicitly bind the add call to the desired this:

render(){
    collection.each(this.add.bind(this));
}

Answer 2

I don't think it's a bug.

You are no longer in the scope of the render() function, but instead in the scope of each().

Try this:

render(){
    var that = this;
    collection.each(that.add);
}