TypeScript: How to use both fat arrow and this?

Question

I'm using the extremely useful local fat arrow to preserve this context in callbacks. However, sometimes I need to access the value that this would've had if I hadn't used the fat arrow.

One example are event callbacks, where this has the value of the element that the event happened on (I'm aware that in this particular example you could use event.currentTarget, but lets assume you can't for the sake of an example):

function callback() {
    // How to access the button that was clicked?
}

$('.button').click(() => { callback() });

Note: I've come across this question which deals with this exact same issue, but in CoffeeScript.

Answer 1

You could write a decorator function that wraps a fat-arrow function inside another function which allows the access to the usual this and passes that value to the fat-arrow function as an additional argument:

function thisAsThat (callback) {
    return function () {
        return callback.apply(null, [this].concat(arguments));
    }
}

So when you call thisAsThat with a fat-arrow function, this basically returns a different callback function that, when called, calls the fat-arrow function with all the arguments but adds this as an argument in the front. Since you cannot bind fat-arrow functions, you can just call bind and apply on it without having to worry about losing the value.

You can then use it like this:

element.addEventListener('click', thisAsThat((that, evt) => console.log(this, that, evt)));

This will log the this of the current scope (as per fat-arrow rules), the this of the callback function as that (pointing to the element for event handlers), and the event itself (but basically, all arguments are still passed on).