Call/Apply

example.js

// You wrote this line
Function.prototype.call(current[ev], this, arguments);


// This line does what I want, calling the method on the current tool and passing the arguments.
current[ev].apply(this, arguments);

// Not sure what you were aiming for with the other line?

I meant to use apply. Invoking the function via Function.prototype helps guard against Cannot call method 'apply' of undefined errors if the object assigned to current doesn't have the event in question for whatever reason.

> current = {}
Object
> current['bogus'].apply(this, [])
TypeError: Cannot call method 'apply' of undefined
> Function.prototype.apply(current['bogus'], this, [])
undefined

Caveat that I ran into today: using call/apply on Function.prototype invokes it as expected, but returns undefined.