One of the real difficult things to deal with in JavaScript is understanding exactly to what the variable “this” currently refers.  ”this” is a scope variable which means that it can change from line to line. In most languages this wouldn’t be a big deal because the number of scopes is small but with JavaScript so much is done with anonymous functions that things become confusing quickly.

In TypeScript many of these internal function can be replaced with what I would call lambdas but I believe might also be known as “Fat Arrow Functions”. These are taken directly from ECMAScript 6.0. However there is a key difference between the new lambda functions and the current function denoted functions: the value of “this”. In a fad arrow function the value of “this” is bound to the outer scope, the lexical scope.

So if you’re at all familiar with d3.js which I’ve been using a lot as of late the “on” function requires that “this” be permitted to be set by d3.

TypeScript forces this to be bound to the outer context by replacing our call to this with one to _this which is a new variable that TypeScript creates. Obviously this doesn’t work for our case as we expect this to be boud whatever d3 has found during selection.

There are a number of possible fixes on StackOverflow but they seem over complicated to me and some of them are jQuery specific. Instead I recommend simply using a traditional function instead of the fat arrow function.