Should creating a variable named _this conflict with this?

Question

This is a very contrived example, but let's suppose we create a variable _this somewhere in a class function.

class Person {
  constructor (public name : string) {}
  changeName(name) {
    var _this = {};
    (() => {
      this.name = name;
    })();
  }
}

This will not work as expected when we call the changeName function because the relevant part of the compiled code looks like this:

var _this = this;
var _this = {};
(function () {
  _this.name = name;
})();

This is bad Javascript: we have two var declarations overwriting each other. The _this created by the compiler is being overwritten by my _this.

As far as I can see, this behavior isn't specified in the TypeScript spec.

Why should they conflict? Can't the compiler detect if I have created a variable named _this and name the automatically generated one something else, like _this2 to keep my variables and the compiler-generated ones from trampling on each other?

Answer 1

The compiler will automatically create _this as a reference to this to facilitate the closure that will be created by using lambda syntax. I'm pretty sure I read this in the TypeScript specification somewhere, but I'd agree that the compiler should emit an error in this case.

I don't like the idea of the compiler varying how it emits javascript as this conflicts with the stated goal of generating "idiomatic" (and therefore predictable) javascript.

Answer 2

Isn't it what you would call a reserved variable? I guess you wouldn't declare a variable called "this" or "document" either.

As long as MS is clear as to what the reserved variables are, I don't think there's a major problem. They could perhaps improve the compiler to output warnings though.