'this' implicitly has type 'any' because it does not have a type annotation

Issue

When I enable noImplicitThis in tsconfig.json, I get this error for the following code:

'this' implicitly has type 'any' because it does not have a type annotation.

class Foo implements EventEmitter {
  on(name: string, fn: Function) { }
  emit(name: string) { }
}

const foo = new Foo();
foo.on('error', function(err: any) {
  console.log(err);
  this.emit('end');  // error: `this` implicitly has type `any`
});

Adding a typed this to the callback parameters results in the same error:

foo.on('error', (this: Foo, err: any) => { // error: `this` implicitly has type `any`

A workaround is to replace this with the object:

foo.on('error', (err: any) => {
  console.log(err);
  foo.emit('end');
});

But what is the proper fix for this error?

---

UPDATE: It turns out adding a typed this to the callback indeed addresses the error. I was seeing the error because I was using an arrow function with a type annotation for this:

Solution

The error is indeed fixed by inserting this with a type annotation as the first callback parameter. My attempt to do that was botched by simultaneously changing the callback into an arrow-function:

foo.on('error', (this: Foo, err: any) => { // DON'T DO THIS

It should've been this:

foo.on('error', function(this: Foo, err: any) {

or this:

foo.on('error', function(this: typeof foo, err: any) {

---

A GitHub issue was created to improve the compiler's error message and highlight the actual grammar error with this and arrow-functions.

Answered By - tony19