Meta-issue: this disambiguator

[RyanCavanaugh]

Do not discuss the details of these in this bug! Go to the appropriate linked issue to discuss them

This issue is to track all the various this proposals floating around and to distinguish them. Any comments here should be clarifying comments/questions.

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Specify this as a function return value

#285

Let type definitions specify that the type of a field (or maybe just function return values) is the same as the type that contains the property.

Example:

interface Animal {
  clone(): this; // Today: clone(): Animal;
  self: this;
}
interface Dog extends Animal { woof(); }
interface Cat extends Animal { meow(); }
var c: Cat;
var cc = c.clone(); // Desired: cc: Cat
cc.meow(); // Desired: OK. Today: Fails
var ccc = cc.self; // Desired: ccc: Cat

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Specify, as part of a function, the type of this inside the function body

#229

Allow a syntax to specify the type of this as part of a function expression.

Example:

function fn(this: HTMLElement) {
  this.focus(); // OK
}
fn(); // 'this' argument not visible externally. May or may not be error

---

Specify, as part of a function type, the type of this inside a function body

(no issue yet, please leave a comment if you create one)
Allow a syntax to specify that when a function is invoked, this will be of a certain type

Example:

// http://api.jquery.com/jquery.each/
function jquery_each<T>(items: T[], callback: (this: T, index: number, value: T) { /* ... */ }
jquery_each(['foo', 'bar'], () => console.log(this.substr(1));

---

Specify the what valid this values are during function invocation

#15

Add type system logic to detect when a method is being invoked without a proper this context

class Foo {
  bar = 5;
  printBar() { console.log(this.bar); }
}
var f = new Foo();
// prints 'undefined', not '5'
window.setImmediate(f.printBar); // Flag this as an error

[LPGhatguy]

It appears most all of these suggestions have merged into #3694 except for the last one.

[sandersn]

#3694 also covers the last one -- once you specify a this type for a function, function invocation needs to check that the caller satisfies the type.

[mhegazy]

#6739 covers most of the remaining pieces.