Suggestion: contextually type this in class methods

[OliverJAsh]

Search Terms

inference class methods contextually this parameter

Suggestion

Potentially related, but not the same (AFAICS): #1373

Inside of class methods, TypeScript knows what the type of this will be:

class Foo {
  value = 2
  add(number: number) {
    return this.value + number;
  }
}

const foo = new Foo();

const result = foo.add(1);

However, when the class method is passed around, TypeScript does not prescribe the this type:

declare function pipeWith<A, B>(a: A, ab: (this: void, a: A) => B): B;

// Runtime exception
// Uncaught TypeError: Cannot read property 'value' of undefined
// Expected type error but got none ❌ 
const result = pipeWith(1, foo.add);

For this to error as expected, it is necessary to explicitly define the type of this in the method signature:

-  add(number: number) {
+  add(this: Foo, number: number) {

// Type error ✅ 
const result = pipeWith(1, foo.add);

Suggestion: TypeScript could contextually annotate the this type of class methods, so users don't need to remember to manually define it.

Use Cases

Examples

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[OliverJAsh]

Annnddd as soon as I filed this, I discovered an existing issue: #35451