Deprecation Guide for Prototype Function Listeners

Currently, function listeners and string listeners behave identically to each other. Their inheritance and removal structure is the same, and they can be used interchangeably for the most part. However, function listeners can be much more expensive as they maintain a reference to the function.

Function listeners also have limited utility outside of per instance usage. Consider the following example which the same listener using strings and using function references:

class Foo {
  method() {}
}

addListener(Foo, 'event', null, 'method');
addListener(Foo, 'event', null, Foo.prototype.method);

It's clear that the string version is much more succinct and preferable. A more common alternative would be adding the listener to the instance in the constructor:

class Foo {
  constructor() {
    addListener(this, 'event', this, this.method);
  }

  method() {}
}

But in this case, the listener doesn't need to be applied to the prototype either.

Updating

In cases where function listeners have been added to a prototype, and those functions do exist on the prototype, replace them with string listeners:

Before:

class Foo {
  method() {}
}

addListener(Foo, 'event', null, Foo.prototype.method);

After:

class Foo {
  method() {}
}

addListener(Foo, 'event', null, 'method');

In cases where function listeners have been added to a prototype for arbitrary functions which do not exist on the prototype, you can convert the function to a method, create a wrapper function, or add the listener on the instance instead:

Before:

function foo() {}

class Foo {}

addListener(Foo, 'event', null, foo);

After:

class Foo {
  foo() {}
}

addListener(Foo, 'event', null, 'foo');

// OR
function originalFoo() {}

class Foo {
  foo() {
    originalFoo();
  }
}

addListener(Foo, 'event', null, 'foo');

// OR
function foo() {}

class Foo {
  constructor() {
    addListener(this, 'event', this, foo);
  }
}