declaring callback scope members


Many JavaScript libraries have a usage pattern where you call a method and pass a callback, and later it gets called with both argument and a custom scope for this (eg: via Function.call() or apply()).

This is a sneaky way to pass implicit state while keeping the signature clean. It is arguable not ideal but it is used in many popular libraries so we'd like to be able to describe this.

Simple examples:
Vue.directive('my-directive', function (value: any) {
    this.el.innerHTML = 'foo: ' + this.id;
    // ...
    .node('!foo', function (value) {
        if (value == bad) {
        // ...
In case of the first snippet you could have something like:
declare module Vue {
    function directive: (name: string, callback: DirectiveCallback): void;
interface DirectiveCallback {
    (value: any): void # {
        id: string;
        el: DOMElement;
// or split it up:
interface DirectiveCallback {
    (value: any): void $ DirectiveCallbackScope;
interface DirectiveCallbackScope {
    id: string;
    el: DOMElement;
It uses the callable signature with a interface body (with a prefix maybe, like # or $ to break pattern and make it explicit the this is about the this scope).
This would work for any callable signature declaration. (interface member, callable-interface, module function and variable or return types etc).
Closed Jul 28, 2014 at 11:18 PM by jonturner
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jamesnw wrote Apr 3, 2014 at 10:54 PM

I think this may be solved via this issue: (no pun intended! LOL)

Bartvds wrote Apr 3, 2014 at 11:24 PM

Hah, this is exactly what I meant!

So this is a duplicate: let's close it (but I cannot close my own tickets so somebody else needs to do it)