Suggestion : "automatic" context binding

# Ludwig GUERIN (21 days ago)

Sometimes, you need to pass a callback as a parameter of a function.

Sometimes, you would like to pass in a method as the said parameter but you may encounter the common issue that the this keyword is not bound to the correct context anymore ; therefore you need to bind the correct context.

Let's says that you have :

  • a function f which receives a callback
  • and an object obj which has a method method which requires the this keyword to be bound to obj in order to work properly as intended.

You will probably call f this way : f(obj.method.bind(obj));

This is perfectly fine but sometimes can lead to a very large amount of extra "effort" for something that should not require that much "effort" and tends to take away a lot of space and clarity : f(obj.prop.method.bind(obj.prop));

My suggestion to solve this issue would be to use a new notation based around what C++ developers call the scope resolution operator, aka ::.

Here are details on how this should work :

  • {object} refers to an object (it can be either obj, or obj.method, or obj.prop, etc...)
  • {method} refers to one of {object}'s methods (if {object} is obj and we have obj.method then {method} would be method)
  • The "old-fashioned" way would look like {object}.{method}.bind({object})
  • With this suggestion, it would look like {object}::{method}

One way to implement such a functionality would be to establish a function autoBindContext which would guarantee the following properties :

  • {object}::{method} <=> autoBindContext({object}.{method}, {object})
  • Given any function/method {fn}(either f or obj.method), autoBindContext({fn}, null) === {fn}
  • {object}::{method} <=> ({object})::{method}
  • const a = {object}::{method};
    const b = {object}::{method};
    a==b; //is true
    a===b; //is true

Below is one implementation of autoBindContext :

const autoBindContext = (method, context=null) => {
  if(typeof method !== "function")
      throw new TypeError("Can only bind the context of a function/method");
  if(context === null)
   return method; //Do not bind if the context is null
  if(!(context instanceof Object))
   throw new TypeError("Can only bind a function's/method's context to an object");
  const self = autoBindContext;
  if(!self.cache.hasBinding(method, context))
   self.cache.addBinding(method, context);
  return self.cache.getBinding(method, context);
autoBindContext.cache = {
   binding: {context, method, bound}
 bindings: [],
  lookForBinding(method, context=null){
   return this.bindings.find(binding => {
      return binding.context === context
      && binding.method === method;
  hasBinding(method, context=null){
   return this.lookForBinding(method, context) !== undefined;
  getBound(method, context=null){
   if(this.hasBinding(method, context))
     return this.lookForBinding(method, context).bound;
   return null;
  getBinding(method, context=null){
   return this.getBound(method, context);
  addBinding(method, context=null){
   if(!this.hasBinding(method, context)){
        bound: method.bind(context)
    return this;
# T.J. Crowder (21 days ago)

This already has a Stage 0 proposal here: tc39/proposal-bind-operator.

You can find all proposals at tc39/proposals in (currently) three categories: That page itself, which lists proposals for Stages 1, 2, and 3; and the linked lists of Stage 0 proposals, Inactive proposals, and Finished proposals.

-- T.J. Crowder