What Are the Differences (If Any) Between Es6 Arrow Functions and Functions Bound with Function.Prototype.Bind

Is there any meaningful difference bind() vs arrow function + call()?

let a = function(){};
let b = function(){};

a.m = 1
b.m = 2
a.someMethod = function(x, y, z){ return this.m + x + y + z }

let x = a.someMethod.bind(b, 1, 2, 3);
let y = () => a.someMethod.call(b, 1, 2, 3)

console.log( x(1,2,3) )
console.log( y(1,2,3) )

function goBind() {
    for (var i = 0; i < 1000000; i++) {
        x(1,2,3)
    }
}

function goArrow() {
    for (var i = 0; i < 1000000; i++) {
        y(1,2,3)
    }

}

function race() {
  var start = performance.now();
  goBind();
  console.log('bind: ' + (performance.now() - start));
  start = performance.now();
  goArrow()
  console.log('arrow: ' + (performance.now() - start));
  start = performance.now();
  goBind();
  console.log('bind: ' + (performance.now() - start));
  start = performance.now();
  goArrow()
  console.log('arrow: ' + (performance.now() - start));
  console.log('------');
}

<button onclick="race()">RACE!</button>

How does ecmascript-6 Function.prototype.bind() handle a class constructor?

This doesn't have anything to do with classes specifically but with how .bind works.

You have been on the right track. The most relevan section here is 9.4.1.2.

Just as a recap: ECMAScript distinguishes between two types of functions: callable functions and constructable functions. Function expressions/declarations are both, whereas e.g. class constructors are only constructable and arrow functions are only callable.

In the specification this is represented by function's internal [[Call]] and [[Construct]] methods.

new will trigger the invocation of the internal [[Construct]] method.

.bind will return a new function object with different implementations for [[Call]] and [[Construct]]. So what does the "bound" version of [[Construct]] look like?

9.4.1.2 [[Construct]] ( argumentsList, newTarget )

When the [[Construct]] internal method of a bound function exotic object, F that was created using the bind function is called with a list of arguments argumentsList and newTarget, the following steps are taken:

1. Let target be F.[[BoundTargetFunction]].
2. Assert: IsConstructor(target) is true.
3. Let boundArgs be F.[[BoundArguments]].
4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
5. If SameValue(F, newTarget) is true, set newTarget to target.
6. Return ? Construct(target, args, newTarget).

What this means is that the bound constructor will "construct" the original function (F.[[BoundTargetFunction]]) with the bound arguments (F.[[BoundArguments]]) and the passed in arguments (argumentsList), but it completely ignores the bound this value (which would be F.[[BoundThis]]).

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but are there other sorts of functions that are being fed something else than the value passed to bind() ?

Yes, arrow functions. Arrow functions do not have their own this binding (the value from the closest this providing environment is used instead), so bound arrow functions also ignore the bound this value.

ES6 Arrow functions vs ES5: how to know which function to bind `this` to when using ES5 non-arrow function

Why do you use bind(this) with the callback to setTimeout rather than with the counter function?

Because the counter function (which works like a method of the obj object) already has the proper this because you call it like obj.counter() so it gets this from calling it as obj.counter(). Assuming you call counter as obj.counter(), then if you do console.log(this.id) on the first line of the counter() function, it will properly show the id value.

The callback you pass to setTimeout() however has no natural this value unless you use an arrow function or .bind() on the callback function itself because when setTimeout() calls your callback it does not set a specific this value (it just calls your callback as a normal function), therefore the this value goes to the default. That means this will be undefined if running strict mode or the global object if running in loosey-goosey mode inside of the setTimeout() callback.

See the 6 ways that the value of this is set here when calling a function.

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I should also mention that if you did what you were proposing like this:

var obj = {
  id: 42,
  counter: function counter() {
    setTimeout(function() {
      console.log(this.id);
    }, 1000);
  }.bind(this);
};

Not only would it not help the setTimeout() callback at all, but it would also bind the wrong value of this to the counter() method. You would get whatever this was before the var obj definition (also known as the lexical value of this).

What is the difference between () = {} and function() {} in react-native javascript?

The () => {} is called an arrow function. They are, as you said, part of ES6. From the linked page:

An arrow function expression has a shorter syntax compared to function expressions and lexically binds the this value (does not bind its own this, arguments, super, or new.target). Arrow functions are always anonymous.

Are 'Arrow Functions' and 'Functions' equivalent / interchangeable?

tl;dr: No! Arrow functions and function declarations / expressions are not equivalent and cannot be replaced blindly.

If the function you want to replace does not use this, arguments and is not called with new, then yes.

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As so often: it depends. Arrow functions have different behavior than function declarations / expressions, so let's have a look at the differences first:

1. Lexical this and arguments

Arrow functions don't have their own this or arguments binding. Instead, those identifiers are resolved in the lexical scope like any other variable. That means that inside an arrow function, this and arguments refer to the values of this and arguments in the environment the arrow function is defined in (i.e. "outside" the arrow function):

// Example using a function expression
function createObject() {
  console.log('Inside `createObject`:', this.foo);
  return {
    foo: 42,
    bar: function() {
      console.log('Inside `bar`:', this.foo);
    },
  };
}

createObject.call({foo: 21}).bar(); // override `this` inside createObject

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