https://tc39.es/ecma262/multipage/fundamental-objects.html#sec-function.prototype.bind:
I can't understand the notes:
Note 1 Function objects created using
Function.prototype.bindare exotic objects. They also do not have a "prototype" property.Note 2 If Target is either an arrow function or a bound function exotic object, then the thisArg passed to this method will not be used by subsequent calls to F.
Can someone give some examples?
Regarding Note 1:
const bound = (function(){}).bind();
console.log(bound instanceof Function);
console.log(!("prototype" in bound));
// being an "exotic object" just means that it behaves unusual - in this case,
// having a special [[call]] operation
Regarding Note 2:
function example() { "use strict"; console.log(this); }
const bound = example.bind("Hello");
const boundAgain = bound.bind("World");
console.log(bound(), boundAgain()); // Hello Hello
function makeArrow() { "use strict"; return () => console.log(this); }
const arrow = makeArrow.call("Hello");
const boundArrow = bound.bind("World");
console.log(arrow(), boundArrow()); // Hello Hello
Note 1: Bound functions do not have prototypes
const target = { foo: "bar" };
const f = function() {};
const bound = f.bind(target);
console.log("f.prototype", f.prototype);
console.log("bound.prototype", bound.prototype);
VM439:4 f.prototype {constructor: ƒ}
VM439:5 bound.prototype undefined
Note 2: Arrow functions cannot be bound
const target = { foo: "bar", toString: function() { return "foobar"; } };
const normal = function() { return this; };
const arrow = () => { return this; };
const boundNormal = normal.bind(target);
const boundArrow = arrow.bind(target);
console.log("normal()", normal());
console.log("boundNormal()", boundNormal());
console.log("arrow()", arrow());
console.log("boundArrow()", boundArrow());
console.log("normal.call(target)", normal.call(target));
console.log("arrow.call(target)", arrow.call(target));
normal() Window {postMessage: ƒ, blur: ƒ, focus: ƒ, close: ƒ, parent: Window, …}
boundNormal() {foo: "bar", toString: ƒ}
arrow() Window {postMessage: ƒ, blur: ƒ, focus: ƒ, close: ƒ, parent: Window, …}
boundArrow() Window {postMessage: ƒ, blur: ƒ, focus: ƒ, close: ƒ, parent: Window, …}
normal.call(target) {foo: "bar", toString: ƒ}
arrow.call(target) Window {postMessage: ƒ, blur: ƒ, focus: ƒ, close: ƒ, parent: Window, …}
boundArrow in your example), but they of course ignore the bound this argument as they still use lexical scoping.