javascript - this pointing problem in js extended prototype method

Question

The book JavaScript Ninja Secrets has a prototype library and an example of function bind code:

Function.prototype.bind = function(){
            var fn = this, args = Array.prototype.slice.call(arguments),
                object = args.shift();
            return function(){
                  return fn.apply(object,args.concat(Array.prototype.slice.call(arguments)));
            };
        }; 
            var myObject = {a:"1"};
            function myFunction(){
                return this == myObject;
            };
            var aFunction = myFunction.bind(myObject);

            aFunction();

I use breakpoints to check the functionbindThe fn inside points to myFunctionI don’t quite understand this function. My understanding is that just use Function.prototype Prototype extension method The variable var fn=this; in the declared variable fn=this; points to the function using this method, just like myFunction in this example. bind(myObject);Call the bin method, fn points to myFunction I don’t know if this function is correct or not

Answer 1

Actually, I don’t quite understand your question, so let’s walk through the code with ideas

myFunction.bind(myObject) enters the prototype’s bind function

1. Assign myFunction to fn, convert the parameters into an array args, delete the first parameter and assign it to object.

2. Return a function, then think about closures, and then you will understand. The work done by this function can be simply understood as myFunction.apply(myObject,[...here are other parameters])

aFunction()

It is myFunction.apply(myObject,[...there are no parameters here])
Then enter the inside of myFunction, because of the relationship between apply, this is myObject, and then the work is myObject==myObject
Return true