I tried to recreate javascript promise, following this tutorial, this is the complete code from the tutorial:

function myPromise(callback) {

    let resolve = function(value){
        this.resolveCallback(value);
    }
    callback(resolve)

}

// .then
myPromise.prototype.then = function(resolveCallback){
    attachResolveCallback(resolveCallback);
}

const attachResolveCallback = function(resolveCallback){
  this.resolveCallback = resolveCallback;
}

im confused about the part that implements .then():

// .then
myPromise.prototype.then = function(resolveCallback){
    attachResolveCallback(resolveCallback);
}

const attachResolveCallback = function(resolveCallback){
  this.resolveCallback = resolveCallback;
}

Why is const attachResolveCallback needed, instead of just using this keyword in the prototype function?

So I replaced the .then part of the code with:

myPromise.prototype.then = function(resolveCallback){
    this.resolveCallback = resolveCallback;
}

But the code does not work and returns an error:

promise.js:4 Uncaught TypeError: this.resolveCallback is not a function
    at resolve (promise.js:4:14)
    at promise.js:25:9

I suspect this problem is due to usage of 'this' when assigning properties to functions, but im not sure about the specific reason here. Could someone enlighten me on this problem?