JavaScript Promise Declaration Difference

Question

I am a JavaScript learner and have an issue about Promises.

When I use function resolve of Promise object, I got no errors as expected. Like :

var promise1 = Promise.resolve(3);

However, when I use following declaration:

var promise2 = new Promise(function(resolve, reject) {
    setTimeout(resolve, 1000, 'foo');
});
promise2.resolve(4)

Isn't this the same thing? Or if promise2 is also a Promise class object, why do I get an error like :

TypeError: promise2.resolve is not a function
    at Object.<anonymous>

Answer 1

No, they are not the same thing.

The resolve method on the Promise (with a capital P) constructor function creates a new promise and immediately resolves it.

Existing promises don't have a resolve method. They can only be resolved by calling the function passed to the first argument of the function you pass to the Promise constructor function.

---

Or if promise2 is also a Promise class object, why do I get an error like

promise2 is an instance of Promise.

Promise is a constructor function used to create promises, it isn't a promise itself.

Consider the difference between a motor car and a Ford factory.