Promise constructor with reject call vs throwing error

Is there any difference between using reject (in p2) from the Promise api, and throwing an error (in p1) using throw?

Yes, you cannot use throw asynchronously, while reject is a callback. For example, some timeout:

new Promise(_, reject) {
    setTimeout(reject, 1000);
});

Its exactly the same?

No, at least not when other code follows your statement. throw immediately completes the resolver function, while calling reject continues execution normally – after having “marked” the promise as rejected.

Also, engines might provide different exception debugging information if you throw error objects.

For your specific example, you are right that p1 and p2 are indistinguishable from the outside.