How does this Javascript code works?

Question

let promise = new Promise(function(resolve, reject) {
resolve(1);

setTimeout(() => resolve(2), 1000);
});

promise.then(alert);//should not it be promise.then((message)=>alert(message))

Answer 1

It alerts with 1 immediately because only the first resolve or reject call matters in a Promise.

The call to resolve(2) is ignored since the promise is already resolved at that point.

In other words, your code's behaviour simplifies to:
let promise = new Promise(function(resolve, reject) {
resolve(1);
});
promise.then(alert);

or simplifies more to:
alert(1);