When exactly is a promise triggered and can it (theoretically) execute faster than .then is called?

Question

JavaScript

In the subject of the question is its essence. I am not a strong JavaScript expert, I study promises, I read in the documentation that the promise code starts executing immediately after the creation of the promise object. But we make calls to .then and .error AFTER the Promise object is created. Example:

fetchResult(query) // возвращает промис
    .then(function(result) {
        return processResult(result);
    })
    .then(function(processedResult) {
        console.log('processed result', processedResult);
    });

QUESTION: Can it happen that a promise is executed faster than .then is called?

Answer 1

Promise can be completed faster. It's right.
But then will also be executed.
You can see for yourself by running the code

var promise = new Promise(function(resolve, reject) {
  console.log('I\'m promise');
  resolve('resolve');
});

for (var i=0; i<10000; i++) {
  console.log(1);
}

promise.then( function (result) {
  console.log(result);
  return 'f1';
});

promise.then( function (result) {
  console.log(result);
  return 'f2';
});

Console output

I'm promise
(10000 раз) 1
resolve
resolve

Answer 2

First, the query will be executed, then, if there were no errors, the first then, then, if there were no errors, the second. For the sake of observing this sequence, as it were, everything was conceived.