Promise constructor is synchronous

⬅️ Execute Program JavaScript Concurrency

Promise constructor is synchronous

• When we do new Promise(...) and pass a callback to the constructor, that cb is ran immediately (even if you wrapped with a setTimeout)

const array = [];
array.push('before');
 
new Promise(resolve => {
  array.push('constructed');
});
 
array.push('after');
array;
 
// Result:
['before', 'constructed', 'after']

• but then always works async
• 💡 Constructor callbacks always run immediately and synchronously, while then callbacks are scheduled to run asynchronously.