promise_chain_delay_example.js

// won't work correctly

let counter = 0;

function delay(ms) {
  return new Promise((resolve, reject) => {
    
    setTimeout(function() {
      console.log(++counter)
      resolve()
    }, ms);
  });
}

delay(1000)
  .then(delay(1000))
  .then(delay(2000))
  .then(() => console.log("Hello!"))
  
// ---

// work correctly

let counter = 0;

function delay(ms) {
  return new Promise((resolve, reject) => {
    
    setTimeout(function() {
      console.log(++counter)
      resolve()
    }, ms);
  });
}

delay(1000)
  .then(() => delay(1000))
  .then(() => delay(2000))
  .then(() => console.log("Hello!"))