How to use .then() in node.js?

Question

I'm a complete beginner in node.js. I've just read we can use .then() function for executing several functions in particular order. I was going to write the code this way:

function one(){
  console.log("one")
}
function two(){
  console.log("two")
}
function three(){
  console.log("three")
}
one().then(two()).then(three())

But I'm getting this error:

TypeError: Cannot read property 'then' of undefined
at Object.<anonymous> (C:\chat\test.js:10:6)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.runMain (module.js:604:10)
at run (bootstrap_node.js:389:7)
at startup (bootstrap_node.js:149:9)
at bootstrap_node.js:502:3

Answer 1

.then is a method that exists on Promises and is a mechanism for code synchronization. Your code is not asynchronous, so you wouldn't need to use promises. You can just call

one();
two();
three();

If your code does something asynchronous, then you can use promises and .then. Asynchronous operations are things like reading/writing files, http requests, timers, and many more.

Just as an example, we can use the built in Promise to create our own asynchronous operations:

I don't recommend you do this normally. We're just using it as an example. In most cases you can call functions that already return promises for you.

function one() {
  return new Promise(resolve => {
    console.log("one");
    resolve();
  });
}

function two() {
  return new Promise(resolve => {
    console.log("two");
    resolve();
  });
}

function three(){
   console.log("three")
}

one().then(() => two()).then(() => three());

Also note that when you use .then, you need to pass a callback. two() calls the two function immediately, so it's not the same as () => two().

---

Next, you can often use async/await instead of .then which I think makes your code easier to reason about in most cases.

async function run() {
  await one();
  await two();
  three();
}
run();

This is the same as the second example rewritten to use await instead of .then. You can think of everything after await as being inside of a .then chained to the expression after await.

---

Finally, you should handle errors by either chaining .catch to the promises or using the normal try/catch inside of async functions.