javascript - How does .then(console.log()) and .then(() => console.log()) in a promise chain differ in execution

Question

Is there any difference in efficiency? Will the behavior be any different if a setTimeout is used instead of console.log()

Answer 1

You can basically do these three things

.then(console.log())

This calls the console.log immediately, without waiting until the promise is resolved, so it is not probably something that you would want to do.

---

.then(console.log)

This executes the console.log only after the promise has successfully resolved (requires one function call) and implicitly pass the result of the promise to to the console.log function.

---

.then(() => console.log())

Same as before, requires 2 function calls but you can easily pass some other arguments to it.

---

To pass additional argument to the console.log in the second case, you need to use Function.prototype.bind method.

const promise = new Promise((resolve, reject) => {
  resolve('');
});
promise.then(console.log.bind(console, 'new arg'));