When do plain code, Promise.then and setTimeout run?

First the current synchronous code finishes. Reactions to settled Promises go to the so-called microtask queue, which is processed before many other kinds of work. A timer can be handled only after its time has passed and when the environment gets a chance.

In the simple example console.log('A'); Promise.resolve().then(() => console.log('B')); setTimeout(() => console.log('C'), 0) we see A, then B and C. A zero delay on a timer doesn't mean “immediately”. For Node's extra mechanisms, such as nextTick and setImmediate, you need to take the context and version into account. Without that, don't recite one order for every program.