Promise Then Chain

Trace ordered Promise callbacks as a single-thread event-loop sequence.

promise_then_chain.js

const base = 6;
let first = 0;
let second = 0;

function addStep(value) {
  first = value + 4;
  return first;
}

function finishStep(value) {
  second = value * 2;
  console.log("base=" + base);
  console.log("first=" + first);
  console.log("second=" + second);
}

Promise.resolve(base)
  .then(addStep)
  .then(finishStep);

promise-chain A Promise chain runs callbacks later, but still in a deterministic order. Replay shows each callback updating the same source panel.