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JavaScript Brain Teasers: Exercise Your Mind

Discussion: Waiting in Line

Explore how JavaScript handles asynchronous tasks through the event loop by analyzing a puzzle involving setTimeout and Promise. Understand the order of execution and how microtasks and the event queue affect output timing, helping you master asynchronous code behavior.

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Verifying the output

Now, it’s time to execute the code and observe the output.

Javascript (babel-node-es2024)
console.log("Start");
setTimeout(() => {
  console.log("Timeout");
}, 0);
Promise.resolve().then(() => {
  console.log("Promise");
});
console.log("End");

Understanding the output

Here, we have a mixture of synchronous and asynchronous operations and use console.log() to output different messages at various points in the code. Since the setTimeout() is set to 0 milliseconds, you might expect the messages to appear in the order of “Start,” “Timeout,” “Promise,” and “End.” However, the actual output is different because of the event loop and how JavaScript ...