# JSON, Regex & the Event Loop — JavaScript

Source: https://www.geekswithgeeks.com/en/javascript/js-json-regex-loop

> Serializing data, matching patterns, and why async doesn't block.

## JSON.stringify & parse

`stringify` turns a JS value into a JSON string; `parse` does the reverse.

```js
const user = { name: "Ada", age: 36 };
const json = JSON.stringify(user);
console.log(json);              // '{"name":"Ada","age":36}'
console.log(JSON.parse(json));  // back to an object
```

## Basic regex

`.test()` checks for a match and returns a boolean; `.match()` returns the actual matched text.

```js
const pattern = /^\d{3}-\d{4}$/;
console.log(pattern.test("555-1234"));       // true
console.log("Call 555-1234 now".match(/\d{3}-\d{4}/)[0]);  // "555-1234"
```

## The event loop, at a glance

JS runs on **one thread**, but async work (timers, fetch) is handed off to the browser/Node, and its callback is queued to run only after the current call stack empties — that's why async code doesn't block.

**Quiz:** Why doesn't setTimeout(fn, 0) run fn immediately?

- [x] Because it queues fn to run after the current call stack finishes
- [ ] Because 0ms actually means never
- [ ] Because setTimeout is fully synchronous

*Answer:* Because it queues fn to run after the current call stack finishes. Its callback still waits for the call stack to clear, even with a 0ms delay.