JSON, Timers and CLI Tools

Working with JSON

JSON (JavaScript Object Notation) is commonly used for configuration, data files, and APIs.

Read JSON from a File

const fs = require("fs");

const data = fs.readFileSync("data.json", "utf8");
const parsed = JSON.parse(data);

console.log(parsed.name);

Write JSON to a File

const user = { name: "Alice", age: 25 };
const json = JSON.stringify(user, null, 2); // pretty print

fs.writeFileSync("user.json", json);

Parse and Stringify

const obj = JSON.parse('{"name": "Bob"}');
const str = JSON.stringify(obj);

Timers

Node provides timer functions similar to the browser.

setTimeout

Run a function once after a delay (ms).

setTimeout(() => {
  console.log("Delayed message");
}, 1000);

setInterval

Run a function repeatedly at an interval (ms).

const intervalId = setInterval(() => {
  console.log("Repeats every 2s");
}, 2000);

setTimeout(() => clearInterval(intervalId), 7000); // stop after 7s

setImmediate

Runs a callback after the current event loop phase.

setImmediate(() => {
  console.log("Runs immediately after current task");
});

clearTimeout / clearInterval

Stop scheduled timers.

const id = setTimeout(() => {}, 5000);
clearTimeout(id);

Building CLI Tools

Reading Command-Line Arguments

const args = process.argv.slice(2);
console.log(args); // ["hello", "world"]

Run with:

node script.js hello world

Simple Flag Parser

const flags = {};
process.argv.slice(2).forEach(arg => {
  const [key, value] = arg.split("=");
  flags[key] = value;
});

console.log(flags);
// node app.js name=alice age=30
// Output: { name: 'alice', age: '30' }

Output with process.stdout

process.stdout.write("Type something: ");

Input with process.stdin

process.stdin.on("data", (data) => {
  console.log(`You typed: ${data.toString().trim()}`);
});

Summary

Concept	Method/Function
Read JSON	fs.readFileSync + JSON.parse()
Write JSON	JSON.stringify() + fs.writeFileSync
Delayed Run	setTimeout()
Repeated Run	setInterval()
CLI Args	process.argv
Read from stdin	process.stdin.on("data")