Node.js Input Masterclass for Competitive Programming

1. The "Java Scanner" Fix (Synchronous FS)

This is the absolute best way to handle input for Codeforces or AtCoder. It reads the whole file instantly and creates helper functions like nextInt().

const fs = require("fs");

// 1. Read everything at once (Fastest Method)
const input = fs.readFileSync(0, "utf-8").trim().split(/\s+/);
let cur = 0;

// 2. Helper functions mimicking Java Scanner
function next() { return input[cur++]; }
function nextInt() { return Number(input[cur++]); }
function nextBigInt() { return BigInt(input[cur++]); }

function solve() {
  const n = nextInt(); // 5
  const k = nextInt(); // 3
  const arr = [];
  
  for (let i = 0; i < n; i++) {
    arr.push(nextInt()); // Reads 0, 0, 0, 0, -1
  }

  // Your Logic
  console.log(arr.map(x => x + k).join(" "));
}

solve();

Why use this? It bypasses the Node.js event loop. It is synchronous, just like Java, and it never suffers from "Time Limit Exceeded" errors.

2. Understanding Platform "Driver Codes"

Sometimes you don't need to write input code at all. Platforms are split into three archetypes:

Function Stubs (LeetCode)

You only write the logic inside a specific function. The platform provides all variables as arguments.

function solve(nums, k) { ... }

Provided Main (HackerRank)

The code is pre-filled with a main() function. You only need to edit the result = ... line.

function main() { ... }

Empty File (Codeforces)

The most common. You must provide everything from require("fs") to the final output logic.

const fs = require("fs"); ...

3. String to Number Live Lab

JavaScript is loose with types. Choosing the wrong conversion method can break your solution. Test how different strings convert live below:

Input String Value

Try values like "", "100", or "08".

Number(s)

parseInt(s, 10)

+s (Unary)

~~s (Double NOT)

4. The "Cheat Code": JSON.parse()

Sometimes strings look like this: [[1,2],[3,4]]. Instead of writing loops to find brackets, use JSON magic.

Complex String Input

{"n": 5, "coords": [[10,20], [30,40]]}

Native JS Object Output

5. Understanding the "Event" Vocabulary

Node.js is asynchronous. Here is what those confusing keywords mean:

on

.on("line", ...)

This is an Event Listener. Every time a user presses Enter, the "line" event fires, and Node.js gives you that text. Think of it like a trigger.

cl

.on("close", ...)

This event fires when there is no more text left to read from the file. This is the signal that it's safe to run your main math logic.

en

.on("end", ...)

Identical to "close", but used for lower-level streams. It means the End of File (EOF) has been reached.

qu

.question(...)

A special helper that pauses execution, asks a question, and waits for a single response. Great for interviews, bad for huge CP data.

6. The Complete Toolbox (All 5 Ways)

Every way to get input into Node.js, summarized for your reference.

const readline = require("readline/promises");
const rl = readline.createInterface({ input: process.stdin, output: process.stdout });

async function main() {
  const line1 = await rl.question("");
  const n = Number(line1);
  rl.close();
}
main();

const readline = require("readline");
const rl = readline.createInterface({ input: process.stdin, output: process.stdout });

rl.question("", (n) => {
  rl.question("", (arr) => {
    console.log(arr.split(" "));
    rl.close();
  });
});

const readline = require("readline");
const rl = readline.createInterface({ input: process.stdin, output: process.stdout });
const lines = [];

rl.on("line", (l) => lines.push(l));
rl.on("close", () => {
  console.log("Input finished!", lines);
});

process.stdin.resume();
process.stdin.setEncoding("utf-8");
let data = "";
process.stdin.on("data", c => data += c);
process.stdin.on("end", () => {
  const lines = data.split("\n");
  console.log(lines);
});