JavaScript Input & Output

Table of Contents

• Intro
• Basic Output in JavaScript
• Taking Input in JavaScript
• Input & Output in Node.js
• Quick Comparison
• Best Practices
• Interview Q&A
• Interview MCQ

Introduction

JavaScript handles input and output differently depending on where your code runs. In a browser, you interact with users through dialogs or the page itself. In environments like Node.js, you use the console or standard input.

1. Basic Output in JavaScript

Console Output (Most Common)

console.log("Hello, World!");
console.error("Error message");
console.warn("Warning message");

This prints output to the browser console (press F12 -> Console).

Output to Web Page

document.write("Hello, User!"); // not recommended for modern apps

document.getElementById("output").innerText = "Hello!";

<p id="output"></p>

Alert Box Output

alert("Welcome!");

2. Taking Input in JavaScript

Using prompt() (Simple Input)

let name = prompt("Enter your name:");
console.log("Hello " + name);

Using HTML Input Field (Best Practice)

<input type="text" id="username" placeholder="Enter name">
<button onclick="getInput()">Submit</button>
<p id="result"></p>

function getInput() {
    let name = document.getElementById("username").value;
    document.getElementById("result").innerText = "Hello " + name;
}

3. Input & Output in Node.js

Output

console.log("Hello from Node.js");

Input (Using readline)

const readline = require("readline");

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

rl.question("Enter your name: ", function(name) {
    console.log("Hello " + name);
    rl.close();
});

4. Quick Comparison

Best Practices

• Validate user input before use.
• Convert string inputs to number using Number().
• Avoid excessive alert() in production UI.
• Use console for debugging, DOM for user-facing output.

10 Input/Output Interview Q&A

10 Input/Output MCQs