Node.js 20.17.0 (V8 Engine)Isolated Sandbox

Online JavaScript Compiler — Run JS Code in Browser

Veyrai's Online JavaScript Compiler runs on Node.js 20 (LTS) powered by Google's V8 engine. It supports modern ECMAScript features including async/await, optional chaining, nullish coalescing, arrow functions, and destructuring. Write algorithmic solutions, inspect console outputs, and practice frontend/backend JavaScript interview questions.

Example:

Execution Output

Click "Run (Ctrl+Enter)" to compile and execute your JavaScript program in the isolated sandbox.

How to Use the Online JavaScript Compiler

Modern ECMAScript Support

You can freely use modern syntax (const/let, arrow functions, template literals, Map, Set, Promises). Node.js 20 executes standard server-side JS.

Print Output with console.log()

Use console.log(), console.info(), or console.error(). All output streams are captured and displayed in the Execution Console below.

Asynchronous Execution

Our runner supports top-level async functions and Promises. The sandbox waits for event loop completion before returning output.

Defend Your Code in AI Interviews

Prepare for frontend engineering interviews: transition from testing JavaScript algorithms here to explaining event loop mechanics in a live AI mock interview.

JavaScript Code Examples & Algorithms

Each example is verified to run in our production Node.js 20.17.0 (V8 Engine) runtime. Click "Load in Editor" to test and modify any example above.

1. Data Transformation with Map, Filter & Reduce

Transform and aggregate an array of candidate records using functional JavaScript idioms.

Time: O(N)

const candidates = [
  { name: "Aarav", role: "Backend Engineer", score: 92, verified: true },
  { name: "Priya", role: "Frontend Developer", score: 88, verified: true },
  { name: "Rohan", role: "Data Scientist", score: 74, verified: false },
  { name: "Ananya", role: "AI Engineer", score: 96, verified: true },
];

// Pipeline: Filter verified, map names & scores, calculate average
const topPerformers = candidates
  .filter(c => c.verified && c.score >= 85)
  .map(c => ({ candidate: c.name, score: c.score }));

const avgScore = topPerformers.reduce((acc, curr) => acc + curr.score, 0) / topPerformers.length;

console.log("Top Performers:", topPerformers);
console.log("Average Score:", avgScore.toFixed(2));

2. Deep Flatten Nested Arrays (Recursion vs Iteration)

Implement array flattening across arbitrary nesting depths without using Array.prototype.flat().

Time: O(N)

function flattenArray(arr) {
  const result = [];
  
  function helper(subArr) {
    for (const item of subArr) {
      if (Array.isArray(item)) {
        helper(item);
      } else {
        result.push(item);
      }
    }
  }
  
  helper(arr);
  return result;
}

const deeplyNested = [1, [2, [3, 4], 5], [6, [7, [8, [9]]]]];
console.log("Original Array:", JSON.stringify(deeplyNested));
console.log("Flattened Result:", flattenArray(deeplyNested));

3. Asynchronous Concurrency with Promise.all()

Simulate concurrent API requests with latency, demonstrating promise resolution order.

Time: O(1) async

async function fetchServiceHealth(serviceName, delayMs) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve({ service: serviceName, latencyMs: delayMs, status: "HEALTHY" });
    }, delayMs);
  });
}

async function runHealthCheck() {
  console.log("Initiating microservice health checks...");
  const start = Date.now();
  
  const results = await Promise.all([
    fetchServiceHealth("AuthService", 120),
    fetchServiceHealth("PaymentGateway", 80),
    fetchServiceHealth("CodeSandbox", 150),
  ]);
  
  console.log("All services responded in", Date.now() - start, "ms");
  console.table(results);
}

runHealthCheck();

Common JavaScript Beginner Mistakes & Fixes

Loose Equality ('==') vs Strict Equality ('===')

Loose equality coerces types implicitly (e.g. '0 == false' is true, 'null == undefined' is true), causing subtle logic bugs.

Fix: Always default to strict equality ('===') and strict inequality ('!==').

Unhandled Asynchronous Rejections in Promises

Failing to attach .catch() or wrapping await in try/catch triggers UnhandledPromiseRejection errors.

Fix: Always wrap await statements in 'try { ... } catch (err) { ... }' blocks.

Shallow Copy vs Deep Copy with Object.assign / Spread

Using {...obj} or Object.assign({}, obj) only copies top-level properties; nested objects remain referenced by memory address.

Fix: Use 'structuredClone(obj)' in modern JavaScript for true deep copies.

Environment Specifications

Language: JavaScript
Compiler / Engine: Node.js 20.17.0 (V8 Engine)
Standard Input: Supported (stdin)
Isolation Layer: Dockerized Sandbox
Execution Timeout: 8s to 25s max
Guest Access: Allowed (Rate limited)

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