JavaScript Code Examples

A clean, minimal JavaScript scratchpad with zero boilerplate. Write custom scripts, experiment with algorithms, and run code instantly in a client-side Web Worker.

Experiment with the standard JavaScript Console API. Test console.log, console.warn, console.error, and multi-argument message formatting in a live runner.

A starter template preloaded with numeric and object arrays. Test modern array transformations, higher-order methods, sorting, and destructuring in real time.

Test asynchronous JavaScript flows, Promise resolution, timeouts, and async/await chains in an isolated client-side execution environment.

A focused environment for developing and benchmarking JavaScript algorithms, data structures, recursion, and interview practice challenges.

The JavaScript Hello World program is the foundational entry point for learning JavaScript. It uses the standard console.log() API to print text to the debugging console without requiring any external libraries or browser DOM manipulation.

JavaScript variables are named memory containers used to hold and manipulate data values throughout runtime execution. Modern JavaScript uses const for fixed identifier bindings and let for mutable variables, enforcing block scope and eliminating var.

JavaScript template literals are string literals delimited by backticks (`) that enable embedded expressions, multi-line string authoring, and string interpolation using the ${expression} syntax without cumbersome plus-sign concatenation.

JavaScript conditional statements execute distinct blocks of code based on whether a specified condition evaluates to truthy or falsy. The if...else syntax provides structured branching control flow across all ECMAScript environments.

A JavaScript function declaration defines a reusable, parameterized subroutine using the function keyword. In JavaScript, functions are first-class values that can be assigned to variables, passed as arguments, and returned from other functions.

JavaScript arrow functions provide a concise syntax for writing function expressions using the => token. Unlike traditional functions, arrow functions do not bind their own this, arguments, or super, lexically capturing them from their enclosing scope.

JavaScript default parameters allow formal function parameters to be initialized with default values if no value or undefined is passed during invocation, preventing unexpected errors from missing arguments without defensive boilerplate.

The JavaScript Array map() method creates a new array populated with the results of calling a provided callback function on every element in the calling array. It performs immutable data transformation without modifying the original array.

The JavaScript Array filter() method creates a shallow copy of an array containing only the elements that pass the test implemented by the provided predicate callback function. Elements that evaluate to truthy are retained, while others are omitted.

The JavaScript Array reduce() method executes a reducer callback function on each array element, passing the accumulated result from preceding calculations forward. It aggregates arrays into numbers, objects, lookups, or transformed collections.

The JavaScript Array find() method returns the first element in an array that satisfies a provided testing callback function. If no element satisfies the testing condition, it returns undefined, enabling fast linear searches across collections.

A JavaScript object is a standalone entity consisting of key-value properties. Property keys are strings or Symbols, and values can be any JavaScript data type, including primitive numbers, strings, nested objects, and executable method functions.

JavaScript object destructuring is an ECMAScript 2015 expression syntax that unpacks properties from objects directly into distinct standalone variables. It supports property renaming, nested extraction, and default fallback assignments.

The JavaScript object spread operator (...) copies enumerable own properties from one or more source objects into a new target object literal. It is standard across modern state management libraries for immutable updates and shallow cloning.

JavaScript optional chaining (?.) permits reading nested properties without manually checking each reference in the chain for null or undefined. Nullish coalescing (??) provides a fallback value only when a value is strictly null or undefined.

A JavaScript Promise is a proxy object representing the eventual completion or failure of an asynchronous operation and its resulting value. Promises eliminate nested callback hell with chainable .then(), .catch(), and .finally() methods.

The async and await keywords allow developers to write promise-based asynchronous code in a sequential, synchronous-looking style. An async function always returns a Promise, and await pauses execution until the awaited Promise settles.

The JavaScript setTimeout() function schedules a callback function to execute after a minimum delay in milliseconds. Because JavaScript runs on a single-threaded event loop, setTimeout places callbacks in the task queue after the main thread clears.

The JavaScript try...catch statement marks a block of statements to test for exceptions, with a designated handler block to execute if an error occurs. It prevents uncaught runtime exceptions from crashing applications and enables resilient recovery.

The JavaScript throw statement generates a user-defined exception, pausing the current execution thread and passing control to the nearest enclosing catch block. Instantiating Error objects provides critical stack trace context for debugging.

Reversing a string in JavaScript is a core algorithmic problem. The standard idiom splits the string into an array of characters, invokes Array.prototype.reverse(), and joins the array elements back into a string with O(n) linear time complexity.

A palindrome is a sequence that reads identically forwards and backwards. In JavaScript, a palindrome checker verifies symmetry by cleaning characters and comparing the normalized string against its reversed counterpart or using two-pointer index comparison.

FizzBuzz is a classic coding interview problem that iterates through integers, printing Fizz for multiples of 3, Buzz for multiples of 5, and FizzBuzz for multiples of both 3 and 5 (15). It tests modulo arithmetic and correct evaluation ordering.

The factorial of a non-negative integer n is the product of all positive integers less than or equal to n, denoted as n!. In JavaScript, factorials are calculated using recursive functions with defined base cases or iterative accumulator loops.