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JavaScript code examples · runnable

20 runnable JavaScript code examples.

Open any JavaScript example to explore the code and run it online. No setup, no signup.Explore practical JavaScript code examples with clear explanations. Open any example to edit and run it online — no setup, no signup.

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JavaScript

How to remove an item from an array in JavaScriptRemove a specific item from a JavaScript array: indexOf + splice in place, toSpliced() for a new array, filter() for every match or a condition, why delete leaves a hole, and how to remove items inside a loop without skipping any.How to remove a property from an object in JavaScriptRemove a property from a JavaScript object with delete, or build a copy without it using rest destructuring — plus dropping several keys at once, why setting a key to undefined is not the same, and the frozen-object and Map edge cases.How to check if a string contains a substring in JavaScriptCheck whether a JavaScript string contains a substring with includes(), test the start or end with startsWith() and endsWith(), find the position with indexOf(), ignore case, and match patterns with a regex.How to loop through an array in JavaScriptLoop through an array in JavaScript with for...of, get the index with entries() or forEach, stop early with break, some() or find(), use the classic for loop for backwards and stepped walks, and see why for...in is the wrong tool for arrays.How to replace all occurrences of a string in JavaScriptReplace every occurrence of a substring in JavaScript with replaceAll() — plus the global regex, how to put a variable in a RegExp safely with RegExp.escape, and the $& / $1 replacement-pattern gotcha.How to capitalize the first letter of a string in JavaScriptCapitalize the first letter of a string with charAt(0).toUpperCase() + slice(1), plus why str[0] throws on an empty string, how to capitalize every word, emoji and surrogate pairs, and locale-aware toLocaleUpperCase().How to deep clone an object in JavaScriptDeep clone an object or array with structuredClone, see why spread and Object.assign only copy one level, what JSON.parse(JSON.stringify()) silently breaks, and what structuredClone cannot copy.How to check if an array contains a value in JavaScriptCheck if a JavaScript array contains a value with includes(), the older indexOf() !== -1 test and its NaN blind spot, some() for objects and custom tests, and a Set when you look up many values.How to round to 2 decimal places in JavaScriptRound a number to 2 decimal places in JavaScript with Math.round(x * 100) / 100, fix the 1.005 floating-point trap, show exactly two decimals with toFixed or Intl.NumberFormat, and turn a float into a whole number with Math.trunc, floor, ceil or round.How to sort an array of objects by property in JavaScriptSort an array of objects by a property: subtract for numbers, localeCompare for strings, Date.parse for dates, || for tie-breaks, and toSorted() when the original must not change.How to check if a key exists in an object in JavaScriptCheck if a key exists in a JavaScript object with Object.hasOwn or the in operator, why obj.key !== undefined gives the wrong answer, hasOwnProperty in older code, Map.has, and optional chaining for nested keys.How to loop through an object in JavaScriptLoop through a JavaScript object with for...of over Object.entries(obj), plus Object.keys and Object.values, why for...in also visits inherited keys, the order keys come back in, and mapping an object with Object.fromEntries.How to merge two objects in JavaScriptMerge two JavaScript objects with spread syntax or Object.assign, see which one mutates and which key wins, why nested objects get replaced instead of merged, a small recursive deep merge, and how to add a property only when a condition holds.How to use async/await in a loop in JavaScriptUse async/await in a JavaScript loop: why forEach does not wait, for...of with await for one-at-a-time work, Promise.all with map for parallel work, Promise.allSettled when some fail, for await...of, and a sleep() helper built from setTimeout.How to remove duplicates from an array in JavaScriptRemove duplicates from a JavaScript array with [...new Set(arr)], the older filter + indexOf, a Map keyed by id for arrays of objects, a case-insensitive seen-set, and merge-then-dedupe with spread or the ES2025 Set.prototype.union().How to format a number with commas in JavaScriptFormat a number with commas as thousands separators in JavaScript: toLocaleString('en-US'), a reusable Intl.NumberFormat with fixed decimals, currency (USD, EUR, JPY), other locales' separators, compact 1.2K notation, BigInt, and the regex fallback with its limits.How to convert a string to a number in JavaScriptConvert a string to a number with Number() or unary +, read a number off the front of text with parseInt and parseFloat (always with a radix), avoid the Number("") === 0 trap, validate with Number.isFinite, and use BigInt past 2^53.How to flatten an array in JavaScriptFlatten an array of arrays in JavaScript: flat() for one level, flat(Infinity) for any depth, flatMap() to map and flatten in one pass, the older concat and reduce answers, and a recursive flatten for custom rules.How to find an object in an array by property in JavaScriptFind an object in a JavaScript array by a property value with find(), get its position with findIndex() or search from the end with findLast(), collect every match with filter(), pluck one property with map(), and build a Map or Object.groupBy index for repeated lookups.How to check for null or undefined in JavaScriptCheck for null or undefined in JavaScript with value == null, the one good use of loose equality: plus null vs undefined, typeof for undeclared variables, the truthiness traps (0, "", false, NaN), ?? vs ||, ??=, optional chaining and a blank-string check.

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