Free developer tools

How to, step by step

• Free HTML, CSS and JavaScript playground with live preview

  Write HTML, CSS and JavaScript in your browser and see the page as you type, with a console, errors and line numbers. Nothing uploaded, no account.

• Online HTML editor with live preview, free and private

  Write or open an HTML file and see the page beside it as you type, with CSS and JavaScript editors and a console. Save it back as one file. Free, no account.

• CSS playground: try CSS online with a live preview

  Try flexbox, grid, animations and custom properties on a page as you type, with SCSS compiled on your device. Free, no account, nothing uploaded.

• How to run JavaScript online, with a real console

  Run JavaScript in your browser with a real console: logs, tables, errors with their line, endless loops stopped, and packages by name. Free, no account.

• How to try React online without installing anything

  Write React components in JSX and see them run as you type. JSX is compiled on your device and React comes from esm.sh. Free, no account, no install.

• How to run TypeScript in the browser, with no install

  Write TypeScript and see it run as you type, turned into JavaScript on your device line for line. Free, no account, no install.

• How to try Tailwind CSS online, with no build step

  Style a page with Tailwind CSS classes and @apply and see it as you type, with Tailwind's own browser build served by KeepSimp. Free, no account.

• Test a website on different screen sizes, free

  Type your site's address or your localhost server and see it at the sizes of phones, tablets and laptops side by side, loaded by your own browser.

• Mobile view tester: see how a website looks on a phone

  See your website on an iPhone, an iPhone SE, a Pixel, a Galaxy and a Galaxy Z Flip side by side, each at the size its browser shows a page. Free.

• Tablet view tester: see a website on an iPad

  See your website on an iPad mini, an iPad, an iPad Pro, a Galaxy Tab and an open Galaxy Z Fold at once, and turn them all with one key. Free.

• Screen size tester: test a website at different resolutions

  See your website at 375, 440, 768, 1280, 1440 and 1920 pixels wide at once, each marked with its Tailwind or Bootstrap breakpoint. Free, no account.

• How to test localhost at mobile and tablet sizes

  See your development server at localhost on a phone, a tablet and a laptop at once, and keep them in sync as you scroll and click. Free, nothing uploaded.

• Test a REST API online, free, from your browser

  Send GET, POST and every other request to an API from your browser, with headers, JSON or form bodies, sign-in and cURL, and see exactly why one is blocked.

• Convert curl to Python, JavaScript, Go and more

  Paste a cURL command and get the same request in Python, fetch, axios, Node, Go, Rust, Java, C#, PHP, Ruby, Swift, Kotlin, Dart, PowerShell, Wget or HTTPie.

• How to send a GraphQL query from your browser

  Send GraphQL queries from your browser with the API's schema read for suggestions, checks and docs, variables as JSON and the operation you choose.

• WebSocket tester online: connect and send messages

  Connect to a ws:// or wss:// WebSocket from your browser, send text or JSON, and read every message in a log with its time.

• CORS error explained, and how to test an API's CORS

  What a CORS error and a preflight are, why the browser only says a request failed, and how to test whether an API lets a web page read it.

• Test an OpenAPI or Swagger file online

  Open an OpenAPI 3 or Swagger 2 file, in JSON or YAML, and get a collection of its requests with example bodies, ready to send or save for Postman.

• Encode and decode Base64, URL, HTML and Unicode online

  Paste text and see whether it is Base64, percent-encoding, HTML entities, Unicode escapes or punycode; read it in chained steps and look inside every character.

• Base64 encode and decode online: text, files and images

  Encode and decode Base64 and Base64url with or without padding, in MIME lines, and files as data URLs; see what a decode passed over.

• URL encode and decode online: percent-encoding explained

  Percent-encode text for a part of an address, a whole address or a form, decode text encoded several times, and take an address apart.

• HTML entity encoder and decoder: escape and unescape HTML

  Escape text for HTML with named, decimal or hex character references, or decode them as a browser does, with HTML's full list of names.

• Unicode converter: code points, escapes, hidden characters

  See each character's code point, name and UTF-8 bytes, write escapes for your language, normalise text, and find hidden and look-alike characters.

• UUID v4 vs v7: which to use, with a generator

  Make UUIDs of every version in RFC 9562 by the thousand, and read any UUID back to its version and, for versions 1, 6 and 7, the time it was made.

• ULID generator and decoder, and ULID vs UUID

  Make ULIDs that sort by time, and read a ULID, KSUID, ObjectId or Discord or X Snowflake ID back to the moment it was made.

• Hash generator: SHA-256, MD5 and check a file checksum

  Hash text or files of any size with SHA-256, MD5, SHA-3, BLAKE3 and 23 more, make HMACs, and check a download against its checksum or a SHA256SUMS file.

• How to make and check a bcrypt or Argon2 hash

  Make bcrypt, Argon2id and scrypt password hashes with the settings you choose, see how long each takes, and check a password against a stored hash.

• Fake test data as JSON, CSV or SQL, made in your browser

  Make test data in your browser from fields, a JSON example or a JSON Schema: names, addresses, IDs and dates as JSON, CSV, SQL, XML or YAML. Nothing uploaded.

• Test a regular expression online: JS, PCRE2 and Go

  Test a regex in your browser, nothing uploaded: JavaScript, PCRE2 or Go, matches marked as you type, each part explained and drawn, and backtracking found.

• JWT decoder that runs on your device: decode and verify

  Decode a JWT in your browser, nothing sent: read its claims and expiry, verify it with a secret, PEM key or JWKS, and see warnings such as alg none.

• Cron expression generator: build, explain and convert cron

  Build or paste a cron schedule in your browser, read it in words, see its next runs in any time zone, and convert it for GitHub Actions, Quartz and more.

• Cron every 5 minutes: */5 and other common schedules

  Write cron for every 5, 15 or 30 minutes or every hour, check its next runs in your browser, and see how Quartz, Spring and GitHub Actions differ.

• Cron every day at midnight, 9am or on weekdays

  Write cron for midnight, 9am, weekdays or the 1st of the month, check its runs in your own time zone in your browser, and avoid the day-field trap.

• Is it safe to paste a JWT into an online decoder?

  A JWT is often a live key to an account. How to read one in your browser without sending it anywhere, what decoding shows, and how to check a page yourself.

• JWT expired? How to read exp, iat and nbf

  Read a JWT's exp, iat and nbf claims as dates in your browser, see why a token is refused as expired or not yet valid, and what clock skew does to it.

• Is it safe to paste JSON into an online formatter?

  Some online formatters keep what you paste. How to format and validate JSON in your browser with nothing uploaded, and how to check any page yourself.

• Why is my JSON invalid? Common errors and how to fix them

  Find out why JSON will not parse in your browser, nothing uploaded: trailing commas, single quotes, comments, unquoted names. Go to the line and fix it.

• How to convert JSON to CSV and open it in Excel

  Turn a JSON array into a CSV file in your browser, nothing uploaded, then open it in Excel without losing leading zeros or long IDs to its guesses.

• Convert CSV to JSON online without uploading it

  Turn a CSV file into a JSON array of objects in your browser, nothing uploaded, with every cell kept as text so leading zeros and long IDs stay as written.

• Compare two JSON files, ignoring key order

  Compare two JSON documents in your browser by their values, not their spacing or key order, and get each change as a path and as a JSON Patch.

• Regex for email, phone number and URL, tested

  Tested regex for email addresses, phone numbers, URLs, IP addresses, UUIDs and dates, with what each accepts. Try them in your browser in three engines.

• How to turn a Base64 string into a PDF or image

  Decode a Base64 string or data URL back into the PNG, JPEG or PDF it holds, in your browser with nothing uploaded, then preview it and save the file.

• How to convert a curl command to JavaScript fetch

  Turn a curl command into JavaScript fetch, Node.js or axios code in your browser, nothing uploaded, and see which options a web page cannot carry over.

• A free Postman alternative that runs in your browser

  Send API requests from your browser with collections, environments, tests and runs. Open your Postman files, and keep it all on your device, with no account.

• JSON formatter and validator online, free, no login

  Format, minify and validate JSON in your browser, nothing uploaded, keeping large numbers, escaped strings and key order exactly as written.

• JSONPath tester online, with examples and JSON Pointer

  Query nested JSON with JSONPath, select an exact value with JSON Pointer, and export results without rounding large IDs.

• Repair broken JSON online, with a preview

  Repair broken JSON in your browser with a preview, compare values instead of spacing, and make or apply all six JSON Patch operations.

• JSON Schema validator online, data kept on your device

  Validate JSON Schema draft-07, 2019-09 and 2020-12, inspect error paths, use local references, and infer a starting schema.

• Convert JSON to YAML, CSV, JSON Lines or TypeScript

  Convert JSON locally, keep CSV identifiers as strings, review spreadsheet formula protection, and generate sample types and schemas.

• JSON to TypeScript: make types from a JSON sample

  Make TypeScript types from a JSON sample in your browser, with nothing uploaded, then check what one sample cannot know: optional keys, nulls and big numbers.