Line At A Time

Learn to code, one line at a time

Type real, correct code with instant feedback, see it "run" in a terminal, and read a plain-English explanation of every single line.

Browse languages

Languages

Java
Object-orientedStatically typedCompiled
Android apps, Minecraft and big business systems.
Python
Multi-paradigmDynamically typedInterpreted
AI, science, automation and web servers.
JavaScript
Multi-paradigmDynamically typedInterpreted
Websites, web apps and browser games.
HTML
Markup languageRuns in every browserNo installs needed
The tags every web page is built from.
Go
ProceduralStatically typedCompiled
Cloud services, servers and command-line tools.
Swift
Multi-paradigmStatically typedCompiled
iPhone, iPad, Mac and Apple Watch apps.
Kotlin
Multi-paradigmStatically typedCompiled
Modern Android apps and Java-platform software.

Tools

Git
Version controlA tool, not a languageFree & open source
The save-and-undo system used for almost all software.
Bash
Command lineShell & scriptingFree & open source
The command line: type an instruction, get an answer.
MS-DOS
Command lineOperating systemComputer history
The prompt every PC started at, before Windows.

Frameworks

Spring Boot
FrameworkFor JavaServer-side
Java's toolkit for building web servers and APIs.
React
FrameworkFor JavaScriptFront-end
The most popular way to build web app interfaces.

Sandbox

Java sandbox
Java 8 to 25Real JVMJUnit 5
Write and run real Java: packages, Scanner input, records, sealed types and pattern matching, JUnit 5 tests, with live error checking as you type.
Python sandbox
Python 3.12numpy, pandas & moreMulti-file
Write and run real Python: modules and packages, input(), classes, f-strings, the standard library plus numpy, pandas, sympy and more, with live syntax checking.
JavaScript sandbox
Runs in your browserNode-styleMulti-file
Write and run real JavaScript: import/export and require between files, async/await, input from the terminal, with live syntax checking. Nothing leaves your browser.
TypeScript sandbox
Runs in your browserType-checkedMulti-file
Write and run real TypeScript: interfaces, generics and modules, type errors flagged as you type, then compiled and run without leaving your browser.