Hello, World
def main():
print("Hello, World!")main.mojo
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Mojo 1.0.0b2Readymain.mojoLn 1, Col 1Spaces: 4UTF-8
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main.mojo · output appears here
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Output streams here as your program runs.
Mojo is a programming language created by Modular that aims to combine the usability of Python with the performance of systems languages like C++ and Rust. Designed as a superset of Python, it adds strong static typing, compile-time metaprogramming, ownership-based memory management, and direct access to hardware features, targeting AI and high-performance numerical workloads while letting developers reuse the existing Python ecosystem.
CompileBytes runs Mojo version 0.26.1. Mojo introduces constructs like `fn` for strongly typed functions, `var` for mutable bindings, and `struct` for value types alongside Python-style `def`. The language is still evolving rapidly, so syntax can change between releases; the online environment lets you try the core language and standard library quickly without setting up the Modular toolchain or MAX platform locally.
main.mojo is the entry point for each run.
Use the Input panel to pipe standard input to your program when it needs data.
Run sends your code to a fresh hosted runtime and streams the result into the output panel.
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def main():
print("Hello, World!")fn add(a: Int, b: Int) -> Int:
return a + b
def main():
print(add(2, 3))def main():
var total = 0
for i in range(1, 6):
total += i
print("sum:", total)struct Point:
var x: Int
var y: Int
fn __init__(out self, x: Int, y: Int):
self.x = x
self.y = y
def main():
var p = Point(1, 2)
print(p.x, p.y)fn classify(n: Int) -> String:
if n > 0:
return "positive"
elif n < 0:
return "negative"
else:
return "zero"
def main():
print(classify(-5))fn factorial(n: Int) -> Int:
if n <= 1:
return 1
return n * factorial(n - 1)
def main():
print(factorial(5))struct Rect:
var w: Int
var h: Int
fn __init__(out self, w: Int, h: Int):
self.w = w
self.h = h
fn area(self) -> Int:
return self.w * self.h
def main():
var r = Rect(3, 4)
print("area:", r.area())def main():
var n = 1
var product = 1
while n <= 5:
product *= n
n += 1
print("5! =", product)The editor uses Monaco, the engine behind VS Code, with language-aware editing tools and familiar keyboard controls.
Type or paste your code into the editor, then click Run (or press Ctrl/Cmd + Enter) to execute it.
Output streams into the console panel, including runtime errors, exit status, and execution time.
Nothing to install and no account to create — code runs in an isolated hosted runtime.
Language-aware autocomplete and IntelliSense suggestions where the editor has a matching language service
Syntax highlighting, with native compiler and runtime errors shown in the console
Bracket matching, auto-indentation, code folding, and a minimap
Multi-cursor editing plus find & replace (Ctrl/Cmd + F)
Full undo/redo history and copy to clipboard
Built on Monaco — the same editor that powers VS Code
Run on demand, or turn on Auto-run to execute on every change
Stop long-running or infinite programs at any time
A real console shows standard output, errors, exit status, and execution time
Pass standard input (stdin) to your program from the Input panel — one value per line
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Run code: Ctrl/Cmd + Enter
Find & replace: Ctrl/Cmd + F
Open the full shortcut list from the keyboard icon in the toolbar
Mojo is one of 46 single-file compiler scratchpads. Each run submits only main.mojo; local sibling-file imports are not enabled.
Linked-project languages (23): Node.js, TypeScript, Deno, Python, C, Go, Dart, Swift, Rust, Ruby, PHP, Java, Kotlin, C++, C#, Nim, Crystal, Haxe, Zig, Scala, D, Bun, Objective-C.
Single-file scratchpads (46): Matplotlib, NumPy, pandas, Haskell, Clojure, Perl, Lua, Tcl, jq, Raku, Elixir, Bash, Zsh, Nushell, R, Assembly (NASM), Julia, Octave, Groovy, PowerShell, Visual Basic .NET, Odin, Pascal, Lisp, Erlang, CoffeeScript, COBOL, BASIC, V, Racket, OCaml, Prolog, AWK, Emacs Lisp, Emojicode, Forth, F#, Fortran, Assembly (NASM64), Solidity, Mojo, Ada, WASM (WAT), Gleam, Shell Script (sh), Scheme.
Download file exports the active Mojo source file.