JavaScript¶

Dynamic, high-level programming language for web and server-side development

Overview¶

JavaScript is the language of the web, powering interactive websites and modern web applications. With run, you can execute JavaScript code using Node.js without setting up npm projects or managing package.json files.

The JavaScript engine in run uses Node.js as the runtime, providing access to all Node.js built-in modules and APIs. The REPL mode is stateful, meaning variables and functions persist across commands within the same session.

Language Aliases¶

You can invoke JavaScript using any of these aliases:

run javascript "console.log('Hello')"
run js "console.log('Hello')"
run node "console.log('Hello')"
run nodejs "console.log('Hello')"
# Output: Hello (x4)

Basic Usage - Inline Code¶

Execute JavaScript code directly from the command line:

Simple Console Log¶

run js "console.log('Hello, World!')"
# Output: Hello, World!

Arithmetic and Variables¶

run js "const x = 10; const y = 20; console.log(x + y)"
# Output: 30

Template Literals¶

run js "const name = 'Alice'; console.log(\`Hello, \${name}!\`)"
# Output: Hello, Alice!

Arrow Functions¶

run js "const square = x => x * x; console.log(square(5))"
# Output: 25

Array Methods¶

run js "const nums = [1,2,3,4,5]; console.log(nums.map(x => x * 2))"
# Output: [ 2, 4, 6, 8, 10 ]

Multi-line Code¶

Best Practice: Use Heredoc

For multi-line JavaScript code, always use heredoc to avoid shell quoting issues:

# RECOMMENDED: Heredoc
run js << 'EOF'
const users = [
    { name: 'Alice', age: 30 },
    { name: 'Bob', age: 25 }
];
users.forEach(u => console.log(`${u.name}: ${u.age}`));
EOF

# OK: Single-line with semicolons
run js "const x = [1,2,3]; x.forEach(n => console.log(n * 2));"

# AVOID: Multi-line strings (quote/escaping issues)
run js "
const users = [{name: 'Alice'}];
console.log(users);
"

File Execution¶

Execute JavaScript files with Node.js:

# Create a JavaScript file
echo "console.log('Hello from file!')" > hello.js

# Execute with language specified
run js hello.js

# Or let run auto-detect from extension
run hello.js
# Output: Hello from file!

Module Example¶

# Create a module file
cat > math.js << 'EOF'
function factorial(n) {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}

for (let i = 0; i <= 10; i++) {
  console.log(`${i}! = ${factorial(i)}`);
}
EOF

run js math.js
# Output:
# 0! = 1
# 1! = 1
# 2! = 2
# 3! = 6
# 4! = 24
# 5! = 120
# 6! = 720
# 7! = 5040
# 8! = 40320
# 9! = 362880
# 10! = 3628800

REPL Mode - Interactive JavaScript¶

Start an interactive JavaScript REPL with 'run js'. The REPL is stateful within the session - variables and functions persist across commands at the js>>> prompt:

$ run js
run universal REPL. Type :help for commands.
js>>> let x = 10
undefined
js>>> let y = 20
undefined
js>>> x + y
30
js>>> const greet = name => `Hello, ${name}!`
undefined
js>>> greet('World')
'Hello, World!'
js>>> const numbers = [1, 2, 3, 4, 5]
undefined
js>>> numbers.reduce((a, b) => a + b, 0)
15

REPL Behavior - Stateful¶

JavaScript's REPL is STATEFUL within a single session:

Start REPL once with 'run js'
Variables and functions persist at the js>>> prompt
Each separate 'run js "code"' command is independent

Node.js Built-in Modules¶

Access all Node.js built-in modules without installation:

File System Operations¶

run js "
const fs = require('fs');
const files = fs.readdirSync('.');
console.log(\`Found \${files.length} files\`);
files.slice(0, 5).forEach(f => console.log(\`  - \${f}\`));
"
# Output:
# Found 12 files
#   - README.md
#   - src
#   - Cargo.toml
#   - target
#   - .git

Path Operations¶

run js "
const path = require('path');
const file = '/home/user/documents/file.txt';
console.log('Directory:', path.dirname(file));
console.log('Filename:', path.basename(file));
console.log('Extension:', path.extname(file));
"
# Output:
# Directory: /home/user/documents
# Filename: file.txt
# Extension: .txt

OS Information¶

run js "
const os = require('os');
console.log('Platform:', os.platform());
console.log('CPU Cores:', os.cpus().length);
console.log('Total Memory:', Math.round(os.totalmem() / 1024 / 1024 / 1024) + ' GB');
"
# Output:
# Platform: linux
# CPU Cores: 8
# Total Memory: 16 GB

Modern JavaScript Features¶

Use ES6+ features including async/await, destructuring, and more:

Async/Await¶

run js "
const delay = ms => new Promise(resolve => setTimeout(resolve, ms));
(async () => {
  console.log('Starting...');
  await delay(1000);
  console.log('Done after 1 second!');
})();
"
# Output:
# Starting...
# Done after 1 second!

Destructuring¶

run js "
const person = { name: 'Alice', age: 30, city: 'NYC' };
const { name, age } = person;
console.log(\`\${name} is \${age} years old\`);
"
# Output: Alice is 30 years old

Spread Operator¶

run js "
const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const combined = [...arr1, ...arr2];
console.log(combined);
"
# Output: [ 1, 2, 3, 4, 5, 6 ]

Common Use Cases¶

Quick JavaScript prototyping and testing
Node.js script development
JSON data processing and manipulation
File system operations and automation
Testing algorithms and data structures
Learning modern JavaScript features
API testing and HTTP requests
String manipulation and parsing

Limitations¶

External npm packages must be installed separately
No automatic package.json or node_modules management
Browser-specific APIs (DOM, window) are not available
Some async operations may require explicit handling