deno eval Evaluate JavaScript or TypeScript code directly from the command line without creating a file.
Basic usage deno eval "console.log('Hello, world!')"
Syntax deno eval [OPTIONS] < CODE >
The JavaScript or TypeScript code to evaluate.
Options Print the result of the evaluated expression.
Permission flags All standard permission flags are supported. See the permissions guide for details.
Common options
--ext <EXT> - Set content type (ts, tsx, js, jsx)--no-check - Skip type checking--import-map <FILE> - Use import map--env <FILE> - Load environment variables from file
Examples Simple expression deno eval "console.log(1 + 2)" # Output: 3
Print result deno eval --print "1 + 2" # Output: 3
Using with permissions # Read a file deno eval --allow-read "console.log(Deno.readTextFileSync('README.md'))" # Fetch from URL deno eval --allow-net "const res = await fetch('https://example.com'); console.log(await res.text())"
TypeScript code deno eval "const x: number = 42; console.log(x * 2)" # Output: 84
Multi-line code deno eval ' const items = [1, 2, 3, 4, 5]; const sum = items.reduce((a, b) => a + b, 0); console.log("Sum:", sum); ' # Output: Sum: 15
JSON processing # Parse JSON deno eval --print 'JSON.parse(`{"name":"Deno","version":"2.0"}`).name' # Output: Deno # Create JSON deno eval --print 'JSON.stringify({a: 1, b: 2})' # Output: {"a":1,"b":2}
Environment variables deno eval --allow-env 'console.log("User:", Deno.env.get("USER"))'
Using Deno APIs # Get Deno version deno eval 'console.log(Deno.version)' # List directory deno eval --allow-read ' for await (const entry of Deno.readDir(".")) { console.log(entry.name); } ' # HTTP server (quick test) deno eval --allow-net 'Deno.serve(() => new Response("Hello!"))'
Working with dates # Current timestamp deno eval --print 'Date.now()' # Formatted date deno eval --print 'new Date().toISOString()'
Use cases
deno eval --print "Math.sqrt(144)" deno eval --print "(100 * 1.15).toFixed(2)"
deno eval "const arr = [1,2,3]; console.log(arr.map(x => x * 2))"
deno eval --allow-read --allow-write ' const data = Deno.readTextFileSync("input.txt"); const processed = data.toUpperCase(); Deno.writeTextFileSync("output.txt", processed); '
echo '{"value": 42}' | deno eval --print 'JSON.parse(await Deno.stdin.readAll()).value'
Tips
Use single quotes to avoid shell interpolation issues
Use --print to automatically print the result without console.log()
TypeScript is supported by default
Top-level await is supported
See also