The js utility is a command-line JavaScript interpreter that implements the ECMAScript specification. It provides both a REPL (Read-Eval-Print Loop) for interactive JavaScript execution and script execution capabilities.
Usage
Basic syntax
js [options] [scripts...]
Interactive REPL
Launch the REPL by running js without arguments:
The REPL provides:
- Line editing with syntax highlighting
- Multi-line input support
- Command history (saved to
~/.js-history)
- Auto-completion for variables and properties
Execute scripts
js script.js
js script1.js script2.js # Concatenates sources
Evaluate inline code
js -c "console.log('Hello, World!')"
Command-line options
Script execution options
| Option | Description |
|---|
-c, --evaluate <script> | Evaluate the provided string as JavaScript code |
-m, --as-module | Treat input as an ES module instead of a script |
-p, --parse-only | Parse the script without executing it |
-l, --print-last-result | Print the last expression result |
Debugging options
| Option | Description |
|---|
-A, --dump-ast | Print the Abstract Syntax Tree |
-d, --dump-bytecode | Print the compiled bytecode |
-i, --disable-ansi-colors | Disable colored output |
-h, --disable-source-location-hints | Disable source location hints in errors |
-g, --gc-on-every-allocation | Run garbage collection after every allocation (for testing) |
--disable-debug-output | Disable debug output |
Display options
| Option | Description |
|---|
-s, --no-syntax-highlight | Disable live syntax highlighting in REPL |
-r, --raw-strings | Display strings without quotes or escape sequences |
Test262 support
| Option | Description |
|---|
--use-test262-global | Use Test262 global object ($262) for test compatibility |
The --use-test262-global option enables the Test262 global object, which provides functions like $262.createRealm() and $262.evalScript() required by the official ECMAScript Test262 suite.
REPL commands
The REPL provides several built-in commands:
exit([code])
Exit the REPL with an optional exit code (defaults to 0).
exit()
exit(1) // Exit with error code
help()
Display available REPL commands.
print(value)
Pretty-print a JavaScript value.
print({ name: "Ladybird", version: 1.0 })
loadINI(file)
Load an INI file and return its contents as a JavaScript object.
const config = loadINI("config.ini")
loadJSON(file)
Load and parse a JSON file.
const data = loadJSON("data.json")
save(file)
Save REPL input history to a file.
Special variables
Access the result of the last expression:
Assigning to _ disables the automatic last value tracking. You’ll need to restart the REPL to re-enable it.
global
Reference to the global object:
Examples
Running a JavaScript file
Executing code with modules
Debugging bytecode
js --dump-bytecode --dump-ast script.js
Testing with raw output
js --disable-ansi-colors --print-last-result script.js
Interactive exploration
js
> const arr = [1, 2, 3, 4, 5]
> arr.map(x => x * 2)
[2, 4, 6, 8, 10]
> _
[2, 4, 6, 8, 10]
> save("my-session.js")
Features
Syntax highlighting
The REPL automatically highlights JavaScript syntax:
- Keywords in blue
- Strings in green
- Numbers in magenta
- Control keywords in cyan
- Identifiers in white
Disable syntax highlighting with the --no-syntax-highlight option if you’re piping output or prefer plain text.
Error reporting
Detailed error messages with source location:
> function test() { return x; }
> test()
Uncaught exception: ReferenceError: x is not defined
at test (REPL:1:24)
> function factorial(n) {
. if (n <= 1) return 1;
. return n * factorial(n - 1);
. }
> factorial(5)
120
Promise tracking
The interpreter warns about unhandled promise rejections:
> Promise.reject("error")
WARNING: A promise was rejected without any handlers (result: error)
Implementation details
Global objects
The js utility provides two global object types:
- ReplObject - Used in REPL mode, includes REPL-specific functions
- ScriptObject - Used in script mode, minimal global environment
JavaScript engine
The utility uses LibJS, Ladybird’s JavaScript engine:
- Full ECMAScript support
- Bytecode compilation and interpretation
- Garbage collection
- ES modules support
Console object
Full console API support:
console.log("Message")
console.warn("Warning")
console.error("Error")
console.table([{a: 1}, {a: 2}])
console.group("Group")
Windows: REPL functionality is not supported on Windows due to LibLine dependency. Script execution works normally.
- test262-runner - Automated Test262 test suite runner
- wasm - WebAssembly runtime with JavaScript integration
Source code
Source file: Utilities/js.cpp