"Ex argumentis, veritas"
by @boneskull
npm install @boneskull/bargsMost argument parsers make you choose: either a simple API with weak types, or a complex and overengineered DSL. bargs provides a combinator-style API for building type-safe CLIs—composable schema definitions with full type inference.
Also: this is the only argument parser I know of that comes with frickin' themes. Themes, dogg.
A CLI with an optional command and a couple options:
import{bargs,opt,pos}from'@boneskull/bargs';awaitbargs('greet',{version: '1.0.0'}).globals(opt.options({name: opt.string({default: 'world'}),loud: opt.boolean({aliases: ['l']}),}),).command('say',pos.positionals(pos.string({name: 'message',required: true})),({ positionals, values })=>{const[message]=positionals;constgreeting=`${message}, ${values.name}!`;console.log(values.loud ? greeting.toUpperCase() : greeting);},'Say a greeting',).defaultCommand('say').parseAsync();$ greet Hello --name Alice --loud
HELLO, ALICE!Each helper returns a fully-typed definition:
import{opt,pos}from'@boneskull/bargs';constverbose=opt.boolean({aliases: ['v']});// Type: BooleanOption & { aliases: ['v'] }constlevel=opt.enum(['low','medium','high'],{default: 'medium'});// Type: EnumOption<'low' | 'medium' | 'high'> & { default: 'medium' }constfile=pos.string({name: 'file',required: true});// Type: StringPositional & { name: 'file', required: true }When you build a CLI with these, the result types flow through automatically—options with defaults or required: true are non-nullable.
Options and positionals can be merged using callable parsers:
import{opt,pos}from'@boneskull/bargs';// Create separate parsersconstoptions=opt.options({verbose: opt.boolean({aliases: ['v']}),output: opt.string({aliases: ['o'],default: 'stdout'}),});constpositionals=pos.positionals(pos.string({name: 'input',required: true}),);// Merge them: positionals(options) combines bothconstparser=positionals(options);// Type: Parser<{ verbose: boolean | undefined, output: string }, [string]>For a CLI without subcommands, use .globals() with merged options and positionals, then handle the result yourself:
import{bargs,opt,pos}from'@boneskull/bargs';// Merge options and positionals into one parser// when a positional is variadic, it becomes an array within the resultconstparser=pos.positionals(pos.variadic('string',{name: 'text'}))(opt.options({uppercase: opt.boolean({aliases: ['u'],default: false}),}),);const{ values, positionals }=awaitbargs('echo',{description: 'Echo text to stdout',version: '1.0.0',}).globals(parser).parseAsync();const[words]=positionals;consttext=words.join(' ');console.log(values.uppercase ? text.toUpperCase() : text);For a CLI with multiple subcommands:
import{bargs,merge,opt,pos}from'@boneskull/bargs';awaitbargs('tasks',{description: 'A task manager',version: '1.0.0',}).globals(opt.options({verbose: opt.boolean({aliases: ['v'],default: false}),}),).command('add',// Use merge() to combine positionals with command-specific optionsmerge(opt.options({priority: opt.enum(['low','medium','high'],{default: 'medium'}),}),pos.positionals(pos.string({name: 'text',required: true})),),({ positionals, values })=>{const[text]=positionals;console.log(`Adding ${values.priority} priority task: ${text}`);if(values.verbose)console.log('Verbose mode enabled');},'Add a task',).command('list',opt.options({all: opt.boolean({default: false}),}),({ values })=>{console.log(values.all ? 'All tasks' : 'Pending tasks');},'List tasks',).defaultCommand('list').parseAsync();$ tasks add "Buy groceries" --priority high --verbose
Adding high priority task: Buy groceries
Verbose mode enabled
$ tasks list --all
All tasksCommands can be nested to arbitrary depth. The only limit is your contempt for your users. Use the factory pattern:
import{bargs,opt,pos}from'@boneskull/bargs';awaitbargs('git').globals(opt.options({verbose: opt.boolean({aliases: ['v']})}))// Factory pattern: receives a builder with parent globals already typed.command('remote',(remote)=>remote.command('add',pos.positionals(pos.string({name: 'name',required: true}),pos.string({name: 'url',required: true}),),({ positionals, values })=>{const[name,url]=positionals;// values.verbose is fully typed! (from parent globals)if(values.verbose)console.log(`Adding ${name}: ${url}`);},'Add a remote',).command('remove'/* ... */).defaultCommand('add'),'Manage remotes',).command('commit',commitParser,commitHandler)// Regular command.parseAsync();$ git --verbose remote add origin https://github.com/...
Adding origin: https://github.com/...
$ git remote remove originThe factory function receives a CliBuilder that already has parent globals typed, so all nested command handlers get full type inference for merged global + command options. See examples/nested-commands.ts for a full example.
Create a CLI builder.
| Option | Type | Description |
|---|---|---|
description | string | Description shown in help |
version | string | Enables --version flag |
epilog | string or false | Footer text in help (see Epilog) |
theme | Theme | Help color theme (see Theming) |
Set global options and transforms that apply to all commands.
bargs('my-cli').globals(opt.options({verbose: opt.boolean()}));// ...Register a command. The handler receives merged global + command types.
.command('build',opt.options({watch: opt.boolean()}),({ values })=>{// values has both global options AND { watch: boolean }console.log(values.verbose,values.watch);},'Build the project',)Register a nested command group using a factory function. The factory receives a builder that already has parent globals typed, giving full type inference in nested handlers.
bargs('main').globals(opt.options({verbose: opt.boolean()})).command('nested',(nested)=>nested.command('foo',fooParser,({ values })=>{// values.verbose is typed correctly!}).command('bar',barParser,barHandler),'Nested commands',).parseAsync();Or
.defaultCommand(parser, handler)
Set the command that runs when no command is specified.
// Reference an existing command by name.defaultCommand('list')// Or define an inline default.defaultCommand(pos.positionals(pos.string({name: 'file'})),({ positionals })=>console.log(positionals[0]),)Parse arguments and execute handlers.
.parse()- Synchronous. Throws if any transform or handler returns a Promise..parseAsync()- Asynchronous. Supports async transforms and handlers.
// Async (supports async transforms/handlers)constresult=awaitbargs('my-cli').globals(...).parseAsync();console.log(result.values,result.positionals,result.command);// Sync (no async transforms/handlers)constresult=bargs('my-cli').globals(...).parse();import{opt}from'@boneskull/bargs';opt.string({default: 'value'});// --name valueopt.number({default: 42});// --count 42opt.boolean({aliases: ['v']});// --verbose, -vopt.boolean({aliases: ['v','verb']});// --verbose, --verb, -vopt.enum(['a','b','c']);// --level aopt.array('string');// --file x --file yopt.array(['low','medium','high']);// --priority low --priority highopt.count();// -vvv → 3| Property | Type | Description |
|---|---|---|
aliases | string[] | Short (['v'] for -v) or long aliases (['verb'] for --verb) |
default | varies | Default value (makes the option non-nullable) |
description | string | Help text description |
group | string | Groups options under a custom section header |
hidden | boolean | Hide from --help output |
required | boolean | Mark as required (makes the option non-nullable) |
Options can have both short (single-character) and long (multi-character) aliases:
opt.options({verbose: opt.boolean({aliases: ['v','verb']}),output: opt.string({aliases: ['o','out']}),});All of these are equivalent:
$ my-cli -v # verbose: true
$ my-cli --verb # verbose: true
$ my-cli --verbose # verbose: true
$ my-cli -o file.txt # output: "file.txt"
$ my-cli --out file.txt # output: "file.txt"
$ my-cli --output file.txt # output: "file.txt"For non-array options, using both an alias and the canonical name throws an error:
$ my-cli --verb --verbose
Error: Conflicting options: --verb and --verbose cannot both be specifiedFor array options, values from all aliases are merged. Single-character aliases and the canonical name are processed first (in command-line order), then multi-character aliases are appended:
opt.options({files: opt.array('string',{aliases: ['f','file']}),});$ my-cli --file a.txt -f b.txt --files c.txt
# files: ["b.txt", "c.txt", "a.txt"]# (-f and --files first, then --file appended)All boolean options automatically support a negated form --no-<flag> to explicitly set the option to false:
$ my-cli --verbose # verbose: true
$ my-cli --no-verbose # verbose: false
$ my-cli # verbose: undefined (or default)If both --flag and --no-flag are specified, bargs throws an error:
$ my-cli --verbose --no-verbose
Error: Conflicting options: --verbose and --no-verbose cannot both be specifiedIn help output, booleans with default: true display as --no-<flag> (since that's how users would turn them off):
opt.options({colors: opt.boolean({default: true,description: 'Use colors'}),});// Help output shows: --no-colors Use colors [boolean] default: trueCreate a parser from an options schema:
constparser=opt.options({verbose: opt.boolean({aliases: ['v']}),output: opt.string({default: 'out.txt'}),});// Type: Parser<{ verbose: boolean | undefined, output: string }, []>import{pos}from'@boneskull/bargs';pos.string({required: true});// <file>pos.number({default: 8080});// [port]pos.enum(['dev','prod']);// [env]pos.variadic('string');// [files...]| Property | Type | Description |
|---|---|---|
default | varies | Default value |
description | string | Help text description |
name | string | Display name in help (defaults to arg0, arg1, ...) |
required | boolean | Mark as required (shown as <name> vs [name]) |
Create a parser from positional definitions:
constparser=pos.positionals(pos.string({name: 'source',required: true}),pos.string({name: 'dest',required: true}),);// Type: Parser<{}, [string, string]>Use variadic for rest arguments (must be last):
constparser=pos.positionals(pos.variadic('string',{name: 'files'}));// Type: Parser<{}, [string[]]>Use merge() to combine multiple parsers into one:
import{merge,opt,pos}from'@boneskull/bargs';constcombined=merge(opt.options({priority: opt.enum(['low','medium','high'],{default: 'medium'}),}),pos.positionals(pos.string({name: 'task',required: true})),);// Type: Parser<{ priority: 'low' | 'medium' | 'high' }, [string]>You can merge as many parsers as needed—options are merged (later overrides earlier), and positionals are concatenated.
Alternatively, parsers can be merged by calling one with the other:
constoptions=opt.options({priority: opt.enum(['low','medium','high'])});constpositionals=pos.positionals(pos.string({name: 'task',required: true}),);// These are equivalent:constcombined1=positionals(options);constcombined2=options(positionals);Use whichever style you find more readable.
Use map() to transform parsed values before they reach your handler:
import{bargs,map,opt}from'@boneskull/bargs';constglobals=map(opt.options({config: opt.string(),verbose: opt.boolean({default: false}),}),({ values, positionals })=>({
positionals,values: {
...values,// Add computed propertiestimestamp: newDate().toISOString(),configLoaded: !!values.config,},}),);awaitbargs('my-cli').globals(globals).command('info',opt.options({}),({ values })=>{// values.timestamp and values.configLoaded are availableconsole.log(values.timestamp);},'Show info',).parseAsync();Transforms are fully type-safe—the return type becomes the type available in handlers.
Transforms can be async:
constglobals=map(opt.options({url: opt.string({required: true})}),async({ values, positionals })=>{constresponse=awaitfetch(values.url);return{
positionals,values: {
...values,data: awaitresponse.json(),},};},);If you prefer camelCase property names instead of kebab-case, use the camelCaseValues transform:
import{bargs,map,opt,camelCaseValues}from'@boneskull/bargs';const{ values }=awaitbargs('my-cli').globals(map(opt.options({'output-dir': opt.string({default: '/tmp'}),'dry-run': opt.boolean(),}),camelCaseValues,),).parseAsync(['--output-dir','./dist','--dry-run']);console.log(values.outputDir);// './dist'console.log(values.dryRun);// trueThe camelCaseValues transform:
- Converts all kebab-case keys to camelCase (
output-dir→outputDir) - Preserves keys that are already camelCase or have no hyphens
- Is fully type-safe—TypeScript knows the transformed key names
By default, bargs displays your package's homepage and repository URLs (from package.json) at the end of help output. URLs become clickable hyperlinks in supported terminals.
// Custom epilogbargs('my-cli',{epilog: 'For more info, visit https://example.com',});// Disable epilog entirelybargs('my-cli',{epilog: false});Customize help output colors with built-in themes or your own:
// Use a built-in theme: 'default', 'mono', 'ocean', 'warm'bargs('my-cli',{theme: 'ocean'});// Disable colors entirelybargs('my-cli',{theme: 'mono'});The ansi export provides common ANSI escape codes for styled terminal output:
import{ansi}from'@boneskull/bargs';bargs('my-cli',{theme: {colors: {command: ansi.bold,flag: ansi.brightCyan,positional: ansi.magenta,// ...},},});Available theme color slots:
| Slot | What it styles |
|---|---|
command | Command names (e.g., init, build) |
defaultText | The default: label |
defaultValue | Default value (e.g., false, "hello") |
description | Description text for options and commands |
epilog | Footer text (homepage, repository) |
example | Example code/commands |
flag | Flag names (e.g., --verbose, -v) |
positional | Positional argument names (e.g., <file>) |
scriptName | CLI name shown in header |
sectionHeader | Section headers (e.g., USAGE, OPTIONS) |
type | Type annotations (e.g., [string], [number]) |
url | URLs (for clickable hyperlinks) |
usage | The usage line text |
Tip
You don't need to specify all color slots. Missing colors fall back to the default theme.
bargs can generate shell completion scripts for bash, zsh, and fish. Enable it with the completion option:
bargs('my-cli',{completion: true,version: '1.0.0',});Then generate and install the completion script for your shell:
# Add to ~/.bashrc (or ~/.bash_profile on macOS)
my-cli --completion-script bash >>~/.bashrc
source~/.bashrc# Add to ~/.zshrc
my-cli --completion-script zsh >>~/.zshrc
source~/.zshrc
# Or save to a file in your $fpath
my-cli --completion-script zsh >~/.zsh/completions/_my-cli# Save to completions directory
my-cli --completion-script fish >~/.config/fish/completions/my-cli.fishOnce installed, pressing Tab will complete:
- Commands and subcommands (including nested commands and aliases)
- Options (
--verbose,-v,--no-verbosefor booleans) - Enum values for options and positionals with defined choices
- Global options at any command level
$ my-cli <TAB>
build test lint
$ my-cli build --target <TAB>
dev staging prod
$ my-cli --<TAB>
--verbose --config --help --versionSee examples/completion.ts for a complete example.
bargs automatically terminates the process (via process.exit()) in certain scenarios. This is standard CLI behavior—users expect these flags to print output and exit immediately:
| Scenario | Exit Code | Output |
|---|---|---|
--help / -h | 0 | Help text to stdout |
--version | 0 | Version string to stdout |
--completion-script | 0 | Shell completion script to stdout |
| Unknown command | 1 | Error + help text to stderr |
| Missing required command | 1 | Error + help text to stderr |
--get-bargs-completions | 0 | Completion candidates to stdout |
For testing, you can mock process.exit to capture the exit code:
// Sentinel error to distinguish process.exit from other errorsclassProcessExitErrorextendsError{constructor(publiccode: number){super(`process.exit(${code})`);}}constoriginalExit=process.exit;letexitCode: number|undefined;process.exit=((code?: number)=>{exitCode=code??0;thrownewProcessExitError(exitCode);})astypeofprocess.exit;try{cli.parse(['--help']);}catch(err){if(!(errinstanceofProcessExitError)){throwerr;// Re-throw unexpected errors}}finally{process.exit=originalExit;// Always restore}console.log(exitCode);// 0bargs exports some Error subclasses for errors that don't cause automatic process termination:
import{bargs,BargsError,ValidationError}from'@boneskull/bargs';try{awaitbargs('my-cli').parseAsync();}catch(error){if(errorinstanceofValidationError){// Config validation failed (e.g., invalid schema)// i.e., "you screwed up"console.error(`Config error at "${error.path}": ${error.message}`);}elseif(errorinstanceofBargsError){// General bargs errorconsole.error(error.message);}}Generate help text programmatically:
import{generateHelp,generateCommandHelp}from'@boneskull/bargs';// These require the internal config structure—see source for detailsconsthelpText=generateHelp(config);constcommandHelp=generateCommandHelp(config,'migrate');Create clickable terminal hyperlinks (OSC 8):
import{link,linkifyUrls,supportsHyperlinks}from'@boneskull/bargs';// Check if terminal supports hyperlinksif(supportsHyperlinks()){// Create a hyperlinkconsole.log(link('Click me','https://example.com'));// Auto-linkify URLs in textconsole.log(linkifyUrls('Visit https://example.com for more info'));}Tip
bargs already automatically links URLs in --help output if the terminal supports hyperlinks.
import{ansi,// ANSI escape codescreateStyler,// Create a styler from a themedefaultTheme,// The default theme objectstripAnsi,// Remove ANSI codes from stringthemes,// All built-in themes}from'@boneskull/bargs';// Create a custom stylerconststyler=createStyler({colors: {flag: ansi.green}});console.log(styler.flag('--verbose'));// Strip ANSI codes for plain text outputconstplain=stripAnsi('\x1b[32m--verbose\x1b[0m');// '--verbose'The handle(parser, fn) function is exported for advanced use cases where you need to create a Command object outside the fluent builder. It's mostly superseded by .command(name, parser, handler).
bargs has zero (0) dependencies. Only Node.js v20+.
I've always reached for yargs in my CLI projects. However, I find myself repeatedly doing the same things; I have a sort of boilerplate in my head, ready to go (requiresArg: true and nargs: 1, amirite?). I don't want boilerplate in my head. I wanted to distill my chosen subset of yargs' behavior into a composable API. And so bargs was begat.
Copyright © 2025 Christopher "boneskull" Hiller. Licensed under the Blue Oak Model License 1.0.0.