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Oppa is an options parser (also known as arguments parser).

It is very easy to use, yet powerful, and if used from TypeScript, the parsed result will have types matching the parse configuration.

By default, oppa creates --help (and -h) arguments which it uses to produce a help output, describing the arguments. If a version is provided, it creates --version and -v too.


  • Typesafe parse result (unique feature)
  • Handles --long and -short names
    • (the second is equivalent to -s -h -o -r -t)
  • Aliases (e.g. --force = -f)
  • Auto generated --help (-h) page
  • Auto generated --version (-v)
  • Powerful argument value handling
    • Supports strings, booleans and numbers
    • Auto-allows no- prefixes (e.g. --no-keep) to boolean arguments
    • Multi value argument --find foo bar --baz (find = [ 'foo', 'bar' ])
    • Typesafe validators
    • Specific values, individually documented in --help
  • Understands dash-dash separation (--)
  • Smart defaults (what appears as default in --help can be dynamic at run-time)


import { oppa } from 'oppa'
const result =
    oppa( {
        name: 'myapp',
        version: '1.2.3',
    } )
    .add( {
        name: 'file',
        alias: 'f',
        type: 'string',
        description: 'The file to write to',
    } )
    .add( {
        name: 'retry',
        alias: 'r',
        type: 'number',
        description: 'Retry <retry> times before bailing',
    } )
    .add( {
        name: 'force',
        type: 'boolean',
        default: true,
        description: 'Force writing to the file',
    } )
    // Defaults to process.argv if no array is provided:
    .parse( [ '-f', 'test.json', '--no-force', 'foo' ] );
expectDeepEqual( result, {
    args: {
        file: 'test.json',
        force: false,
    unknown: [ ],
    rest: [ 'foo' ],
    dashdash: [ ],
} );
// If using TypeScript, the following will not compile since
// <force> is a boolean (even at compile-time):
result.args.force.toLowerCase( ); // !

--help will write:

   Usage: myapp [options]


      -h, --help            Print (this) help screen
      -v, --version         Print the program version
      -f, --file <file>     The file to write to
      -r, --retry <retry>   Retry <retry> times before bailing
      --(no-)force          Force writing to the file (default: true)



The oppa( ) function creates an object on which arguments can be configured. It can then be used to parse an array of strings, and also to print a help screen.

The options to the oppa( ) function are:

    name: string;
    version: string;
    usage: string;
    description: Description;
    noHelp: boolean;
    noHelpAlias: boolean;
    noVersionAlias: boolean;
    allowUnknown: boolean;
    throwOnError: boolean;
    noExit: boolean;

name, version, usage and description are used for the auto-generated --help and --version options.

If noHelp is true, a --help will not be auto-generated.

noHelpAlias and noVersionAlias will prevent the auto-generated --version and --help to have shortcut aliases -v and -h, so they can be used for other options, e.g. verbose and host.

allowUnknown will cause oppa not to fail on unknown arguments, but simply keep them as booleans or strings (in its own array of { name, value } objects called unknown). --foo will add a boolean foo and set it to true. --bar=baz will set the name bar to the string baz.

throwOnError will, instead of printing a help screen on invalid arguments, throw an error so that you can control the flow (sometimes useful in unit tests).

noExit causes the program not to exit after having printed the help screen or version (sometimes useful in unit tests).


The result of oppa( ) is an object on which you can add arguments using add which takes the following options:

    oppa( )
    .add( {
        // These are required:
        name: 'force',   // The long-versioned name
        type: 'boolean', // 'string', 'boolean' or 'number'
        // These are optional
        alias: 'f',        // An alias (or array of aliases)
        multi: false,      // Whether this is a multi-valued argument,
                           // only applicable to 'string' and 'number'.
        description: [     // A string or array of strings
            'Force action',
            'Will overwrite foo if bar'
        negatable: false,  // Adds 'no-' alternative (only applicable
                           // to boolean and true by default)
    } )
    .add( {
        name: 'file',
        type: 'string',
        // More optional, mostly for 'string' and 'number' arguments
        default: 'out.json', // The default-value (printed to --help)
        realDefault:         // The run-time default, unless <default>
            path.join( __dirname, 'out.json' ),
        values: [            // A list of valid values, other will
                             // fail. Will be printed to --help too.
            { 'file': 'Filename of the file to write to' },
            { '-': 'Write to stdout' }
        example: [           // A list of example arguments
            { '-f my-file.json': 'Write to my-file.json' },
            { '-f -': 'Write to stdout' },
        match: ( file ) =>   // Custom validation. Overwrites <values>
            file === '-' || file.endsWith( '.json' ),
    } )

These will cause the following --help output:

   Usage: [options]


      -h, --help      Print (this) help screen
      -f, --force     Force action
                      Will overwrite foo if bar
      --file <file>    (default: out.json)

                         file   Filename of the file to write to
                         -      Write to stdout

                         -f my-file.json   Write to my-file.json
                         -f -              Write to stdout


For sub-commands (e.g. like git), oppa is designed not to support this as a single-step parsing solution. Instead, you're encouraged to run oppa again. Oppa saves no global state, everything is local to an oppa( ) context.

    const mainResult =
        oppa( )
        .add( /* ... */ )
        .add( /* ... */ )
        .parse( ); // Will parse process.argv
    // Applies to the entire application, e.g. verbosity, debug
    const globalOptions = mainResult.args;
    const [ subCommand, ...subArgs ] = mainResult.rest;
    switch ( subCommand ) // The first non-option argument
        case 'init': return runInit( );
        case 'push': return runPush( );
    // ...
    function runInit( )
        // Parse <init> arguments:
        const initOptions = oppa( )
            .add( /* ... */ )
            .add( /* ... */ )
            .parse( subArgs );
        // ...
    function runPush( )
        // ...


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