index.js

module.exports = function (args, opts) {
    if (!opts) opts = {};
    
    var flags = { bools : {}, strings : {}, unknownFn: null };

    if (typeof opts['unknown'] === 'function') {
        flags.unknownFn = opts['unknown'];
    }

    if (typeof opts['boolean'] === 'boolean' && opts['boolean']) {
      flags.allBools = true;
    } else {
      [].concat(opts['boolean']).filter(Boolean).forEach(function (key) {
          flags.bools[key] = true;
      });
    }
    
    var aliases = {};
    Object.keys(opts.alias || {}).forEach(function (key) {
        aliases[key] = [].concat(opts.alias[key]);
        aliases[key].forEach(function (x) {
            aliases[x] = [key].concat(aliases[key].filter(function (y) {
                return x !== y;
            }));
        });
    });

    [].concat(opts.string).filter(Boolean).forEach(function (key) {
        flags.strings[key] = true;
        if (aliases[key]) {
            flags.strings[aliases[key]] = true;
        }
     });

    var defaults = opts['default'] || {};
    
    var argv = { _ : [] };
    Object.keys(flags.bools).forEach(function (key) {
        setArg(key, defaults[key] === undefined ? false : defaults[key]);
    });
    
    var notFlags = [];

    if (args.indexOf('--') !== -1) {
        notFlags = args.slice(args.indexOf('--')+1);
        args = args.slice(0, args.indexOf('--'));
    }

    function argDefined(key, arg) {
        return (flags.allBools && /^--[^=]+$/.test(arg)) ||
            flags.strings[key] || flags.bools[key] || aliases[key];
    }

    function setArg (key, val, arg) {
        if (arg && flags.unknownFn && !argDefined(key, arg)) {
            if (flags.unknownFn(arg) === false) return;
        }

        var value = !flags.strings[key] && isNumber(val)
            ? Number(val) : val
        ;
        setKey(argv, key.split('.'), value);
        
        (aliases[key] || []).forEach(function (x) {
            setKey(argv, x.split('.'), value);
        });
    }

    function setKey (obj, keys, value) {
        var o = obj;
        keys.slice(0,-1).forEach(function (key) {
            if (o[key] === undefined) o[key] = {};
            o = o[key];
        });

        var key = keys[keys.length - 1];
        if (o[key] === undefined || flags.bools[key] || typeof o[key] === 'boolean') {
            o[key] = value;
        }
        else if (Array.isArray(o[key])) {
            o[key].push(value);
        }
        else {
            o[key] = [ o[key], value ];
        }
    }
    
    for (var i = 0; i < args.length; i++) {
        var arg = args[i];
        
        if (/^--.+=/.test(arg)) {
            // Using [\s\S] instead of . because js doesn't support the
            // 'dotall' regex modifier. See:
            // http://stackoverflow.com/a/1068308/13216
            var m = arg.match(/^--([^=]+)=([\s\S]*)$/);
            setArg(m[1], m[2], arg);
        }
        else if (/^--no-.+/.test(arg)) {
            var key = arg.match(/^--no-(.+)/)[1];
            setArg(key, false, arg);
        }
        else if (/^--.+/.test(arg)) {
            var key = arg.match(/^--(.+)/)[1];
            var next = args[i + 1];
            if (next !== undefined && !/^-/.test(next)
            && !flags.bools[key]
            && !flags.allBools
            && (aliases[key] ? !flags.bools[aliases[key]] : true)) {
                setArg(key, next, arg);
                i++;
            }
            else if (/^(true|false)$/.test(next)) {
                setArg(key, next === 'true', arg);
                i++;
            }
            else {
                setArg(key, flags.strings[key] ? '' : true, arg);
            }
        }
        else if (/^-[^-]+/.test(arg)) {
            var letters = arg.slice(1,-1).split('');
            
            var broken = false;
            for (var j = 0; j < letters.length; j++) {
                var next = arg.slice(j+2);
                
                if (next === '-') {
                    setArg(letters[j], next, arg)
                    continue;
                }
                
                if (/[A-Za-z]/.test(letters[j])
                && /-?\d+(\.\d*)?(e-?\d+)?$/.test(next)) {
                    setArg(letters[j], next, arg);
                    broken = true;
                    break;
                }
                
                if (letters[j+1] && letters[j+1].match(/\W/)) {
                    setArg(letters[j], arg.slice(j+2), arg);
                    broken = true;
                    break;
                }
                else {
                    setArg(letters[j], flags.strings[letters[j]] ? '' : true, arg);
                }
            }
            
            var key = arg.slice(-1)[0];
            if (!broken && key !== '-') {
                if (args[i+1] && !/^(-|--)[^-]/.test(args[i+1])
                && !flags.bools[key]
                && (aliases[key] ? !flags.bools[aliases[key]] : true)) {
                    setArg(key, args[i+1], arg);
                    i++;
                }
                else if (args[i+1] && /true|false/.test(args[i+1])) {
                    setArg(key, args[i+1] === 'true', arg);
                    i++;
                }
                else {
                    setArg(key, flags.strings[key] ? '' : true, arg);
                }
            }
        }
        else {
            if (!flags.unknownFn || flags.unknownFn(arg) !== false) {
                argv._.push(
                    flags.strings['_'] || !isNumber(arg) ? arg : Number(arg)
                );
            }
            if (opts.stopEarly) {
                argv._.push.apply(argv._, args.slice(i + 1));
                break;
            }
        }
    }
    
    Object.keys(defaults).forEach(function (key) {
        if (!hasKey(argv, key.split('.'))) {
            setKey(argv, key.split('.'), defaults[key]);
            
            (aliases[key] || []).forEach(function (x) {
                setKey(argv, x.split('.'), defaults[key]);
            });
        }
    });
    
    if (opts['--']) {
        argv['--'] = new Array();
        notFlags.forEach(function(key) {
            argv['--'].push(key);
        });
    }
    else {
        notFlags.forEach(function(key) {
            argv._.push(key);
        });
    }

    return argv;
};

function hasKey (obj, keys) {
    var o = obj;
    keys.slice(0,-1).forEach(function (key) {
        o = (o[key] || {});
    });

    var key = keys[keys.length - 1];
    return key in o;
}

function isNumber (x) {
    if (typeof x === 'number') return true;
    if (/^0x[0-9a-f]+$/i.test(x)) return true;
    return /^[-+]?(?:\d+(?:\.\d*)?|\.\d+)(e[-+]?\d+)?$/.test(x);
}