Complete rule set for direct implementation in JS. Apply the “i” flag (ignoreCase) to all operations.

/**
 * Stemming logic converted from Python.
 * Uses .test() for searches and .replace() for substitutions.
 */
 
function getCauserStem(spoken) {
    if (/[!?()]/.test(spoken)) return '';
    let s = spoken.replace(/^-/, '').toLowerCase();
    // Count hyphens
    const hyphenCount = (s.match(/-/g) || []).length;
    if (hyphenCount >= 2) {
        return s.replace(/-([^-]+)-.*$/, '$1');
    }
    return s.replace(/[,-].*$/, '');
}
 
function getMedianStem(spoken) {
    if (/[!?()]/.test(spoken)) return '';
    let s = spoken.replace(/^-/, '').toLowerCase();
 
    // Attempt multi-hyphen match
    let m = s.match(/(.*)-(.*)-(.*)/);
    if (m) return m[1] + m[3];
 
    // Attempt single-hyphen match
    m = s.match(/(.*)-([^ ]*)(.*)/);
    if (m) return m[1] + m[2];
 
    return s.replace(/,.*$/, '');
}

Apply these to the Causer stem. Replace using $1, $2 syntax.

const REDUP_PAIRS = [
    [/^[td]h?([sz]h?)([aeiouy]*)([aeiou])/i, "t$3d$1$2$3"],
    [/^([bdgptkszfv])(h?)([lrsfzv])/i,       "$1e$1$2$3"],
    [/^([sz])(h?)([pbkgtd])/i,               "s$2ez$3"],
    [/^[fv]([pbkgtd])/i,                     "fev$1"],
    [/^[sz]([aeiou]*)([aeiou])/i,            "s$2z$1$2"],
    [/^[fv]([aeiou]*)([aeiou])/i,            "f$2v$1$2"],
    [/^[sz]h([aeiou]*)([aeiou])/i,           "sh$2zh$1$2"],
    [/^(h?)([uwo]*)([ou])/i,                 "$1owo"],
    [/^(h?)([iy]*)([aeoiu])/i,               "$1iya"],
    [/^[pb]h?([aeiou]*)([aeiou])/i,          "p$2b$1$2"],
    [/^([nml])([aeiou]*)([aeiou])/i,         "$1$3$1$2$3"],
    [/^[kg]h?([aeiou]*)([aeiou])/i,          "k$2g$1$2"],
    [/^[fv]([nml])/i,                        "fav$1"],
    [/^[sz]([nml])/i,                        "saz$1"],
    [/^(h?)([ea]*)/i,                        "$1ea"],
    [/^/i,                                   "hee"]
];
 
function applyReduplicated(stem) {
    for (const [pat, repl] of REDUP_PAIRS) {
        if (pat.test(stem)) {
            return stem.replace(pat, repl);
        }
    }
    return stem;
}

• All patterns should be instantiated as new RegExp(pattern, 'i').
• In the replacements below, $1 corresponds to the first capture group (…).

const PATTERNS = [
    "et(t?)$", "[ou]y$", "(y[ou]+)([sf]h?)?$", "([w]+)[ae]+$", 
    "([yi]+)([aeiou]+)([bdgptkmnlrfscvzjh]+)$", 
    "([wuo]+)([aeiou]*)([bdgptkmnrfscvzjh]+)$",
    "([bdgptkmnlrfscvzjh]+)(([aeiou])([aeiou]))([lr])$",
    "([bdgptkmnlrfscvzjh]+)([aeiou])([lr])$",
    "([bdgptkmnlrfscvzjh']+)([ea])([ea]*)$",
    "([bdgptkmnlrfscvzjh]+)([aeiou]*)([ouw]+)$",
    "([bdgptkmnlrfscvzjhw]+)([aeiou]*)([iy])$",
    "([bdgptkmnlrfscvzjh]+)([aeiou]+)([bdgptkmnrfscvzjh]+)$",
    "()([ae])y()$", "(y)([ae]+)()$", "(^|[''\\-])([aeoiu]+)([bpkgtd]+)$",
    "([''\\-])([aeoiu]+)([lr])$", "([''\\-])([aeoiu]+)([lr][lr])$", "$"
];
 
// Example: Actor-There Replacements
const REPL_ACTOR_THERE = [
    "et$1a", "$1ya", "$1'a$2", "wawa", "$1$2$3e", "$1$2$3e",
    "$1$2ra", "$1$2ra", "$1$2wa", "$1$2wa", "$1$2$3ya", "$1$2$3e",
    "$2ye", "$1$2$3'a", "$1$2$3a", "$1$2ra", "$1$2$3a", "a"
];

Apply these after the spatial/case form is generated.

const ME_PAIRS = [
    [/([ao]|[ae]e)$/i, "$1ni"], 
    [/ii$/i,           "iin"], 
    [/[uw]$/i,         "win"], 
    [/e?$/i,           "in"]
];
 
const YOU_PAIRS = [
    [/([^aeiouwyn])$/i, "$1ets"],
    [/([^aeiou]*[aeiou]+[^aeiou]+[aeiou]+[^aeiou]*[aeiou]+)$/i, "$1ts"],
    [/$/i, "tse"]
];
 
const THEM_PAIRS = [
    [/([aeou])$/i, "$1rh"], 
    [/([^i][wyi])$/i, "$1irh"], 
    [/$/i, "erh"]
];

const PORTMANTEAUS = {
    'me_there':   { pat: /in$/i,       rep: 'inia' },
    'you_there':  { pat: /tse$/i,      rep: 'tsa' },
    'them_there': { pat: /e?[ei]rh$/i, rep: 'earh' }
    // Add others following the same pattern
};
 
function applyPortmanteau(form, person, direction) {
    const key = `${person}_${direction}`;
    if (PORTMANTEAUS[key]) {
        const entry = PORTMANTEAUS[key];
        if (entry.pat.test(form)) return form.replace(entry.pat, entry.rep);
    }
    // Fallback if not in table
    const fallbacks = { 'there': 'a', 'hither': 'i', 'hence': 'oy' };
    return form + (fallbacks[direction] || '');
}

Copy this into a browser console or a .js file to test a few rules.

// Simple demo of the "Cheers" and "Actor-Me" rules
const cheersRules = [
    [/([aeou])[iy]$/i, "$1iyets!"],
    [/([^aeiou])$/i,   "$1eyets!"],
    [/[ou]+$/i,        "oyets!"],
    [/$/i,             "eyets!"]
];
 
function makeCheers(word) {
    for (const [pat, repl] of cheersRules) {
        if (pat.test(word)) return word.replace(pat, repl);
    }
}
 
function makeActorMe(stem) {
    // Basic version of the "Me" rules
    if (/ii$/i.test(stem)) return stem.replace(/ii$/i, "iin");
    if (/[uw]$/i.test(stem)) return stem + "win";
    return stem + "in";
}
 
console.log(makeCheers("sharu"));   // "sharoyets!"
console.log(makeActorMe("shaaru")); // "shaarwin"