A diacritic is a glyph added to a letter or basic glyph. This function undoes it.

undiacritic.ts

export default (sentence: string) => {
  let dict = [
    {
      base: 'A',
      letters: /[\u0041\u24B6\uFF21\u00C0\u00C1\u00C2\u1EA6\u1EA4\u1EAA\u1EA8\u00C3\u0100\u0102\u1EB0\u1EAE\u1EB4\u1EB2\u0226\u01E0\u00C4\u01DE\u1EA2\u00C5\u01FA\u01CD\u0200\u0202\u1EA0\u1EAC\u1EB6\u1E00\u0104\u023A\u2C6F]/g,
    },
    { base: 'AA', letters: /[\uA732]/g },
    { base: 'AE', letters: /[\u00C6\u01FC\u01E2]/g },
    { base: 'AO', letters: /[\uA734]/g },
    { base: 'AU', letters: /[\uA736]/g },
    { base: 'AV', letters: /[\uA738\uA73A]/g },
    { base: 'AY', letters: /[\uA73C]/g },
    {
      base: 'B',
      letters: /[\u0042\u24B7\uFF22\u1E02\u1E04\u1E06\u0243\u0182\u0181]/g,
    },
    {
      base: 'C',
      letters: /[\u0043\u24B8\uFF23\u0106\u0108\u010A\u010C\u00C7\u1E08\u0187\u023B\uA73E]/g,
    },
    {
      base: 'D',
      letters: /[\u0044\u24B9\uFF24\u1E0A\u010E\u1E0C\u1E10\u1E12\u1E0E\u0110\u018B\u018A\u0189\uA779]/g,
    },
    { base: 'DZ', letters: /[\u01F1\u01C4]/g },
    { base: 'Dz', letters: /[\u01F2\u01C5]/g },
    {
      base: 'E',
      letters: /[\u0045\u24BA\uFF25\u00C8\u00C9\u00CA\u1EC0\u1EBE\u1EC4\u1EC2\u1EBC\u0112\u1E14\u1E16\u0114\u0116\u00CB\u1EBA\u011A\u0204\u0206\u1EB8\u1EC6\u0228\u1E1C\u0118\u1E18\u1E1A\u0190\u018E]/g,
    },
    { base: 'F', letters: /[\u0046\u24BB\uFF26\u1E1E\u0191\uA77B]/g },
    {
      base: 'G',
      letters: /[\u0047\u24BC\uFF27\u01F4\u011C\u1E20\u011E\u0120\u01E6\u0122\u01E4\u0193\uA7A0\uA77D\uA77E]/g,
    },
    {
      base: 'H',
      letters: /[\u0048\u24BD\uFF28\u0124\u1E22\u1E26\u021E\u1E24\u1E28\u1E2A\u0126\u2C67\u2C75\uA78D]/g,
    },
    {
      base: 'I',
      letters: /[\u0049\u24BE\uFF29\u00CC\u00CD\u00CE\u0128\u012A\u012C\u0130\u00CF\u1E2E\u1EC8\u01CF\u0208\u020A\u1ECA\u012E\u1E2C\u0197]/g,
    },
    { base: 'J', letters: /[\u004A\u24BF\uFF2A\u0134\u0248]/g },
    {
      base: 'K',
      letters: /[\u004B\u24C0\uFF2B\u1E30\u01E8\u1E32\u0136\u1E34\u0198\u2C69\uA740\uA742\uA744\uA7A2]/g,
    },
    {
      base: 'L',
      letters: /[\u004C\u24C1\uFF2C\u013F\u0139\u013D\u1E36\u1E38\u013B\u1E3C\u1E3A\u0141\u023D\u2C62\u2C60\uA748\uA746\uA780]/g,
    },
    { base: 'LJ', letters: /[\u01C7]/g },
    { base: 'Lj', letters: /[\u01C8]/g },
    {
      base: 'M',
      letters: /[\u004D\u24C2\uFF2D\u1E3E\u1E40\u1E42\u2C6E\u019C]/g,
    },
    {
      base: 'N',
      letters: /[\u004E\u24C3\uFF2E\u01F8\u0143\u00D1\u1E44\u0147\u1E46\u0145\u1E4A\u1E48\u0220\u019D\uA790\uA7A4]/g,
    },
    { base: 'NJ', letters: /[\u01CA]/g },
    { base: 'Nj', letters: /[\u01CB]/g },
    {
      base: 'O',
      letters: /[\u004F\u24C4\uFF2F\u00D2\u00D3\u00D4\u1ED2\u1ED0\u1ED6\u1ED4\u00D5\u1E4C\u022C\u1E4E\u014C\u1E50\u1E52\u014E\u022E\u0230\u00D6\u022A\u1ECE\u0150\u01D1\u020C\u020E\u01A0\u1EDC\u1EDA\u1EE0\u1EDE\u1EE2\u1ECC\u1ED8\u01EA\u01EC\u00D8\u01FE\u0186\u019F\uA74A\uA74C]/g,
    },
    { base: 'OI', letters: /[\u01A2]/g },
    { base: 'OO', letters: /[\uA74E]/g },
    { base: 'OU', letters: /[\u0222]/g },
    {
      base: 'P',
      letters: /[\u0050\u24C5\uFF30\u1E54\u1E56\u01A4\u2C63\uA750\uA752\uA754]/g,
    },
    { base: 'Q', letters: /[\u0051\u24C6\uFF31\uA756\uA758\u024A]/g },
    {
      base: 'R',
      letters: /[\u0052\u24C7\uFF32\u0154\u1E58\u0158\u0210\u0212\u1E5A\u1E5C\u0156\u1E5E\u024C\u2C64\uA75A\uA7A6\uA782]/g,
    },
    {
      base: 'S',
      letters: /[\u0053\u24C8\uFF33\u1E9E\u015A\u1E64\u015C\u1E60\u0160\u1E66\u1E62\u1E68\u0218\u015E\u2C7E\uA7A8\uA784]/g,
    },
    {
      base: 'T',
      letters: /[\u0054\u24C9\uFF34\u1E6A\u0164\u1E6C\u021A\u0162\u1E70\u1E6E\u0166\u01AC\u01AE\u023E\uA786]/g,
    },
    { base: 'TZ', letters: /[\uA728]/g },
    {
      base: 'U',
      letters: /[\u0055\u24CA\uFF35\u00D9\u00DA\u00DB\u0168\u1E78\u016A\u1E7A\u016C\u00DC\u01DB\u01D7\u01D5\u01D9\u1EE6\u016E\u0170\u01D3\u0214\u0216\u01AF\u1EEA\u1EE8\u1EEE\u1EEC\u1EF0\u1EE4\u1E72\u0172\u1E76\u1E74\u0244]/g,
    },
    {
      base: 'V',
      letters: /[\u0056\u24CB\uFF36\u1E7C\u1E7E\u01B2\uA75E\u0245]/g,
    },
    { base: 'VY', letters: /[\uA760]/g },
    {
      base: 'W',
      letters: /[\u0057\u24CC\uFF37\u1E80\u1E82\u0174\u1E86\u1E84\u1E88\u2C72]/g,
    },
    { base: 'X', letters: /[\u0058\u24CD\uFF38\u1E8A\u1E8C]/g },
    {
      base: 'Y',
      letters: /[\u0059\u24CE\uFF39\u1EF2\u00DD\u0176\u1EF8\u0232\u1E8E\u0178\u1EF6\u1EF4\u01B3\u024E\u1EFE]/g,
    },
    {
      base: 'Z',
      letters: /[\u005A\u24CF\uFF3A\u0179\u1E90\u017B\u017D\u1E92\u1E94\u01B5\u0224\u2C7F\u2C6B\uA762]/g,
    },
    {
      base: 'a',
      letters: /[\u0061\u24D0\uFF41\u1E9A\u00E0\u00E1\u00E2\u1EA7\u1EA5\u1EAB\u1EA9\u00E3\u0101\u0103\u1EB1\u1EAF\u1EB5\u1EB3\u0227\u01E1\u00E4\u01DF\u1EA3\u00E5\u01FB\u01CE\u0201\u0203\u1EA1\u1EAD\u1EB7\u1E01\u0105\u2C65\u0250]/g,
    },
    { base: 'aa', letters: /[\uA733]/g },
    { base: 'ae', letters: /[\u00E6\u01FD\u01E3]/g },
    { base: 'ao', letters: /[\uA735]/g },
    { base: 'au', letters: /[\uA737]/g },
    { base: 'av', letters: /[\uA739\uA73B]/g },
    { base: 'ay', letters: /[\uA73D]/g },
    {
      base: 'b',
      letters: /[\u0062\u24D1\uFF42\u1E03\u1E05\u1E07\u0180\u0183\u0253]/g,
    },
    {
      base: 'c',
      letters: /[\u0063\u24D2\uFF43\u0107\u0109\u010B\u010D\u00E7\u1E09\u0188\u023C\uA73F\u2184]/g,
    },
    {
      base: 'd',
      letters: /[\u0064\u24D3\uFF44\u1E0B\u010F\u1E0D\u1E11\u1E13\u1E0F\u0111\u018C\u0256\u0257\uA77A]/g,
    },
    { base: 'dz', letters: /[\u01F3\u01C6]/g },
    {
      base: 'e',
      letters: /[\u0065\u24D4\uFF45\u00E8\u00E9\u00EA\u1EC1\u1EBF\u1EC5\u1EC3\u1EBD\u0113\u1E15\u1E17\u0115\u0117\u00EB\u1EBB\u011B\u0205\u0207\u1EB9\u1EC7\u0229\u1E1D\u0119\u1E19\u1E1B\u0247\u025B\u01DD]/g,
    },
    { base: 'f', letters: /[\u0066\u24D5\uFF46\u1E1F\u0192\uA77C]/g },
    {
      base: 'g',
      letters: /[\u0067\u24D6\uFF47\u01F5\u011D\u1E21\u011F\u0121\u01E7\u0123\u01E5\u0260\uA7A1\u1D79\uA77F]/g,
    },
    {
      base: 'h',
      letters: /[\u0068\u24D7\uFF48\u0125\u1E23\u1E27\u021F\u1E25\u1E29\u1E2B\u1E96\u0127\u2C68\u2C76\u0265]/g,
    },
    { base: 'hv', letters: /[\u0195]/g },
    {
      base: 'i',
      letters: /[\u0069\u24D8\uFF49\u00EC\u00ED\u00EE\u0129\u012B\u012D\u00EF\u1E2F\u1EC9\u01D0\u0209\u020B\u1ECB\u012F\u1E2D\u0268\u0131]/g,
    },
    { base: 'j', letters: /[\u006A\u24D9\uFF4A\u0135\u01F0\u0249]/g },
    {
      base: 'k',
      letters: /[\u006B\u24DA\uFF4B\u1E31\u01E9\u1E33\u0137\u1E35\u0199\u2C6A\uA741\uA743\uA745\uA7A3]/g,
    },
    {
      base: 'l',
      letters: /[\u006C\u24DB\uFF4C\u0140\u013A\u013E\u1E37\u1E39\u013C\u1E3D\u1E3B\u017F\u0142\u019A\u026B\u2C61\uA749\uA781\uA747]/g,
    },
    { base: 'lj', letters: /[\u01C9]/g },
    {
      base: 'm',
      letters: /[\u006D\u24DC\uFF4D\u1E3F\u1E41\u1E43\u0271\u026F]/g,
    },
    {
      base: 'n',
      letters: /[\u006E\u24DD\uFF4E\u01F9\u0144\u00F1\u1E45\u0148\u1E47\u0146\u1E4B\u1E49\u019E\u0272\u0149\uA791\uA7A5]/g,
    },
    { base: 'nj', letters: /[\u01CC]/g },
    {
      base: 'o',
      letters: /[\u006F\u24DE\uFF4F\u00F2\u00F3\u00F4\u1ED3\u1ED1\u1ED7\u1ED5\u00F5\u1E4D\u022D\u1E4F\u014D\u1E51\u1E53\u014F\u022F\u0231\u00F6\u022B\u1ECF\u0151\u01D2\u020D\u020F\u01A1\u1EDD\u1EDB\u1EE1\u1EDF\u1EE3\u1ECD\u1ED9\u01EB\u01ED\u00F8\u01FF\u0254\uA74B\uA74D\u0275]/g,
    },
    { base: 'oi', letters: /[\u01A3]/g },
    { base: 'ou', letters: /[\u0223]/g },
    { base: 'oo', letters: /[\uA74F]/g },
    {
      base: 'p',
      letters: /[\u0070\u24DF\uFF50\u1E55\u1E57\u01A5\u1D7D\uA751\uA753\uA755]/g,
    },
    { base: 'q', letters: /[\u0071\u24E0\uFF51\u024B\uA757\uA759]/g },
    {
      base: 'r',
      letters: /[\u0072\u24E1\uFF52\u0155\u1E59\u0159\u0211\u0213\u1E5B\u1E5D\u0157\u1E5F\u024D\u027D\uA75B\uA7A7\uA783]/g,
    },
    {
      base: 's',
      letters: /[\u0073\u24E2\uFF53\u00DF\u015B\u1E65\u015D\u1E61\u0161\u1E67\u1E63\u1E69\u0219\u015F\u023F\uA7A9\uA785\u1E9B]/g,
    },
    {
      base: 't',
      letters: /[\u0074\u24E3\uFF54\u1E6B\u1E97\u0165\u1E6D\u021B\u0163\u1E71\u1E6F\u0167\u01AD\u0288\u2C66\uA787]/g,
    },
    { base: 'tz', letters: /[\uA729]/g },
    {
      base: 'u',
      letters: /[\u0075\u24E4\uFF55\u00F9\u00FA\u00FB\u0169\u1E79\u016B\u1E7B\u016D\u00FC\u01DC\u01D8\u01D6\u01DA\u1EE7\u016F\u0171\u01D4\u0215\u0217\u01B0\u1EEB\u1EE9\u1EEF\u1EED\u1EF1\u1EE5\u1E73\u0173\u1E77\u1E75\u0289]/g,
    },
    {
      base: 'v',
      letters: /[\u0076\u24E5\uFF56\u1E7D\u1E7F\u028B\uA75F\u028C]/g,
    },
    { base: 'vy', letters: /[\uA761]/g },
    {
      base: 'w',
      letters: /[\u0077\u24E6\uFF57\u1E81\u1E83\u0175\u1E87\u1E85\u1E98\u1E89\u2C73]/g,
    },
    { base: 'x', letters: /[\u0078\u24E7\uFF58\u1E8B\u1E8D]/g },
    {
      base: 'y',
      letters: /[\u0079\u24E8\uFF59\u1EF3\u00FD\u0177\u1EF9\u0233\u1E8F\u00FF\u1EF7\u1E99\u1EF5\u01B4\u024F\u1EFF]/g,
    },
    {
      base: 'z',
      letters: /[\u007A\u24E9\uFF5A\u017A\u1E91\u017C\u017E\u1E93\u1E95\u01B6\u0225\u0240\u2C6C\uA763]/g,
    },
  ]

  dict.map((_rule: object, i: number) => {
    sentence = sentence.replace(dict[i].letters, dict[i].base)
  })

  return sentence
}

As seen on and then readapted: http://web.archive.org/web/20120918093154/http://lehelk.com/2011/05/06/script-to-remove-diacritics