Array of uppercase and lowercase letters

alphabet.js

const letters = (() => {
  const caps = [...Array(26)].map((val, i) => String.fromCharCode(i + 65));
  return caps.concat(caps.map(letter => letter.toLowerCase()));
})();

In case anyone needs the full set of lower and upper case Latin characters (all of the various accented versions), built from this table:

const lowercaseString = "abcdefghijklmnopqrstuvwxyzàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿāăąćĉċčďđēĕėęěĝğġģĥħĩīĭįıijĵķĺļľŀłńņňŋōŏőœŕŗřśŝşšţťŧũūŭůűųŵŷźżžƃƅƈƌƒƙơƣƥƨƭưƴƶƹƽdžljnjǎǐǒǔǖǘǚǜǟǡǣǥǧǩǫǭǯdzǵǻǽǿȁȃȅȇȉȋȍȏȑȓȕȗɓɔɗɘəɛɠɣɨɩɯɲɵʃʈʊʋʒḁḃḅḇḉḋḍḏḑḓḕḗḙḛḝḟḡḣḥḧḩḫḭḯḱḳḵḷḹḻḽḿṁṃṅṇṉṋṍṏṑṓṕṗṙṛṝṟṡṣṥṧṩṫṭṯṱṳṵṷṹṻṽṿẁẃẅẇẉẋẍẏẑẓẕạảấầẩẫậắằẳẵặẹẻẽếềểễệỉịọỏốồổỗộớờởỡợụủứừửữựỳỵỷỹ";
const lowercaseChars = [...lowercaseString];
const uppercaseChars = [...lowercaseString.toLocaleUpperCase()];

Upper and lowercase for c:
{'a', 'b','c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's','t', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'}