Simple library

constants.js

export const LOWERCASE_LETTERS = /[a-z]/g;
export const UPPERCASE_LETTERS = /[A-Z]/g;
export const UPPERCASE_OFFSET = 'A'.charCodeAt(0) - 'a'.charCodeAt(0);

lower.js

import {UPPERCASE_LETTERS} from './constants';
import lowerChar from './lowerChar';

export default function (string) {
  return string.replace(UPPERCASE_LETTERS, lowerChar);
}

lowerChar.js

import {UPPERCASE_OFFSET} from './constants';
import shiftChar from './shiftChar';

export default function (char) {
  return shiftChar(char, -UPPERCASE_OFFSET);
}

main.js

export {default as upper} from './upper.js';
export {default as lower} from './lower.js';
export {default as upperFirst} from './upperFirst.js';

shiftChar.js

export default function (char, offset) {
  return String.fromCharCode(char.charCodeAt(0) + offset);
}

upper.js

import {LOWERCASE_LETTERS} from './constants';
import upperChar from './upperChar';

export default function (string) {
  return string.replace(LOWERCASE_LETTERS, upperChar);
}

upperChar.js

import {UPPERCASE_OFFSET} from './constants';
import shiftChar from './shiftChar';

export default function (char) {
  return shiftChar(char, UPPERCASE_OFFSET);
}

upperFirst.js

import upper from './upper';

export default function (string) {
  return upper(string.substring(0, 1)) + string.substring(1);
}