string-transforms.js

const isString = value => typeof value === 'string' || value instanceof String;

/*
camelCase('camel case');  // camelCase
camelCase('camel_case');  // camelCase
camelCase('camel-case');  // camelCase
*/
const camelCase = str => {
  if(!isString(str)) {
    return '';
  }

  const s = str
    .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
    .map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
    .join('');

  return s.slice(0, 1).toLowerCase() + s.slice(1);
};

/*
kebabCase('kebab case');  // kebab-case
kebabCase('kebabCase');  // kebab-case
kebabCase('kebab_case');  // kebab-case
*/
const kebabCase = str =>
  isString(str) ?
    str
      .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
      .map(x => x.toLowerCase())
      .join('-') :
    '';

/*
snakeCase('snake case');  // snake_case
snakeCase('snakeCase');  // snake_case
snakeCase('snake-case');  // snake_case
*/
const snakeCase = str =>
  isString(str) ?
    str
      .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
      .map(x => x.toLowerCase())
      .join('_') :
    '';

/*
titleCase('the old man and the sea');  // The Old Man and the Sea
titleCase('theOldManAndTheSea');  // The Old Man and the Sea
titleCase('the-old-man-and-the-sea');  // The Old Man and the Sea
titleCase('the_old_man_and_the_sea');  // The Old Man and the Sea
*/
const titleCase = str => {
  const lower = [
    'and', 'or', 'nor', 'but', 'a', 'an', 'the', 'as', 'at', 'by', 'for', 'in', 'of', 'on', 'per', 'to', 'is',
  ];

  return isString(str) ?
    str
      .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
      .map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
      .map((x, i) =>
        lower.includes(x.toLowerCase()) && i !== 0 ? x.toLowerCase() : x)
      .join(' ') :
    '';
};