change-case.js

function snakeToCamel (word, upperCamelCase) {
  const sections = word.split('_')
  return sections.map((section, i) => {
    if (upperCamelCase || i > 0) {
      return section[0].toUpperCase() + section.slice(1).toLowerCase()
    }
    return section.toLowerCase()
  }).join('')
}

function camelToSnake (word, constantCase) {
  const sections = word.split(/(?=[A-Z])/)
  const method = constantCase ? 'toUpperCase' : 'toLowerCase'
  return sections.map(section => section[method]()).join('_')
}

console.log(snakeToCamel('foo_bar_baz', false))
console.log(snakeToCamel('foo_bar_baz', true))
console.log(snakeToCamel('FOO_BAR_BAZ', false))
console.log(snakeToCamel('FOO_BAR_BAZ', true))

console.log(camelToSnake('fooBarBaz', false))
console.log(camelToSnake('fooBarBaz', true))
console.log(camelToSnake('FooBarBaz', false))
console.log(camelToSnake('FooBarBaz', true))