Convert between Camel Case (camelCase) and Snake Case (snake_case) in ES6

case.js

export const toCamelCase = (e) => {
  return e.replace(/_([a-z])/g, (g) =>  g[1].toUpperCase());
};

export const toSnakeCase = (e) => {
  return e.match(/([A-Z])/g).reduce(
    (str, c) => str.replace(new RegExp(c), '_' + c.toLowerCase()),
    e
  )
  .substring((e.slice(0, 1).match(/([A-Z])/g)) ? 1 : 0);
};

With the second argument of replace taking a function that gives you the match itself, you can get a one liner for snake case:

export const camelToSnakeCase = e => e.replace(/([A-Z])/g, (_, c) => `_${c.toLowerCase()}`);

similar for camel case:

export const snakeToCamelCase = e => e.replace(/_([a-z])/g, (_, c) => `${c.toUpperCase()}`);