camel-to-snake.ts

const CAMEL_REGEX = /[\w]([A-Z])/g;
const cb = m => m[0] + '_' + m[1];
const camelToSnake1 = (str: string) => str.replace(CAMEL_REGEX, cb).toLowerCase();

const camelToSnake2 = (str: string) => {
  const { length } = str;
  let result = '', code = 0, i = 0;
  for (i = 0; i < length; i++) {
    code = str[i].charCodeAt();
    if (65 <= code && code <= 90) {
      result += '_';
      result += String.fromCharCode(code + 32);
    } else {
      result += str[i];   
    }
  }
  return result.slice(1);
};

const camelToSnake3 = (...str: string[]) => ((c => 65 <= c && c <= 90 ? '' : '_')(str[0].charCodeAt0()) + str.reduce((acc, char) => {
  const code = char.charCodeAt(0);
  return acc + (65 <= code && code <= 90 ? '_' + String.fromCharCode(code + 32) : char);
}, '')).slice(1)
    
// fastest
const camelToSnake4 = (function (str: string) { return str.replace(this.CAMEL_REGEX, this.cb).toLowerCase(); }).bind({
  CamelRegex: /[\w]([A-Z])/g,
  cb: ([a, b]) => a + '_' + b,
});