camel-to-snake-test.js

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

const CAMEL_REGEX = /[\w]([A-Z])/g;
const cb = m => m[0] + '_' + m[1];
const camelToSnake2 = (str) => str.replace(CAMEL_REGEX, cb).toLowerCase();

const camelToSnake3 = (() => {
  const CAMEL_REGEX = /[\w]([A-Z])/g;
  const cb = m => m[0] + '_' + m[1];
  return (str) => str.replace(CAMEL_REGEX, cb).toLowerCase();
})();

const camelToSnake4 = (function (str) { return str.replace(this.CamelRegex, this.cb).toLowerCase(); }).bind({
  CamelRegex: /[\w]([A-Z])/g,
  cb: ([a, b]) => a + '_' + b,
});

const camelToSnake5 = Object.assign((str) => { return str.replace(camelToSnake5.CamelRegex, camelToSnake5.cb).toLowerCase(); }, {
  CamelRegex: /[\w]([A-Z])/g,
  cb: ([a, b]) => a + '_' + b,
});

const camelToSnake6 = (str) => {
  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 camelToSnake7 = (str) => ((c => 65 <= c && c <= 90 ? '' : '_')(str[0].charCodeAt(0)) + [...str].reduce((acc, char) => {
  const code = char.charCodeAt(0);
  return acc + (65 <= code && code <= 90 ? '_' + String.fromCharCode(code + 32) : char);
}, '')).slice(1);

const value = 'AbcdeAbcdeAbcdeAbcdeAbcdeAbcdeAbcdeAbcdeAbcdeAbcdeAbcdeAbcde';
console.time('t1');
camelToSnake1(value);
console.timeEnd('t1');
console.time('t2');
camelToSnake2(value); // fastest
console.timeEnd('t2');
console.time('t3');
camelToSnake3(value); // fastest
console.timeEnd('t3');
console.time('t4');
camelToSnake4(value);
console.timeEnd('t4');
console.time('t5');
camelToSnake5(value);
console.timeEnd('t5');
console.time('t6');
camelToSnake6(value);
console.timeEnd('t6');
console.time('t7');
camelToSnake7(value);
console.timeEnd('t7');

In Chrome, scope access is faster than field access