A simple Javascript snippet to convert any camelCase or PascalCase string to underscore (or custom joiners)

camelcase_pascal_to_underscore.js

var testStrSimple = "MyNameIsAhsanAyaz";
var testStrWhiteSpace = "myName    \nIsAhsan";

console.log(ccpc2und(testStrSimple));
console.log(ccpc2und(testStrWhiteSpace));
console.log(ccpc2und(testStrWhiteSpace, "__:D__"));      // custum joiner


// using LONG METHOD (for no reason)
console.log(_ccpc2und(testStrSimple));
console.log(_ccpc2und(testStrWhiteSpace));
console.log(_ccpc2und(testStrWhiteSpace, "__:LONG:__"));      // custum joiner

/**
*   @author Ahsan Ayaz
*   Removes whitespace, converts camelCase or PascalCase to underscore (or custom joiner)
*   @param str {String} . Source string to process upon. Default ''
*   @param charToJoin {String} . Character to join converting to lower case and joining. Default '_'
*   @return {String}. Processed string
**/
function ccpc2und(str = '', charToJoin = '_'){
  return str.replace(/\s/g,'').replace(/(?:^)?([A-Z])/g, (match) => {
    return `${charToJoin}${match.toLowerCase()}`
  }).replace(new RegExp(`^${charToJoin}`), "");    
}

/**
*   @author Ahsan Ayaz
*   Removes whitespace, converts camelCase or PascalCase to underscore (or custom joiner)
*   @param str {String} . Source string to process upon. Default ''
*   @param charToJoin {String} . Character to join converting to lower case and joining. Default '_'
*   @return {String}. Processed string
**/
function _ccpc2und(str = '', charToJoin = '_'){     // LONG METHOD (for no reason)
  var modArr = str.replace(/\s/g,'')
                .replace(/^([a-zA-z])?/, char => char.toLowerCase())  // turn first char lowercase
                .split(/([a-z][A-Z])/g);    //replace whitespace and splits
  return modArr.map((el)=>{
      return (modArr.indexOf(el) %2 !== 1) ? el : (()=>{
       let elChars = el.split('');
       return `${elChars[0]}${charToJoin}${elChars[1].toLowerCase()}`;
      })();
  }).join("");
}