Uncamelising a string in JavaScript

We are required to write a JavaScript function that takes in a string as the first argument and a separator character as the second argument.

The first string is guaranteed to be a camelCased string. The function should convert the case of the string by separating the words by the separator provided as the second argument.

For example −

If the input string is −

const str = 'thisIsAString';
const separator = '_';

Then the output string should be −

const output = 'this_is_a_string';

Example

Following is the code −

const str = 'thisIsAString';
const separator = '_';
const separateCase = (str = '', separator = ' ') => {
   const arr = [];
   let left = 0, right = 0;
   for(let i = 0; i < str.length; i++){
      const el = str[i];
      const next = str[i + 1];
      if((el.toUpperCase() === el && el.toUpperCase() !== el.toLowerCase()) || !next){
         right = i + Number(!next);
      };
      if(left !== right){
         const sub = str.substring(left, right).toLowerCase();
         arr.push(sub);
         left = right;
      };
   };
   return arr.join(separator);
};
console.log(separateCase(str, separator));

Output

Following is the output on console −

this_is_a_string