string eval

const solve = (expr) => {
   let leftPart;
   let rightPart;
  let operator = findFirstSign(expr)
  if(operator){
      let exprAsArray = expr.split(operator)
      let startIndex = expr.indexOf(operator)
      leftPart =Number(exprAsArray[0]
                  .split('')
                  .filter( el => el !== '(' && el !== ')')
                  .join(''));
      rightPart = expr.slice(startIndex+1)
      if(!findFirstSign(rightPart)){
         rightPart = Number (rightPart.split('')
                              .filter(el => el !== '(' && el !== ')')
                              .join(''));
         switch(operator) {
          case '+':
           return leftPart + rightPart
         case '-':
           return leftPart - rightPart;
         case '*':
           return leftPart*rightPart;
         case '/':
           return leftPart/rightPart;
            }

      } else {

         switch(operator) {
          case '+':
           return leftPart + solve(rightPart)
         case '-':
           return leftPart - solve(rightPart);
         case '*':
           return leftPart*solve(rightPart);
         case '/':
           return leftPart/solve(rightPart);
            }

            }
      } else {
         return Number (expr.split('')
                           .filter(el => el !== '(' && el !== ')')
                           .join(''))
      }

}

const findFirstSign = (expr) => {
   const asArray = expr.split('');
   for(const el of asArray){
      if(el === '+' || el === '-' || el === '/' || el === '*') {
         return el
      }
   }
}

console.log(solve('45+(24*(12+3))'));