Simple expression evaluator in javascript

exp_eval.js

/*
isValid('{{()}}') = true
isValid('{{(]]}') = false
isValid('(({{()}}))') = true
isValid('(({{(}}))') = false
isValid('(({{()()}}))') = true
*/

function isValid(expression){
  let stack = [];
  const charMap = new Map().set('{', '}').set('[', ']').set('(', ')');
  for(i=0; i<expression.length; i++){
    let top;
    let currentChar = expression[i];
    if(stack.length > 0){
      top = stack[stack.length - 1];
      //console.log("top: ", top, ", currentChar: ", currentChar);
      let result =  charMap.get(top) === currentChar;
      //console.log("isExpressionResolved: ", result);
      if(result){
        stack.pop();
      }else{
        stack.push(expression[i]);
      }
    }else{
      stack.push(expression[i]);
    }
  }
  return stack.length === 0;
}

console.log("isValid('{{()}}'): ", isValid('{{()}}'));
console.log("isValid('{{(]]}'): ", isValid('{{(]]}'));
console.log("isValid('(({{()}}))'): ", isValid('(({{()}}))'));
console.log("isValid('(({{(}}))'): ", isValid('(({{(}}))'));
console.log("isValid('(({{()()}}))'): ", isValid('(({{()()}}))'));
console.log("isValid('}{{()()}}'): ", isValid('}{{()()}}'));