Method that returns true when a string is balanced and false otherwise.

String.isBalanced.js

/*
  Task:
  Given an input string return true if the string is balanced, otherwise return false. 
  A string is balanced when every opening bracket "{", square bracket "[", and parenthesis "("
  has a matching closing character.
*/
const string = "{[()]}";

(function(input){

  const openers = ['{', '[' ,'('];
  const closers = ['}', ']' ,')'];

  // If `unmatchedChars` length is non-zero, input is not balanced.
  const unmatchedChars = input.split('').reduce((acc, char, i) => {
    if (openers.includes(char)) {
      acc.push(char)
    }

    if (closers.includes(char)) {
      const indexOfCloser = closers.indexOf(char);
      const opener = openers[indexOfCloser];

      if (acc[acc.length - 1] === opener) {
        acc.pop()
      }
    }

    return acc;
  }, [])

  // console.log(unmatchedChars.length === 0)

  const accumulator = [];
  
  // If `isNotBalanced` is `false`, the `input` IS balanced. Observe the double negative! 
  // Array.some() method ensures early exit if a mismatch if found, so there's no need to parse the rest of the input.
  const isNotBalanced = input.split('').some((char, i) => {
    if (openers.includes(char)) {
      accumulator.push(char);
    }

    if (closers.includes(char)) {
      const closerIndex = closers.indexOf(char);
      const opener = openers[closerIndex];

      if (accumulator[accumulator.length-1] === opener) {
        accumulator.pop();
      } else {
        return true;
      }
    }

    return false;
  })

  // console.log(!isNotBalanced)


})(string)