TypeError: string is not a function when Array.findIdex is used

Not sure if I’m doing an error in the code, but when I try to find the index of the letter z that is in the array I’ve split I will get this error:

TypeError: string "z" is not a function
    at Array.findIndex (<anonymous>)

Here is the code:

const chars: string = "abcdefghilmnopqrstuwxyz0123456789@-_+*(),.#";
let outputMatrix: Array<string> = [];
let outputWord: string = "";

// Split input matrix function 
const splitMatrix = async (inputMatrix: string) => {
  let splitted = inputMatrix.split(''); 
  let uppercaseChars = splitted.findIndex('z');
  console.log(uppercaseChars)
  console.log(splitted)
  return splitted;
}

// Generating random word
const createRandomString = (splittedChars: Array<string>, length?: number) => {
  const defaultLength: number = 12;
  const min: number  = 0;
  const max: number = splittedChars.length;
  let num: number;

  if (length) {
    for(let i: number = 0; i < length; i++) {
      num = Math.floor(Math.random() * (max - min) + min);
      outputWord += splittedChars[num];
    }
    return outputWord;
  } else {
    for(let i: number = 0; i < defaultLength; i++) {
      num = Math.floor(Math.random() * (max - min) + min);
      outputWord += splittedChars[num];
    }
  }
  console.log('Generated password', outputWord);
}

const main = () => {
  splitMatrix(chars)
    .then( (outputMatrix) => {
      createRandomString(outputMatrix);
    });
}

main();

What I want to do is to get the index of the z letter and after this extract all the elements that are before the letter including the letter to uppercase them.