Lessap

const index = str.match(/^(\D*\d)?/)[0].length - 1;

// или

const index = [...str].findIndex(n => !Number.isNaN(Number(n)));

// или

let index = -1;
for (let i = 0; i < str.length; i++) {
  if ('0123456789'.includes(str.charAt(i))) {
    index = i;
    break;
  }
}

// или

const index = (function get(i, n = str[i]) {
  return (
    !n        ? -1 :
    +n === +n ?  i :
                get(-~i)
  );
})(0);