我需要找出字符串js中两个字符之间的距离。我有一个解决方案，但是我不能理解与if语句相关的代码片段

function findDistance(str, char1, char2) {
  let index1 = str.indexOf(char1);
  let index2 = str.indexOf(char2);
  
  if (index1 === -1 || index2 === -1) {
    return "字符不存在";
  }
  
  let distance = Math.abs(index1 - index2) - 1;
  return distance;
}

let str = "js";
let char1 = "j";
let char2 = "s";

let distance = findDistance(str, char1, char2);
console.log("字符之间的距离为：" + distance);