i need to find the distance between two characters in string js. i have a solution but i can't understand the snippet of code related to if statement

The task was to write a function subLength() that takes 2 parameters, a string and a single character. The function should search the string for the two occurrences of the character and return the length between them including the 2 characters. If there are less than 2 or more than 2 occurrences of the character the function should return 0.

const subLength = (str, char) => {
let charCount = 0;
let len = -1;

for (let i=0; i<str.length; i++) {
  if (str[i] == char) {
    charCount++;
    if (charCount > 2) {
      return 0;
    }
// could somebody explain why -1 is equal to len and then len is reassigned to i???       
if (len == -1) {
      len = i;
    } else {
      len = i - len + 1
    }
  }
}
if (charCount < 2) {
  return 0;
}

return len;
};