JS: String.prototype.lastIndexOf

str.lastIndexOf(search_str)

Return the start index of first occurrence of search_str, searching from right to left. Return -1 if not found.

str.lastIndexOf(str, start_search_pos)

Start search at start_search_pos.

const ss = "abcabc";

console.log(ss.lastIndexOf("b") === 4);

// start at index 1 (including index 1)
console.log(ss.lastIndexOf("b", 1) === 1);

// more than 1 char
console.log(ss.lastIndexOf("bc", 2) === 1);

// even if the substring pass start index, it's still considered found
console.log(ss.lastIndexOf("bca", 1) === 1);

// example of not found, returns -1
console.log(ss.lastIndexOf("xy") === -1);