JavaScript: String.prototype.lastIndexOf

str.lastIndexOf(search_str)

Return the start index of first occurrence of search_str, searching from right to left. Returns -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