JS Built-ins
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StringString.prototype.lastIndexOfintermediate

String.prototype.lastIndexOf() — fromIndex limits search

📖 Usage Example
const str = 'abcabc'
str.lastIndexOf('a')    // → 3
str.lastIndexOf('z')    // → -1
str.lastIndexOf('b', 3) // → 1

Use lastIndexOf() to find the index of the last occurrence of a substring, searching from right to left.

String.prototype.lastIndexOf() — fromIndex

The optional second argument fromIndex tells the search to start from that position and go backward.

Challenge: Implement lastBefore(str, sub, pos) using .lastIndexOf(sub, pos).

lastBefore('abcabc', 'a', 4) // → 3
lastBefore('abcabc', 'a', 2) // → 0
💡 Hints (1)
  • • `str.lastIndexOf(sub, fromIndex)` searches backward starting from `fromIndex`.