String.prototype.lastIndexOf()

The lastIndexOf() method, given one argument: a substring to search for, searches the entire calling string, and returns the index of the last occurrence of the specified substring. Given a second argument: a number, the method returns the last occurrence of the specified substring at an index less than or equal to the specified number.

Syntax

lastIndexOf(searchString)
lastIndexOf(searchString, position)

Parameters

• searchString

  • : Substring to search for, coerced to a string.

    If the method is called with no arguments, searchString is coerced to "undefined". Therefore,"undefined".lastIndexOf() returns 0 — because the substring "undefined" is found at position 0 in the string "undefined". But "undefine".lastIndexOf(), returns -1 — because the substring "undefined" is not found in the string "undefine".

• position optional

  • : The method returns the index of the last occurrence of the specified substring at a position less than or equal to position, which defaults to +Infinity. If position is greater than the length of the calling string, the method searches the entire string. If position is less than 0, the behavior is the same as for 0 — that is, the method looks for the specified substring only at index 0.

    • 'hello world hello'.lastIndexOf('world', 4) returns -1 — because, while the substring world does occurs at index 6, that position is not less than or equal to 4.

    • 'hello world hello'.lastIndexOf('hello', 99) returns 12 — because the last occurrence of hello at a position less than or equal to 99 is at position 12.

    • 'hello world hello'.lastIndexOf('hello', 0) and 'hello world hello'.lastIndexOf('hello', -5) both return 0 — because both cause the method to only look for hello at index 0.

Return value

The index of the last occurrence of searchString found, or -1 if not found.

Description

Strings are zero-indexed: The index of a string's first character is 0, and the index of a string's last character is the length of the string minus 1.

"canal".lastIndexOf("a"); // returns 3
"canal".lastIndexOf("a", 2); // returns 1
"canal".lastIndexOf("a", 0); // returns -1
"canal".lastIndexOf("x"); // returns -1
"canal".lastIndexOf("c", -5); // returns 0
"canal".lastIndexOf("c", 0); // returns 0
"canal".lastIndexOf(""); // returns 5
"canal".lastIndexOf("", 2); // returns 2

Case-sensitivity

The lastIndexOf() method is case sensitive. For example, the following expression returns -1:

"Blue Whale, Killer Whale".lastIndexOf("blue"); // returns -1