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.

searchString

Substring to search for.

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.

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

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

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

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

The following example uses indexOf() and lastIndexOf() to locate values in the string "Brave, Brave New World".

let anyString = 'Brave, Brave New World';

console.log('The index of the first "Brave" is ' + anyString.indexOf('Brave'));
// logs 0
console.log('The index of the last "Brave" is ' + anyString.lastIndexOf('Brave'));
// logs 7

• String.prototype.charAt()
• String.prototype.indexOf()
• String.prototype.split()
• Array.prototype.indexOf()
• Array.prototype.lastIndexOf()