Which of the following method call gives the position of the first occurrence of x in the string?

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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".

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

Examples

Using indexOf() and lastIndexOf()

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

const 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

See also