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# 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

```js
lastIndexOf(searchString)
lastIndexOf(searchString, position)
```

### Parameters

- `searchString`

  - : Substring to search for, [coerced to a string](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String#string_coercion).

    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.

```js
"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`:

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