String.prototype.lastIndexOf() - JavaScript | MDN (original) (raw)

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The lastIndexOf() method of String values searches this string and returns the index of the last occurrence of the specified substring. It takes an optional starting position and returns the last occurrence of the specified substring at an index less than or equal to the specified number.

Try it

const paragraph = "I think Ruth's dog is cuter than your dog!";

const searchTerm = "dog";

console.log(
  `Index of the last <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mi>s</mi><mi>e</mi><mi>a</mi><mi>r</mi><mi>c</mi><mi>h</mi><mi>T</mi><mi>e</mi><mi>r</mi><mi>m</mi></mrow><mi>i</mi><mi>s</mi></mrow><annotation encoding="application/x-tex">{searchTerm} is </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord"><span class="mord mathnormal">se</span><span class="mord mathnormal">a</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord mathnormal" style="margin-right:0.13889em;">T</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mord mathnormal">m</span></span><span class="mord mathnormal">i</span><span class="mord mathnormal">s</span></span></span></span>{paragraph.lastIndexOf(searchTerm)}`,
);
// Expected output: "Index of the last "dog" is 38"

Syntax

lastIndexOf(searchString)
lastIndexOf(searchString, position)

Parameters

searchString

Substring to search for. All values are coerced to strings, so omitting it or passing undefined causes lastIndexOf() to search for the string "undefined", which is rarely what you want.

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

Examples

Using indexOf() and lastIndexOf()

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

const anyString = "Brave, Brave New World";

console.log(anyString.indexOf("Brave")); // 0
console.log(anyString.lastIndexOf("Brave")); // 7

Specifications

Browser compatibility

See also