JavaScript String Search Indexof and Lastindexof Guide

JavaScript provides three methods for finding where a substring appears within a string: indexOf, lastIndexOf, and search. They return the character position of a match, or -1 when nothing is found.

javascript

const text = "JavaScript is fun. JavaScript is powerful.";
 
console.log(text.indexOf("JavaScript"));
console.log(text.lastIndexOf("JavaScript"));
console.log(text.search(/power/));

The first prints 0 because "JavaScript" starts at position zero. The second prints 20, the position of the last occurrence. The third prints 28 because the regex /power/ matches "power" starting at position 28.

These methods complement the boolean-checking methods includes, startsWith, and endsWith. See the guide to includes, startsWith, and endsWith for when you only need a yes or no answer.

indexOf() -- find the first match

The indexOf method scans a string from left to right and returns the position of the first match. It accepts an optional second argument: the position to start searching from.

javascript

const sentence = "The quick brown fox jumps over the lazy dog";
 
console.log(sentence.indexOf("the"));
console.log(sentence.indexOf("the", 10));

The first prints 31 because the lowercase "the" first appears before "lazy". The uppercase "The" at position 0 does not match because indexOf is case-sensitive.

The second call starts searching from position 10, skipping past "quick brown fox jumps over". It still finds "the" at position 31.

The most common beginner mistake is treating the return value as a boolean:

javascript

const text = "Hello";
 
if (text.indexOf("H")) {
  console.log("Found");   // Does NOT print!
}

This does not print anything because indexOf returns 0 for "H" at the first position, and 0 is falsy in JavaScript. Always compare explicitly:

javascript

if (text.indexOf("H") !== -1) {
  console.log("Found");   // Prints correctly
}

lastIndexOf() -- find the last match

The lastIndexOf method works the same way as indexOf but scans from right to left, returning the position of the last match.

javascript

const path = "/users/profile/settings/profile";
 
console.log(path.lastIndexOf("profile"));
console.log(path.indexOf("profile"));

The first prints 26 because "profile" appears last at position 26. The second prints 7 because the first "profile" starts at position 7.

lastIndexOf also accepts a starting position, but it searches backward from that position:

javascript

const word = "banana";
 
console.log(word.lastIndexOf("a"));
console.log(word.lastIndexOf("a", 2));

The first prints 5, the position of the last "a". The second prints 1 because it only considers characters up to position 2, and within "ban", the last "a" is at position 1.

search() -- regex-based search

The search method behaves like indexOf but accepts a regular expression instead of a plain string. It returns the position of the first regex match.

javascript

const text = "Order #12345 confirmed";
 
console.log(text.search(/\d/));
console.log(text.search(/\d+/));

Both print 7 because the first digit appears at position 7. The regex /\d/ matches a single digit, and /\d+/ matches one or more digits, but search only cares about the first match position.

Unlike indexOf, search converts a plain string argument into a regex automatically:

javascript

console.log("hello".search("ll"));
console.log("hello".indexOf("ll"));

Both print 2. But search treats "ll" as the regex /ll/, which means special regex characters like the period and asterisk work differently than you might expect. Stick to plain strings with indexOf unless you intentionally want regex behavior.

For a broader overview of all string search and extraction methods, see the JavaScript string methods guide.

When to use which

The most common real-world pattern is using indexOf to check if a string contains something, then using the returned position to extract or manipulate the surrounding text.

Common mistakes

Treating indexOf as a boolean is the most frequent error. The value 0 is a valid position but falsy in JavaScript, so if (str.indexOf("H")) silently fails when the match is at the start. Always write if (str.indexOf("H") !== -1).

Expecting search to handle a plain string the same as indexOf can cause subtle bugs. The call "file.txt".search(".") returns 0, not 4, because . in a regex matches any character. Use indexOf for plain strings.

Forgetting that all three methods are case-sensitive leads to wrong results. The expression "JavaScript".indexOf("java") returns -1. Normalize both sides with toLowerCase when case should not matter.