JavaScript String indexOf() and lastIndexOf()

indexOf() returns the zero-based position of the first occurrence of text inside a string. If the text is not found, it returns -1.

const text = "www.plus2net.com";
console.log(text.indexOf("plus")); // 4

indexOf() Syntax and Return Value Top ↑

string.indexOf(searchString)
string.indexOf(searchString, position)

The optional position controls where the forward search begins.

When indexOf() Does Not Find the Text Top ↑

const text = "www.plus2net.com";
console.log(text.indexOf("python")); // -1

Start Searching from a Specific Position Top ↑

const text = "one two one";
console.log(text.indexOf("one"));    // 0
console.log(text.indexOf("one", 1)); // 8

Find the Last Occurrence with lastIndexOf() Top ↑

The original page also teaches lastIndexOf(), which returns the last matching position.

const text = "Welcome to www.plus2net.com";
console.log(text.lastIndexOf("l")); // last l position

indexOf() vs includes() Top ↑

Use indexOf() when you need the numeric position. Use includes() when you only need a Boolean answer.

const text = "JavaScript tutorial";
console.log(text.indexOf("Script")); // 4
console.log(text.includes("Script")); // true

String Searches Are Case-Sensitive Top ↑

const text = "JavaScript";
console.log(text.indexOf("java")); // -1
console.log(text.toLowerCase().indexOf("java")); // 0

Searching for an Empty String Top ↑

An empty search string is considered to occur at a valid position within the string, so it does not behave like “not found.”

const text = "abc";
console.log(text.indexOf("")); // 0

Practical Example: Find the @ in an Email-Like Value Top ↑

const email = "userid@example.com";
const at = email.indexOf("@");
console.log(at);

See the email user/domain string example for combining index lookup with extraction.

Common indexOf() Mistakes Top ↑

• Using if (text.indexOf(value)); index 0 is falsy and -1 is truthy.
• Forgetting that matching is case-sensitive.
• Expecting a Boolean instead of a numeric index.
• Using indexOf() when a regular-expression search is required; see search().

← String Reference Email String Example →

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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