The String object's indexOf() method

The String object's indexOf() method returns the index of the first occurrence of a specified substring within a string. If the substring is not found, it returns -1.

Syntax

string.indexOf(searchString[, position])

Parameters

searchString:

• The string to search for within the original string

• Case-sensitive search

position (optional):

• The position in the string to begin searching from

• Default is 0 (start of string)

• If position is negative, the search starts at the beginning

• If position >= string length, returns -1

Return value

Returns a number representing:

• The index of the first occurrence of searchString

• -1 if the searchString is not found

Examples

Example 1: Basic substring searching

In the code below, the function indexOfEx1() demonstrates basic usage of indexOf().

function indexOfEx1() {
  let str = "Hello World";
  
  // Find position of "World"
  Logger.log(str.indexOf("World")); // 6
  
  // Case-sensitive search
  Logger.log(str.indexOf("world")); // -1
  
  // Find a single character
  Logger.log(str.indexOf("o")); // 4
  
  // Substring not found
  Logger.log(str.indexOf("xyz")); // -1
}

Output

6

-1

4

-1

Example 2: Using the position parameter

In the code below, the function indexOfEx2() shows how to use the optional starting position.

function indexOfEx2() {
  let str = "Hello World, Hello Apps Script";
  
  // Find first "Hello"
  Logger.log(str.indexOf("Hello")); // 0
  
  // Find second "Hello" by starting after first one
  Logger.log(str.indexOf("Hello", 1)); // 13
  
  // Start searching from middle of string
  Logger.log(str.indexOf("Hello", 15)); // -1
  
  // Negative position starts from beginning
  Logger.log(str.indexOf("Hello", -5)); // 0
}

Output

0

13

-1

0

Example 3: Common practical applications

In the code below, the function indexOfEx3() demonstrates typical use cases for indexOf().

function indexOfEx3() {
  // Check if string contains substring
  let email = "user@example.com";
  let hasAtSign = email.indexOf("@") !== -1;
  Logger.log("Is valid email format: " + hasAtSign);
  
  // Extract domain from email
  let domainStart = email.indexOf("@") + 1;
  let domain = email.slice(domainStart);
  Logger.log("Domain: " + domain);
  
  // Check file extension
  let filename = "document.pdf";
  let isDotPresent = filename.indexOf(".") !== -1;
  Logger.log("Has file extension: " + isDotPresent);
}

Output

Is valid email format: true

Domain: example.com

Has file extension: true

Example 4: Finding all occurrences

In the code below, the function indexOfEx4() shows how to find all occurrences of a substring.

function indexOfEx4() {
  let str = "The quick brown fox jumps over the lazy fox";
  let searchTerm = "fox";
  let positions = [];
  let pos = 0;
  
  // Find all occurrences of "fox"
  while (true) {
    pos = str.indexOf(searchTerm, pos);
    if (pos === -1) break;
    positions.push(pos);
    pos += 1; // Move to next position
  }
  
  Logger.log("Found '" + searchTerm + "' at positions: " + positions);
}

Output

Found 'fox' at positions: 16, 40

if (str.indexOf("search") !== -1) {
  // String contains "search"
}

let email = "user@example.com";
let atPosition = email.indexOf("@");
let domain = email.slice(atPosition + 1); // extracts "example.com"

Common Use Cases

The indexOf() method is frequently used in text processing tasks. Here are some common applications:

• Validation: Check if a string contains specific characters or substrings.

• Parsing: Find the position of delimiters or markers in structured text.

• Text extraction: Locate starting points for substring extraction.

• File processing: Check file names or extensions.

• Email validation: Basic email format checking.

• URL processing: Find components within URLs.

Conclusion

In this tutorial, you learned how to use the indexOf() method to find substrings within strings in Apps Script. This method is essential for string searching, validation, and text processing tasks. While indexOf() is simple to use, it's powerful when combined with other string methods or used in loops to process text systematically. Remember that indexOf() is case-sensitive and only finds the first occurrence of a substring.