In JavaScript, the **`indexOf()` method** takes a string and a substring and returns the starting index of the first occurrence of the given substring in that string.

# Syntax

``` js
str.indexOf(substr, startIndex);
```

**str**: String in which the substring `substr` is searched.

**substr**: Substring whose position is searched in given string `str`.

**startIndex**: Starting index for the search. If no `startIndex` is provided, the search starts at the 0th index.

**Return Value**: Returns the start index of the first occurrence of the `substr` in `str` *or* returns -1 (if no occurrence found).

## Examples

As shown in **Line 1 and 5**, the `indexOf()` method is *case-sensitive*. The same fact is justified by **Line 6**, where on searching "B", -1 is returned since only the lowercase forms of the alphabet exist in the given string.

console.log('To be or not to be'.indexOf('To'));
console.log('To be or not to be'.indexOf(' '));
console.log('To be or not to be'.indexOf('o', 2));
console.log('To be or not to be'.indexOf('be', 4));
console.log('To be or not to be'.indexOf('to'));
console.log('To be or not to be'.indexOf('B'));
console.log('To be or not to be'.indexOf('', 9));

indexOf() method in JavaScript

The indexOf() method in JavaScript finds the first occurrence index of a substring in a string. It is case-sensitive.