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indexOf() JavaScript String Method – How to Get Text Index

Whenever you use indexOf() on a string, the method does the following:

  1. It searches its calling string for the method's first argument.
  2. It returns the index of the first match, or -1 if the method found no match.
note
  • A calling string is a string on which you used indexOf(). So, in "Hello, world!".indexOf("world"), "Hello, world!" is the calling string.
  • indexOf() is sometimes written as String.prototype.indexOf() because it is a method of the String object's prototype property.

Syntax of the indexOf() Method​

indexOf() accepts two arguments. Here is the syntax:

callingString.indexOf(valueToFind, startIndex);

Argument 1: valueToFind​

A valueToFind is the first argument accepted by the indexOf() method. It defines the value you wish to find in the calling string.

Example 1: Find the index of day​

"SunDay, Tuesday, and Friday are good Days".indexOf("day");

// The invocation above will return: 12

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Example 2: Find the index of Day​

"SunDay, Tuesday, and Friday are good Days".indexOf("Day");

// The invocation above will return: 3

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Example 3: Find the index of 3​

"Day1, day-3, and day 6 are good Days".indexOf(3);

// The invocation above will return: 10

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Argument 2: startIndex​

The startIndex argument is optional. It specifies the index where you want the computer to start searching for the valueToFind argument.

Example 1: Find the index of day from the 13th index​

"SunDay, Tuesday, and Friday are good Days".indexOf("day", 13);

// The invocation above will return: 24

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The startIndex argument's default value is 0. Therefore, if omitted or negative, the search will begin at index 0.

Here's an example:

"SunDay, Tuesday, and Friday are good Days".indexOf("day", -13);

// The invocation above will return: 12

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Example 2: Find the index of Day from the 4th index​

"SunDay, Tuesday, and Friday are good Days".indexOf("Day", 4);

// The invocation above will return: 37

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Suppose startIndex is greater than the calling string's length. In such a case, the computer will ignore the search.

Here's an example:

"SunDay, Tuesday, and Friday are good Days".indexOf("Day", 75);

// The invocation above will return: -1

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tip
  • To find the last occurrence of a substring within a string, use lastIndexOf().
  • Suppose you need to use regular expression as the valueToFind argument. In that case, use search().
  • To do a global search for all matches, use match().
  • You can also use indexOf() on an array.

Overview​

indexOf() searches its calling string for the first occurrence of the method's string argument.