lastIndexOf() String Method in JavaScript Explained

Whenever you use lastIndexOf() 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 position of the last match, or -1 if the method found no match.

Note

• A calling string is a string on which you used lastIndexOf(). So, in "Hello, world!".lastIndexOf("world"), "Hello, world!" is the calling string.
• lastIndexOf() is sometimes written as String.prototype.lastIndexOf() because it is a method of the String object’s prototype property.

Syntax of the lastIndexOf() Method

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

callingString.lastIndexOf(valueToFind, startIndex);

Argument 1: valueToFind

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

Example 1: Find the index of the last Tuesday

"Sunday, Tuesday, Friday, Sunday, Tuesday, Friday".lastIndexOf("Tuesday");

// The invocation above will return: 33

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Example 2: Find the index of the last Friday

"Sunday, Tuesday, Friday, Sunday, Tuesday, Friday".lastIndexOf("Friday");

// The invocation above will return: 42

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Example 3: Find the index of the last 7

"1, 3, 5, 7, 9, 1, 3, 5, 7, 9".lastIndexOf(7);

// The invocation above will return: 24

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Argument 2: startIndex

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

Keep in mind that lastIndexOf(), by default, searches backward from a startIndex of +Infinity.

Therefore, if you omit the startIndex argument, the computer will search the entire string backward (right to left).

Example 1: Find the index of the last Tuesday from the 54th index position

let daysOfTheWeek =
  "Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Tuesday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday";

daysOfTheWeek.lastIndexOf("Tuesday", 54);

// The invocation above will return: 16

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The snippet above returned 16 because the search started backward from the fifty-fourth index position.

However, suppose you omit the startIndex argument, the computer will search backward from the last item in the calling string (callingString.length - 1).

Here’s an example:

let daysOfTheWeek =
  "Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Tuesday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday";

daysOfTheWeek.lastIndexOf("Tuesday");

// The invocation above will return: 89

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AD

Example 2: Find the index of the last Tuesday from the negative 54th index position

Note that a negative startIndex argument gives the same result as a zero startIndex. In other words, the computer will search for the valueToFind argument at index 0 only.

let daysOfTheWeek =
  "Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Tuesday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday";

daysOfTheWeek.lastIndexOf("Tuesday", -54);

// The invocation above will return: -1

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The snippet above returned -1 because the computer could not find "Tuesday" at the zeroth index.

Here’s another example:

let daysOfTheWeek =
  "Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Tuesday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday";

daysOfTheWeek.lastIndexOf("Tuesday", 0);

// The invocation above will return: -1

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Suppose startIndex is greater than the calling string’s length. In such a case, the computer will default to callingString.length - 1.

Here’s an example:

let daysOfTheWeek =
  "Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Tuesday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday";

daysOfTheWeek.lastIndexOf("Tuesday", 3000);

// The invocation above will return: 89

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Tip

• To find the first occurrence of a substring within a string, use indexOf().
• You can also use lastIndexOf() on an array.