lastIndexOf() Array Method in JavaScript Explained

Whenever you use lastIndexOf() on an array, the method does the following:

1. It searches its calling array 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 array is an array on which you used lastIndexOf(). So, in bestColorsList.lastIndexOf("white"), bestColorsList is the calling array.
• lastIndexOf() is sometimes written as Array.prototype.lastIndexOf() because it is a method of the Array object’s prototype property.

Use lastIndexOf() to search an array for the index of the last occurrence of a value.

Syntax of the lastIndexOf() Method

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

callingArray.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 array.

Example 1: Find the index of the last Tuesday

let daysOfTheWeek = [
  "Sunday",
  "Tuesday",
  "Friday",
  "Sunday",
  "Tuesday",
  "Friday",
];

daysOfTheWeek.lastIndexOf("Tuesday");

// The invocation above will return: 4

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Example 2: Find the index of the last Friday

let daysOfTheWeek = [
  "Sunday",
  "Tuesday",
  "Friday",
  "Sunday",
  "Tuesday",
  "Friday",
];

daysOfTheWeek.lastIndexOf("Friday");

// The invocation above will return: 5

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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: 8

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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 callingArray.length - 1.

Therefore, if you omit the startIndex argument, the search will begin from the last item in the calling array.

Example 1: Find the index of the last Tuesday from the 6th index position

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

daysOfTheWeek.lastIndexOf("Tuesday", 6);

// The invocation above will return: 2

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The snippet above returned 2 because the search started backward from the sixth index position.

However, suppose you omit the startIndex argument. In that case, the computer will search backward from the last item in the calling array (callingArray.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: 10

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Example 2: Find the index of the last Tuesday from the negative 6th index position

Note that a negative startIndex argument means that the computer should start its index count from the last item in the calling array. But JavaScript will still do a backward (right to left) search.

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

daysOfTheWeek.lastIndexOf("Tuesday", -6);

// The invocation above will return: 7

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Suppose startIndex is greater than the calling array’s length. In such a case, the computer will default to callingArray.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", 35);

// The invocation above will return: 10

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Tip

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