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

Whenever you use indexOf() 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 first match, or -1 if the method found no match.
note

A calling array is an array on which you used indexOf(). So, in bestColorsList.indexOf("white"), bestColorsList is the calling array.

Syntax of the indexOf() Method

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

callingArray.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 array.

Example 1: Find the index of Tuesday

["Sunday", "Tuesday", "Friday"].indexOf("Tuesday");

// The invocation above will return: 1

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Example 2: Find the index of Sunday

["Sunday", "Tuesday", "Friday"].indexOf("Sunday");

// The invocation above will return: 0

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Example 3: Find the index of 5

[1, 3, 5, 7].indexOf(5);

// The invocation above will return: 2

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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.

note
  • The startIndex argument's default value is 0. Therefore, if omitted, the search will begin at index 0.
  • The indexOf() method always starts its search from left to right irrespective of whether startIndex is a positive or negative number.

Example 1: Find the index of Tuesday from the 3rd index position

const daysOfWeek = [
"Sunday",
"Tuesday",
"Thursday",
"Friday",
"Tuesday",
"Sunday",
"Tuesday",
"Friday",
];

daysOfWeek.indexOf("Tuesday", 3);

// The invocation above will return: 4

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Suppose you specify a negative startIndex argument. In that case, the computer will start the index count from the calling array's last element.

Here's an example:

const daysOfWeek = [
"Sunday",
"Tuesday",
"Thursday",
"Friday",
"Tuesday",
"Sunday",
"Tuesday",
"Friday",
];

daysOfWeek.indexOf("Tuesday", -3);

// The invocation above will return: 6

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Example 2: Find the index of Sunday from the 1st index position

const daysOfWeek = [
"Sunday",
"Tuesday",
"Friday",
"Sunday",
"Tuesday",
"Friday",
];

daysOfWeek.indexOf("Sunday", 1);

// The invocation above will return: 3

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Suppose the startIndex argument is greater than or equal to the calling array's length. In such a case, the computer will ignore the search.

Here's an example:

const daysOfWeek = [
"Sunday",
"Tuesday",
"Friday",
"Sunday",
"Tuesday",
"Friday",
];

daysOfWeek.indexOf("Sunday", 75);

// The invocation above will return: -1

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tip
  • To find the last occurrence of a string within an array, use lastIndexOf().
  • You can also use indexOf() on a string. Learn more here.

Overview

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