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 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.
• indexOf() is sometimes written as Array.prototype.indexOf() because it is a method of the Array object’s prototype property.

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

Try Editing It

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

Try Editing It

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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

Try Editing It

Tip

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