How to check if an Array Index exists in JavaScript

Check if an Array Index exists in JavaScript #

To check if an array index exists, access the array at the specific index and check if the result is not equal to undefined. If the result is not equal to undefined the array index exists.

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const arr = ['a', 'b'];

if (arr[3] !== undefined) {
  // 👉️ index 3 exists in the array
}

We access the array at index 3 and check if the result is not equal to undefined.

Since the array only has 2 elements, the last index in the array is 1. Therefore, the condition is never satisfied and the if block doesn't run.

Alternatively, you can check for the array's length.

Use the length property to check if an array index exists, e.g. if (arr.length > 5) {}. If the array has a length greater than N, then index N is guaranteed to exists in the array.

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const arr = ['a', 'b'];

if (arr.length > 5) {
  // 👉️ index 5 exists in the array
}

Indexes are zero-based in JavaScript, so the last index in the array is equal to array.length - 1.

If an array has a length of 10, its last index is 10 - 1 = 9.

A newer way to check if an array index exists is to use optional chaining.

Use the optional chaining operator to check if an array index exists, e.g. const firstIndex = arr?.[1]. The optional chaining operator will return the array element if the index exists, otherwise it returns undefined.

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const arr = ['a', 'b'];

const firstIndex = arr?.[1];
console.log(firstIndex); // 👉️ b

if (firstIndex !== undefined) {
  // 👉️ index 1 exists in the array
}

const fifthIndex = arr?.[5];
console.log(fifthIndex); // 👉️ undefined

We used the optional chaining operator (?.) to to access the array elements at index 1 and 5.

The array element at index 1 exists, therefore its value is assigned to the firstIndex variable.

The array element at index 5 doesn't exist, so the optional chaining operator short-circuits returning undefined.

Further Reading #

• Add Element to Array at specific Index in JavaScript
• Get the index of an Object in an Array in JavaScript
• Check if Array of Strings contains a Substring in JS
• Check if Array Contains an Object in JavaScript
• Make Array.indexOf() case insensitive in JavaScript
• Filter an Array of Objects based on a property - JavaScript
• Filter an Array with Multiple Conditions in JavaScript
• Get the first N elements from an Array in JavaScript
• Get the last N elements from an Array in JavaScript
• How to Remove Element(s) from an Array in JavaScript
• How to Empty an Array in JavaScript