Index Out of Range

Why this happens

• Index is greater than or equal to the array length

• The array is empty but was accessed at index 0

• Off-by-one error in a loop boundary (e.g., <= instead of <)

Minimal example

const arr = [1, 2, 3];
console.log(arr[5]);

const arr = [1, 2, 3];
console.log(arr[2]);

How to diagnose

• Print the array length and the index being accessed

• Check loop bounds — does the index stay within 0 to length - 1?

• Verify the array is not empty before accessing elements

How to fix

• Check index < array.length before accessing the element

• Fix the loop boundary to use < instead of <=

• Use safe access methods like array.at(index) or try/except

How to prevent

• Prefer for...of or for-each loops over manual index loops

• Validate the index against the array length before access

Related Resources