Why JavaScript Array Length Changes When You Add an Element at a Higher Index. JavaScript's Unexpected Behavior When Skipping Indexes. How Adding Elements at a Specific Index Affects Array Length

Lets explain code step by step:

const arr = [1, 2, 3];
arr[5] = 6;
console.log(arr.length);

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Step 1: Array Declaration

const arr = [1, 2, 3];

• We are creating an array arr using the const keyword.
• The array initially contains three elements:

arr = [1, 2, 3]

• The indexes at this point:

Index: 0  1  2

Value: 1  2  3

• Important:

 In JavaScript, const means you cannot reassign the variable reference, but you can still mutate (change) the contents of the array.

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Step 2: Assigning a Value to arr[5]

arr[5] = 6;

• You are directly assigning a value at index 5.
• Current structure after this step:

        Index: 0   1   2     3       4     5
        Value: 1   2   3  <empty> <empty>  6

• What happens to index 3 and index 4?
• They are automatically created as empty slots (also called "sparse elements").
• These empty slots are not undefined values—they are empty, meaning they don't exist in memory but are counted in the array length.

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Step 3: Checking Array Length

console.log(arr.length);

• The .length property of an array in JavaScript is always one more than the highest index.
• In this case, the highest index is 5.
• So, the length becomes 6.

✔ Final Answer:
C) 6
This question appeared in the Capgemini 2nd round of interview, testing the candidate’s knowledge of a array data type and array length calculation in JavaScript.