length and index relationship

Learn the length property and how to access the last element.

Accessing the last element

You access the last element of an array with the index length - 1. In a three-element array length returns 3; the last element is arr[2].

Example: with const notes = [70, 85, 90];, notes[notes.length - 1] returns 90. This pattern keeps working when the array length changes.

Using length - 1 instead of a fixed index is safer; it still finds the last element after you add more elements.

Array bounds and undefined

Accessing an index outside the array bounds is not an error; JavaScript returns undefined. This is unlike most languages.

Example: with const items = ["a", "b"];, items[5] returns undefined. It does not crash the program.

So you must check whether an index is valid yourself. Negative indices also return undefined; items[-1] does not give the last element.

Exercise Print the last element of the scores array

Create an array named scores with at least four numbers. Then print scores[scores.length - 1] with console.log.

Show example solution

Define a scores array with four numbers, then print the last element with length - 1.
Example solution:
const scores = [10, 20, 30, 40];
console.log(scores[scores.length - 1]);

Test yourself

When nums has four elements, what does nums[4] return?
```
const nums = [10, 20, 30, 40];
console.log(nums[4]);
```
- 40
- throws an error
- null
- undefined
Show answer

undefined — Correct. Out-of-bounds access returns undefined, not an error.

Access the last element with length - 1

const prices = [100, 200, 300];
const last = prices[____];
console.log(last);

Show answer

const prices = [100, 200, 300];
const last = prices[prices.length - 1];
console.log(last);

Match expressions to values for <code>const nums = [10, 20, 30]</code>

nums.length

3
nums[0]

10
nums[2]

30
nums[5]

undefined

Next lesson: Multidimensional arrays

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