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
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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