LESSON 99 OF 410 · ARRAYS I

Array Length in JavaScript

We can get the length of an array using the .length property like this:

let ourArray = [50, 40, 30];

// Printing the length:
console.log(ourArray.length); // Output: 3

// Changing the array:
ourArray.pop();

// Printing the new length:
console.log(ourArray.length); // Output: 2

Using the length

We can use the length to access items from the end of an array.

Because arrays are 0-indexed, the index of the last item is length - 1:

let ourArray = [50, 40, 30];

// Printing the last item:
console.log(ourArray[ourArray.length - 1]); // Output: 30

// Printing the second to last item:
console.log(ourArray[ourArray.length - 2]); // Output: 40

Assignment
Follow the Coding Tutorial and let's play with some arrays.

Hint
Look at the examples above if you get stuck.

STEP 1 OF 3

Guided

Ctrl + Enter to check

Array Length in JavaScript, explained

JavaScript array.length: What It Is and How to Use It

Almost every piece of code that works with arrays needs to know how many elements are in them — to loop through them safely, to grab the last item, to validate input, or to guard against empty arrays. The .length property gives you this instantly. It's not a method call (no parentheses needed), it updates automatically as the array changes, and understanding it prevents one of the most common beginner bugs: off-by-one errors.

Reading the Length

const scores = [95, 82, 74, 88, 91];

console.log(scores.length);   // 5

// length updates automatically when the array changes:
scores.push(77);
console.log(scores.length);   // 6

scores.pop();
console.log(scores.length);   // 5

Accessing the Last Element

Because arrays are zero-indexed, the last element lives at index length - 1. This pattern comes up constantly:

const letters = ["a", "b", "c", "d", "e"];

const last   = letters[letters.length - 1];    // "e"
const second = letters[letters.length - 2];    // "d"

// Modern ES2022 shorthand — cleaner for the last element:
console.log(letters.at(-1));   // "e"
console.log(letters.at(-2));   // "d"

Using Length in Loops

The classic for loop uses length as the upper bound. The loop condition i < array.length (strict less-than, not ≤) is what keeps you from accessing an out-of-bounds index:

const products = ["laptop", "phone", "tablet", "monitor"];

for (let i = 0; i < products.length; i++) {
  console.log(`${i}: ${products[i]}`);
}
// 0: laptop
// 1: phone
// 2: tablet
// 3: monitor

// Check before accessing to guard against empty arrays:
if (products.length > 0) {
  console.log(`First item: ${products[0]}`);
}

Real-World Pattern: Progress Tracking

const completedLessons = ["Variables", "Loops", "Functions"];
const totalLessons = 10;

const progress = (completedLessons.length / totalLessons) * 100;
console.log(`Progress: ${progress}%`);   // Progress: 30%

const remaining = totalLessons - completedLessons.length;
console.log(`${remaining} lessons left`);  // 7 lessons left

Common Mistake: Off-by-One

const arr = [10, 20, 30];

// WRONG — accesses index 3, which doesn't exist:
console.log(arr[arr.length]);    // undefined

// CORRECT — last element is at index length - 1:
console.log(arr[arr.length - 1]);   // 30

Key Takeaways

array.length is a property (no parentheses) — it returns the count of elements.
Last element: arr[arr.length - 1] or arr.at(-1) (ES2022).
Loop condition: i < arr.length (strict less-than keeps you in bounds).
length updates automatically when you push(), pop(), or otherwise modify the array.

What to Learn Next

With length solid, you're ready for for...of loops and forEach() — cleaner ways to iterate without managing index variables manually. Then explore array slicing with slice(start, end) to extract sub-arrays using length-relative positions, and array destructuring to pull out multiple elements in a single line.

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