Array length property in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the length property of an array represent in JavaScript?

The length property shows how many elements are in the array. It tells you the total number of items stored.

beginner

If you have <code>let arr = [1, 2, 3, 4];</code>, what is <code>arr.length</code>?

The value of arr.length is 4 because there are 4 elements in the array.

intermediate

Can you change the length property of an array? What happens if you set it to a smaller number?

Yes, you can set length to a smaller number. This will remove elements from the end of the array, shortening it.

intermediate

What happens if you set the length property of an array to a larger number?

The array's size increases, but new elements are empty slots (holes). The array has more space but no new values.

intermediate

Is the length property always equal to the number of elements you see when you print the array?

Not always. length counts all elements including empty slots. Some elements might be undefined or missing but still counted.

What does array.length return?

A The last element in the array

B The number of elements in the array

C The sum of all elements

D The first element in the array

array.length returns how many elements are in the array.

If arr = [10, 20, 30], what is arr.length?

A 30

B 2

C 3

D undefined

There are 3 elements, so arr.length is 3.

What happens if you set arr.length = 2 on arr = [1, 2, 3, 4]?

A Array becomes [1, 2, 3, 4, 2]

B Error is thrown

C Array stays the same

D Array becomes [1, 2]

Setting length smaller removes elements from the end.

What is the value of arr.length after arr.length = 5 if arr = [1, 2]?

A 5

B 3

C Undefined

D 2

Setting length larger increases size with empty slots, so length is 5.

Does length count empty slots in an array?

A Yes, it counts empty slots

B No, it only counts filled elements

C Only counts numbers

D Only counts strings

length counts all slots, even if empty or undefined.

Explain how the length property works in JavaScript arrays and what happens when you change it.

Think about how length relates to array size and content.

Describe a real-life example where changing the array length property might be useful.

Imagine a list of tasks you want to trim or expand.

Practice

1.

What does the length property of a JavaScript array represent?

easy

A. The last element value in the array

B. The highest index number in the array

C. The total size in bytes of the array

D. The number of elements in the array

Solution

1. Step 1: Understand the length property

   The length property counts how many items are stored in the array.

2. Step 2: Differentiate from other properties

   It does not give the highest index or size in bytes, only the count of elements.

3. Final Answer:

   The number of elements in the array -> Option D

4. Quick Check:

   Array length = number of elements [OK]

Hint: Remember: length counts items, not indexes or sizes [OK]

Common Mistakes:

• Confusing length with highest index
• Thinking length gives memory size
• Assuming length returns last element

2.

Which of the following is the correct way to get the length of an array arr in JavaScript?

easy

A. arr.length()

B. arr.length

C. length(arr)

D. arr.size

Solution

1. Step 1: Recall the syntax for accessing length

   The length property is accessed without parentheses because it is a property, not a function.

2. Step 2: Check each option

   arr.length uses arr.length correctly. arr.length() uses parentheses incorrectly. Options C and D are not valid JavaScript syntax for array length.

3. Final Answer:

   arr.length -> Option B

4. Quick Check:

   Use property, not function: arr.length [OK]

Hint: Access length as property, no parentheses [OK]

Common Mistakes:

• Using arr.length() with parentheses
• Trying length(arr) like a function
• Using arr.size which doesn't exist

3.

What will be the output of the following code?

const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.length);

medium

A. undefined

B. 2

C. 3

D. Error

Solution

1. Step 1: Count the elements in the array

   The array fruits has three elements: 'apple', 'banana', and 'cherry'.

2. Step 2: Use the length property

   Accessing fruits.length returns the number of elements, which is 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Array length = 3 [OK]

Hint: Count items inside brackets to find length [OK]

Common Mistakes:

• Counting indexes starting from 0
• Expecting length to be last index
• Confusing length with undefined

4.

Identify the error in the following code snippet:

let numbers = [1, 2, 3, 4];
numbers.length = 2;
console.log(numbers.length);
console.log(numbers[2]);

medium

A. Setting length to 2 removes elements after index 1

B. numbers.length cannot be changed

C. numbers[2] will print 3

D. Syntax error in setting length

Solution

1. Step 1: Understand setting length property

   Setting numbers.length = 2 truncates the array to first two elements, removing elements after index 1.

2. Step 2: Check the output of numbers[2]

   Since the array is truncated, numbers[2] is now undefined, not 3.

3. Final Answer:

   Setting length to 2 removes elements after index 1 -> Option A

4. Quick Check:

   Changing length truncates array [OK]

Hint: Changing length can cut off elements [OK]

Common Mistakes:

• Thinking length is read-only
• Expecting old elements to remain after truncation
• Assuming numbers[2] still holds value

5.

Given the array let data = [10, 20, 30, 40, 50];, which code snippet correctly doubles the array size by adding undefined elements at the end?

// Choose the correct code
A) data.length = data.length * 2;
B) data.length = data.length + data.length;
C) data.length = data.length * 2; data.fill(undefined, 5, data.length);
D) data.length = data.length * 2; data.fill(undefined);

hard

A. data.length = data.length * 2; data.fill(undefined, 5, data.length);

B. data.length = data.length + data.length;

C. data.length = data.length * 2;

D. data.length = data.length * 2; data.fill(undefined);

Solution

1. Step 1: Double the length property

   Setting data.length = data.length * 2 doubles the array size but new slots are empty (holes).

2. Step 2: Fill new slots with undefined

   Using data.fill(undefined, 5, data.length) fills the new slots from index 5 to the new length with undefined.

3. Step 3: Check other options

   A and B only change length but do not fill new slots, leaving holes. D uses data.fill(undefined) which fills from index 0, overwriting original data.

4. Final Answer:

   data.length = data.length * 2; data.fill(undefined, 5, data.length); -> Option A

5. Quick Check:

   Double length then fill new slots [OK]

Hint: Double length then fill new slots with fill(start, end) [OK]

Common Mistakes:

• Not filling new slots, leaving holes
• Overwriting existing elements with fill
• Using fill without end index