Why arrays are needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an array named fruits.

Javascript

const fruits = [1];

Drag options to blanks, or click blank then click option'

A{'apple', 'banana', 'cherry'}

B'apple', 'banana', 'cherry'

C['apple', 'banana', 'cherry']

D(apple, banana, cherry)

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces instead of square brackets

Not using any brackets

Using parentheses instead of brackets

Arrays in JavaScript are created using square brackets []. This lets us store multiple items in one variable.

2fill in blank

medium

Complete the code to access the first fruit in the array.

Javascript

const firstFruit = fruits[1];

Drag options to blanks, or click blank then click option'

A(0)

B[0]

C{0}

D<0>

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets

Using curly braces

Using angle brackets

We use square brackets with the index number to access an item in an array. Indexing starts at 0.

3fill in blank

hard

Fix the error in the code to add a new fruit to the array.

Javascript

fruits[1]('orange');

Drag options to blanks, or click blank then click option'

Apush

Badd

Cappend

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using add which is not a method

Using append which is not valid in JavaScript

Using insert which does not exist

The push method adds a new item to the end of an array in JavaScript.

4fill in blank

hard

Complete the code to create an array of numbers and get the last number.

Javascript

const numbers = [1, 2, 3, 4, 5];
const lastNumber = numbers[numbers[1] - 1];

Drag options to blanks, or click blank then click option'

A[

B]

C.length

D.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using .size which is not valid for arrays

Using parentheses instead of square brackets

Not subtracting 1 from length

To get the last item, use array[array.length - 1]. length gives the number of items, and we subtract 1 for the last index.

5fill in blank

hard

Fill both blanks to create an array of words, filter words longer than 3 letters, and create a new array with their lengths.

Javascript

const words = ['cat', 'elephant', 'dog', 'bird'];
const longWordsLengths = words
  .filter(word => word[1] 3)
  .map(word => word[2]);

console.log(longWordsLengths);

Drag options to blanks, or click blank then click option'

A.length >

B.length

C[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using comparison without .length

Trying to access word as an array incorrectly

Not logging the array correctly

We filter words with length greater than 3 using word.length > 3. Then we map each word to its length with word.length. Finally, we log the resulting array.

Practice

1. Why do we use arrays in JavaScript?

easy

A. To create functions

B. To store only one value at a time

C. To store multiple values in a single variable

D. To write comments

Solution

1. Step 1: Understand the purpose of arrays

   Arrays are used to hold many values together in one place instead of separate variables.

2. Step 2: Compare options with array purpose

   Only To store multiple values in a single variable correctly states that arrays store multiple values in one variable.

3. Final Answer:

   To store multiple values in a single variable -> Option C

4. Quick Check:

   Arrays store many values [OK]

Hint: Arrays hold many items together in one variable [OK]

Common Mistakes:

• Thinking arrays store only one value
• Confusing arrays with functions
• Believing arrays are for comments

2. Which of the following is the correct way to create an array in JavaScript?

easy

A. let arr = (1, 2, 3);

B. let arr = [1, 2, 3];

C. let arr = {1, 2, 3};

D. let arr = <1, 2, 3>;

Solution

1. Step 1: Recall JavaScript array syntax

   Arrays are created using square brackets [] with values separated by commas.

2. Step 2: Check each option's syntax

   let arr = [1, 2, 3]; uses square brackets correctly. Others use parentheses, curly braces, or angle brackets which are incorrect for arrays.

3. Final Answer:

   let arr = [1, 2, 3]; -> Option B

4. Quick Check:

   Arrays use [] brackets [OK]

Hint: Arrays use square brackets [] to hold values [OK]

Common Mistakes:

• Using parentheses () instead of brackets
• Using curly braces {} which create objects
• Using angle brackets <> which are invalid

3. What will be the output of this code?

let fruits = ['apple', 'banana', 'cherry'];
console.log(fruits[1]);

medium

A. banana

B. apple

C. cherry

D. undefined

Solution

1. Step 1: Understand array indexing

   Arrays start counting at 0, so fruits[0] is 'apple', fruits[1] is 'banana'.

2. Step 2: Identify the value at index 1

   The code logs fruits[1], which is 'banana'.

3. Final Answer:

   banana -> Option A

4. Quick Check:

   Index 1 in array = banana [OK]

Hint: Array indexes start at 0; index 1 is second item [OK]

Common Mistakes:

• Thinking array indexes start at 1
• Confusing the order of items
• Expecting the whole array to print

4. Find the error in this code that tries to create an array:

let numbers = 1, 2, 3, 4;

medium

A. Variable name 'numbers' is invalid

B. Using commas instead of semicolons

C. Array values must be strings

D. Missing square brackets [] around values

Solution

1. Step 1: Check array creation syntax

   Arrays require square brackets [] around the list of values.

2. Step 2: Identify the mistake in the code

   The code assigns multiple values separated by commas without brackets, which is invalid syntax.

3. Final Answer:

   Missing square brackets [] around values -> Option D

4. Quick Check:

   Arrays need [] brackets [OK]

Hint: Arrays must be inside [] brackets [OK]

Common Mistakes:

• Forgetting to use brackets for arrays
• Thinking commas alone create arrays
• Believing variable names cause errors here

5. You want to store the names of 5 friends and print them one by one. Which approach is best and why?

hard

A. Use an array to store all friend names together

B. Use a function to store friend names

C. Write all names in a single string separated by commas

D. Create 5 separate variables, one for each friend

Solution

1. Step 1: Consider managing multiple related items

   Storing many related values separately is hard to manage and print repeatedly.

2. Step 2: Choose the best data structure

   Arrays group related items in one variable, making it easy to loop and print each friend.

3. Step 3: Evaluate other options

   Separate variables are inefficient, a single string is hard to split, and functions don't store lists directly.

4. Final Answer:

   Use an array to store all friend names together -> Option A

5. Quick Check:

   Arrays organize lists well [OK]

Hint: Use arrays to handle multiple related items easily [OK]

Common Mistakes:

• Using many separate variables instead of one array
• Storing names in one string without splitting
• Confusing functions with data storage