Why variables are needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to store the number 10 in a variable named score.

Javascript

let score = [1];

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

A'score'

Bten

Cscore

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around 10 makes it a string, not a number.

Using the variable name instead of a value.

We use let score = 10; to store the number 10 in the variable score.

2fill in blank

medium

Complete the code to change the value of the variable score to 20.

Javascript

score [1] 20;

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

A=

B===

C==

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using == or === instead of = causes comparison, not assignment.

Writing 'equals' is not valid JavaScript syntax.

Use = to assign a new value to a variable.

3fill in blank

hard

Fix the error in the code to declare a variable named name with the value 'Alice'.

Javascript

let [1] = 'Alice';

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

A1name

BName

Cname

Dna me

Attempts:

3 left

💡 Hint

Common Mistakes

Starting variable names with numbers.

Using spaces inside variable names.

Variable names cannot start with a number or contain spaces. name is a valid variable name.

4fill in blank

hard

Fill both blanks to create a variable that stores the sum of 5 and 3.

Javascript

let total = [1] [2] 3;

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

A5

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using minus or multiplication instead of addition.

Putting the wrong number or symbol in the blanks.

We add 5 and 3 using the plus sign + and store it in total.

5fill in blank

hard

Fill all three blanks to create an object with a variable key and value.

Javascript

let [1] = 'color';
let [2] = 'blue';
let obj = { [[1]]: [2] };

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

Akey

Bvalue

Ccolor

Dblue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the string 'color' as a variable name directly.

Mixing up variable names and their values.

We create variables key and value to hold strings, then use them to make an object with a key-value pair.

Practice

1. Why do we use variables in JavaScript?
let age = 25;

easy

A. To store information that can be used and changed later

B. To make the program run faster

C. To create permanent unchangeable values

D. To write comments in the code

Solution

1. Step 1: Understand what variables do

   Variables hold data that can be used and updated in the program.

2. Step 2: Analyze the example

   The code let age = 25; stores the number 25 in the variable named age, which can be changed later.

3. Final Answer:

   To store information that can be used and changed later -> Option A

4. Quick Check:

   Variables store and change data [OK]

Hint: Variables hold data you want to reuse or change [OK]

Common Mistakes:

• Thinking variables make code run faster
• Confusing variables with comments
• Believing variables cannot change

2. Which of the following is the correct way to declare a variable in JavaScript?

easy

A. let name = 'John';

B. variable name = 'John';

C. const = age 30;

D. var 2name = 'John';

Solution

1. Step 1: Check JavaScript variable syntax

   Variables can be declared using let, var, or const followed by a valid name.

2. Step 2: Identify valid variable names and syntax

   let name = 'John'; is correct because it uses let, a valid name, and proper assignment.

3. Final Answer:

   let name = 'John'; -> Option A

4. Quick Check:

   Correct variable declaration [OK]

Hint: Variable names can't start with numbers; use let/const/var [OK]

Common Mistakes:

• Starting variable names with numbers
• Using invalid keywords like 'variable'
• Incorrect assignment syntax

3. What will be the output of this code?

let score = 10;
score = score + 5;
console.log(score);

medium

A. 5

B. 15

C. 10

D. undefined

Solution

1. Step 1: Understand variable update

   The variable score starts at 10, then 5 is added to it.

2. Step 2: Calculate the new value

   10 + 5 equals 15, so score becomes 15.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   10 + 5 = 15 [OK]

Hint: Add numbers step-by-step to find variable's new value [OK]

Common Mistakes:

• Forgetting variable was updated
• Confusing initial value with final
• Expecting undefined because of reassignment

4. Find the error in this code:

let total = 20
let total = 30;
console.log(total);

medium

A. Variable name 'total' is invalid

B. Missing semicolon after first line

C. You cannot declare the same variable twice with let

D. console.log is used incorrectly

Solution

1. Step 1: Check variable declarations

   The code declares total twice using let, which is not allowed in the same scope.

2. Step 2: Understand let rules

   let variables cannot be redeclared; you can only assign a new value without let.

3. Final Answer:

   You cannot declare the same variable twice with let -> Option C

4. Quick Check:

   Redeclaring let variable causes error [OK]

Hint: Use let once per variable; assign new values without let [OK]

Common Mistakes:

• Thinking semicolon is mandatory for error
• Believing variable names are invalid
• Assuming console.log causes error

5. You want to store a user's name and age, then update the age after a birthday. Which code correctly uses variables for this?

??

hard

A. let name = 'Alice';
const age = 30;
age = age + 1;
console.log(name, age);

B. let name = 'Alice';
let age = 30;
const age = age + 1;
console.log(name, age);

C. const name = 'Alice';
const age = 30;
age = age + 1;
console.log(name, age);

D. const name = 'Alice';
let age = 30;
age = age + 1;
console.log(name, age);

Solution

1. Step 1: Identify which variables can change

   The user's name does not change, so const is good. Age changes, so let is needed.

2. Step 2: Check code correctness

   const name = 'Alice'; let age = 30; age = age + 1; console.log(name, age); declares name as const and age as let, then updates age correctly.

3. Final Answer:

   const name = 'Alice'; let age = 30; age = age + 1; console.log(name, age); -> Option D

4. Quick Check:

   Use const for fixed, let for changeable variables [OK]

Hint: Use const for fixed, let for values that change [OK]

Common Mistakes:

• Trying to change const variables
• Redeclaring variables with const or let
• Mixing let and const incorrectly