Why conditional logic is needed in Javascript

Introduction

Conditional logic helps a program make decisions. It lets the program choose what to do based on different situations.

When you want to check if a user is logged in before showing content.

When you need to decide what message to show based on a user's age.

When you want to perform different actions depending on the day of the week.

When you want to check if a number is positive, negative, or zero.

When you want to handle errors differently depending on the problem.

Syntax

Javascript

if (condition) {
  // code to run if condition is true
} else {
  // code to run if condition is false
}

The condition is a question that can be true or false.

The code inside the if block runs only if the condition is true.

Examples

This checks if age is 18 or more. It prints a message based on that.

Javascript

if (age >= 18) {
  console.log('You can vote');
} else {
  console.log('You are too young to vote');
}

This decides what message to show depending on the temperature.

Javascript

if (temperature > 30) {
  console.log('It is hot outside');
} else {
  console.log('It is not hot outside');
}

Sample Program

This program checks the time and prints a greeting based on whether it is before noon or not.

Javascript

const hour = 10;

if (hour < 12) {
  console.log('Good morning!');
} else {
  console.log('Good afternoon!');
}

Important Notes

Conditional logic is like asking a question and choosing what to do next.

You can add more conditions using else if for more choices.

Summary

Conditional logic lets programs make choices.

It uses if and else to run different code based on conditions.

This helps programs respond to different situations.

Practice

1. Why do we use conditional logic like if and else in JavaScript?

easy

A. To make decisions and run different code based on conditions

B. To repeat the same code many times

C. To store multiple values in a list

D. To create new variables

Solution

1. Step 1: Understand the purpose of conditional logic

   Conditional logic allows the program to choose between different actions depending on conditions.

2. Step 2: Compare options with this purpose

   Only To make decisions and run different code based on conditions describes making decisions based on conditions, which matches the purpose of if and else.

3. Final Answer:

   To make decisions and run different code based on conditions -> Option A

4. Quick Check:

   Conditional logic = decision making [OK]

Hint: Think: Does it help choose between actions? [OK]

Common Mistakes:

• Confusing conditional logic with loops
• Thinking it stores data
• Believing it only creates variables

2. Which of the following is the correct syntax to check if a variable age is greater than 18 in JavaScript?

easy

A. if (age > 18) { }

B. if (age > 18) then { }

C. if age > 18 then { }

D. if age > 18 { }

Solution

1. Step 1: Recall JavaScript if statement syntax

   JavaScript requires parentheses around the condition and curly braces for the code block: if (condition) { }.

2. Step 2: Check each option

   if (age > 18) { } matches the correct syntax. Options A and C miss parentheses or use invalid keywords like 'then'. if (age > 18) then { } incorrectly uses 'then'.

3. Final Answer:

   if (age > 18) { } -> Option A

4. Quick Check:

   Correct if syntax = parentheses + braces [OK]

Hint: Remember: condition in ( ), code in { } [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using 'then' keyword (not in JavaScript)
• Missing curly braces

3. What will be the output of this code?

let score = 75;
if (score >= 60) {
  console.log('Pass');
} else {
  console.log('Fail');
}

medium

A. Fail

B. No output

C. Error

D. Pass

Solution

1. Step 1: Check the condition score >= 60

   The variable score is 75, which is greater than or equal to 60, so the condition is true.

2. Step 2: Determine which block runs

   Since the condition is true, the code inside the if block runs, printing 'Pass'.

3. Final Answer:

   Pass -> Option D

4. Quick Check:

   75 >= 60 is true, so output = Pass [OK]

Hint: Check if condition is true or false to pick output [OK]

Common Mistakes:

• Confusing which block runs on true condition
• Thinking output is 'Fail' for true condition
• Assuming syntax error

4. Find the error in this code snippet:

let temperature = 30;
if temperature > 25 {
  console.log('Hot');
} else {
  console.log('Cold');
}

medium

A. Using else without if

B. Missing semicolon after variable declaration

C. Missing parentheses around the condition in if statement

D. Incorrect variable name

Solution

1. Step 1: Check the if statement syntax

   JavaScript requires parentheses around the condition in an if statement, but here they are missing.

2. Step 2: Verify other parts

   The variable declaration is correct with semicolon, else is correctly used, and variable name matches.

3. Final Answer:

   Missing parentheses around the condition in if statement -> Option C

4. Quick Check:

   if needs (condition) [OK]

Hint: Always put condition inside ( ) after if [OK]

Common Mistakes:

• Forgetting parentheses in if condition
• Thinking semicolon is mandatory after if
• Confusing else usage

5. You want to write a program that prints 'Good morning' if the hour is less than 12, 'Good afternoon' if the hour is between 12 and 18 (inclusive), and 'Good evening' otherwise. Which code correctly uses conditional logic to do this?

hard

A. if (hour < 12) {
console.log('Good morning');
} else if (hour < 18) {
console.log('Good afternoon');
} else {
console.log('Good evening');
}

B. if (hour < 12) {
console.log('Good morning');
} else if (hour <= 18) {
console.log('Good afternoon');
} else {
console.log('Good evening');
}

C. if (hour <= 12) {
console.log('Good morning');
} else if (hour <= 18) {
console.log('Good afternoon');
} else {
console.log('Good evening');
}

D. if (hour < 12) {
console.log('Good morning');
} else if (hour > 12 && hour < 18) {
console.log('Good afternoon');
} else {
console.log('Good evening');
}

Solution

1. Step 1: Understand the conditions for each greeting

   'Good morning' if hour < 12, 'Good afternoon' if hour between 12 and 18 inclusive, else 'Good evening'.

2. Step 2: Check each option's condition ranges

   if (hour < 12) { console.log('Good morning'); } else if (hour <= 18) { console.log('Good afternoon'); } else { console.log('Good evening'); } correctly uses hour < 12 for morning, hour <= 18 for afternoon (including 12 and 18), else evening. Others miss inclusive 12 or 18 or have wrong ranges.

3. Final Answer:

   if (hour < 12) { console.log('Good morning'); } else if (hour <= 18) { console.log('Good afternoon'); } else { console.log('Good evening'); } -> Option B

4. Quick Check:

   Inclusive afternoon means <= 18, so if (hour < 12) { console.log('Good morning'); } else if (hour <= 18) { console.log('Good afternoon'); } else { console.log('Good evening'); } fits [OK]

Hint: Check if conditions cover all cases without gaps [OK]

Common Mistakes:

• Using < instead of <= for inclusive ranges
• Overlapping or missing ranges
• Wrong logical operators in conditions