Logic Statements

1. Purpose of Logic Statements

Logic statements allow a program to choose between different execution paths based on conditions.

[Condition?]
   |
True   False
 |       |
if block else block

2. if and if...else

Basic if

let temperature = 30;
if (temperature > 25){
    console.log("It is warm outside.");
}

if...else

let isLoggedIn = false;
if(isLoggedIn){
   console.log("Welcome back!");
}else{
   console.log("Please log in first.");
}

3. else if Chain

let score=78;
if(score>=90){
 console.log("Grade A");
}else if(score>=80){
 console.log("Grade B");
}else if(score>=70){
 console.log("Grade C");
}else{
 console.log("Grade D");
}

Conditions are checked from top to bottom. The first true condition executes.

4. Truthy and Falsy

Falsy values:

• false
• 0
• -0
• ""
• null
• undefined
• NaN

let name="";
if(name){
 console.log("Name is provided");
}else{
 console.log("Name is missing");
}

5. Comparison and Logical Operators

Logical Operators

• && AND
• || OR
• ! NOT

let age=20;
let hasID=true;
if(age>=18 && hasID){
 console.log("Access granted");
}

let isAdmin=false;
let isModerator=true;
if(isAdmin || isModerator){
 console.log("You can manage comments.");
}

let isComplete=false;
if(!isComplete){
 console.log("Task is not complete yet.");
}

6. Conditional (Ternary) Operator

let age=17;
let message=(age>=18)?"Adult":"Minor";
console.log(message);

Ternary is a concise alternative to simple if...else statements.

7. switch Statement

let day="Wednesday";
switch(day){
 case "Monday":
   console.log("Start of week");
   break;
 case "Wednesday":
   console.log("Middle of week");
   break;
 default:
   console.log("Another day");
}

Without break, execution falls through to the next case.

8. Choosing Between if and switch

Use if...else for ranges and inequalities. Use switch for matching a single variable against multiple values.

9. Complete Example - Movie Ticket Price Calculator

let age=16;
let isStudent=true;
let dayOfWeek="Wednesday";
let basePrice;

if(age<12){
 basePrice=5;
}else if(age<18){
 basePrice=8;
}else{
 basePrice=10;
}

let discount=isStudent?2:0;
let finalPrice;

switch(dayOfWeek){
 case "Wednesday":
   finalPrice=basePrice-discount-1;
   break;
 default:
   finalPrice=basePrice-discount;
}

console.log(finalPrice);

Summary

• if
• if...else
• else if
• Truthy & Falsy
• Comparison Operators
• Logical Operators
• Ternary Operator
• Switch Statement
• Fall-through
• Choosing between if and switch