What is if-else Statement in JavaScript

What is an if-else Statement in JavaScript?

An if-else statement in JavaScript is a control structure that allows the program to execute one block of code if a condition is true, and a different block if the condition is false. This provides a way to handle two possible outcomes based on a condition.

Syntax of if-else Statement

if (condition) {
    // Code to execute if the condition is true
} else {
    // Code to execute if the condition is false
}

How to Use an if-else Statement

1. Condition Evaluation – The condition inside the if parentheses is checked.
2. True Block –  If the condition evaluates to true, the code inside the if block runs.
3. False Block – If the condition is false, the code within the else block will execute.

Example:

let temperature = 25;

if (temperature > 20) {
    console.log("It's warm outside.");
} else {
    console.log("It's cold outside.");
}

Output:

It's warm outside.

If the temperature were 15, the output would be:

It's cold outside.

Why Use if-else Statements?

• Two-Path Decisions – When you need to execute different code based on a condition.
• User Interaction – Tailor responses according to user input.
• Validation – Handle form input or data validation by providing alternative logic.
• Error Handling – Manage errors or provide fallback options when conditions are not met.

When to Use if-else Statements?

• Binary Decisions – For situations with two possible outcomes (e.g., success or failure).
• Validation Checks – When verifying if a user meets a requirement or not.
• Simple Alternatives – To choose between two options based on variable values.
• Form Submission – Check if a form is filled correctly, and respond accordingly.

Real-World Example:

Checking Age for Voting Eligibility

let age = 17;

if (age >= 18) {
    console.log("You are eligible to vote.");
} else {
    console.log("You are not old enough to vote.");
}

Output:

You are not old enough to vote.

Example: User Login System

let isLoggedIn = false;

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

Output:

Please log in.

Nested if-else Statements

You can place if-else statements inside each other to handle more complex conditions.

let score = 85;

if (score >= 90) {
    console.log("Grade: A");
} else {
    if (score >= 75) {
        console.log("Grade: B");
    } else {
        console.log("Grade: C");
    }
}

Output:

Grade: B

Benefits of if-else Statements

• Simple to Implement – Easy to understand and write.
• Efficient Control Flow – Helps make logical decisions and control program behavior.
• Dynamic Logic – Reacts to changing inputs or data conditions.
• Flexible – Can be nested or combined for complex scenarios.

Common Mistakes to Avoid

1. Forgetting the else Block – If no else block is written, nothing happens when the condition is false.
2. Assignment Instead of Comparison

if (x = 10)  // Incorrect: assigns value instead of comparing

Fix:

if (x === 10)  // Correct: compares values

3. Incorrect Bracket Usage

if (x > 10) 
    console.log("Greater");  // Works, but not recommended
else 
    console.log("Smaller");

Fix:

if (x > 10) {
    console.log("Greater");
} else {
    console.log("Smaller");
}

Ternary Operator (Shorter Alternative to if-else)

For simple if-else logic, a ternary operator can be used to make the code more concise.

let number = 5;
let result = (number % 2 === 0) ? "Even" : "Odd";
console.log(result);  // Odd

Best Practices for Using if-else

2. Keep It Simple – Avoid overly complex if-else chains by breaking down logic into smaller functions.
3. Readable Conditions – Use clear and simple conditions.
4. Avoid Deep Nesting – Excessive nesting can make the code harder to read.
5. Use Logical Operators – Combine conditions using && (AND) or || (OR) to reduce code repetition.

Key Takeaways

• The if-else statement lets you manage two possible outcomes, controlling the execution flow based on conditions.
• A fallback (else) ensures alternative code runs if the primary condition fails.
• Ternary operators provide a shorter alternative for simple if-else logic.