JavaScript Control Flow

Table of Contents

• Intro
• if Statement
• if...else Statement
• else if Ladder
• Nested if
• Important Tips
• Real-Life Example
• When to Use What
• Best Practices
• Interview Q&A
• Interview MCQ

JavaScript Conditional Statements

Conditional statements let your program make decisions. In JavaScript, they control which block of code runs based on conditions that evaluate to true or false.

if Statement (Basic Decision)

Use if when you want to execute code only when a condition is true.

let age = 20;

if (age >= 18) {
    console.log("You are eligible to vote");
}

If the condition is false, nothing happens.

if...else Statement (Two Choices)

Use this when you have exactly two possible outcomes.

let age = 16;

if (age >= 18) {
    console.log("Adult");
} else {
    console.log("Minor");
}

Here, one block will always execute.

else if Ladder (Multiple Conditions)

Use this when you need to check multiple conditions in sequence.

let marks = 75;

if (marks >= 90) {
    console.log("Grade A");
} else if (marks >= 75) {
    console.log("Grade B");
} else if (marks >= 50) {
    console.log("Grade C");
} else {
    console.log("Fail");
}

JavaScript checks conditions from top to bottom and stops at the first true condition.

Nested if (if inside if)

Use nested if when a condition depends on another condition.

let age = 20;
let hasID = true;

if (age >= 18) {
    if (hasID) {
        console.log("Allowed to enter");
    } else {
        console.log("ID required");
    }
} else {
    console.log("Not allowed");
}

5. Important Tips

• Always use { } even for single statements to avoid mistakes.
• Be careful with comparison operators:

==   // compares value
===  // compares value and type (recommended)

if (5 === "5") {
    console.log("Equal");
} else {
    console.log("Not Equal"); // This runs
}

6. Real-Life Example

let username = "admin";
let password = "1234";

if (username === "admin") {
    if (password === "1234") {
        console.log("Login successful");
    } else {
        console.log("Wrong password");
    }
} else {
    console.log("User not found");
}

7. When to Use What

• Use if for a single condition.
• Use if...else for two choices.
• Use else if ladder for multiple conditions.
• Use nested if when conditions depend on each other.

8. Best Practices

• Keep conditions readable and avoid unnecessary deep nesting.
• Use strict checks (===) where possible.
• Extract repeated logic into functions.
• Prefer guard clauses for early exits in long functions.

9. 10 Control Flow Interview Q&A

10. 10 Control Flow MCQs