JavaScript Katana

JavaScript Conditional Statements Explained: How to Use if, else if, and switch

JavaScript Basics & Introduction
佐川 直弘 | Naohiro Sagawa
By 佐川 直弘 | Naohiro Sagawa (運営者)
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目次

1. Introduction: JavaScript Conditional Branching and the Role of “else if”

JavaScript is a programming language widely used to implement dynamic functionality in web development. Among its many features, conditional branching is an essential mechanism that allows different processes to be executed based on specific conditions.

In this article, we focus on one type of conditional branching: the “else if” statement. We will explain its basic syntax and usage in detail, using concrete examples to make it easy for beginners to understand.

The Importance of Conditional Branching in JavaScript

In programming, it is often necessary to perform different actions depending on certain conditions. For example, on an online shopping site, shipping may become free depending on the purchase amount, or coupon codes may be applied only when specific conditions are met.

In such situations, JavaScript’s if...else and else if statements allow you to define flexible conditional logic.

What Is the “else if” Statement?

The else if statement is a syntax used to evaluate multiple conditions sequentially. It is used as follows:

if (condition1) {
  // Process executed when condition1 is true
} else if (condition2) {
  // Process executed when condition2 is true
} else {
  // Process executed when all conditions above are false
}

This structure is particularly useful when you need to check multiple conditions. For example, it can be used to change access permissions based on a user’s age or determine a rank based on a score.

Article Overview

This article covers the following topics in order:

  1. Basic syntax and usage of “if” and “else” statements
  2. Syntax and practical examples of the “else if” statement
  3. Applied techniques using real-world code examples
  4. A comparison between “else if” and “switch” statements
  5. Common errors and troubleshooting tips

By learning these topics, you will strengthen your skills in JavaScript conditional branching and be able to write more advanced programs.

In the next section, we will start by taking a closer look at the basic syntax of “if” and “else” statements.

2. Basics: Syntax and Usage of “if” and “else” Statements

JavaScript conditional branching can be implemented simply using if and else statements. In this section, we explain their basic syntax and concrete usage.

What Is the “if” Statement?

The if statement is a syntax that executes code only when a specified condition evaluates to true. The basic structure is shown below.

if (condition) {
  // Code executed when the condition is true
}

Example of an “if” Statement

The following example branches processing based on the user’s age.

let age = 20;

if (age >= 18) {
  console.log("You are an adult.");
}

In this code, the message “You are an adult.” is displayed when the variable age is 18 or older. If the condition is not met, nothing is executed.

What Is the “else” Statement?

The else statement defines the process executed when the “if” condition is false. It is used in combination with if.

if (condition) {
  // Code executed when the condition is true
} else {
  // Code executed when the condition is false
}

Example of an “else” Statement

The next example displays a different message when the user is under 18 years old.

let age = 16;

if (age >= 18) {
  console.log("You are an adult.");
} else {
  console.log("You are a minor.");
}

In this code, “You are a minor.” is displayed when age is less than 18.

Summary

In this section, we learned the syntax and usage of JavaScript’s basic conditional statements: if and else. These constructs form the foundation not only for simple branching, but also for building more complex logic.

In the next section, we will take a deeper look at the else if statement, which allows you to handle multiple conditions. We will explore practical examples and applied techniques.

3. Intermediate: “else if” Syntax and Examples for Beginners

In this section, we explain complex conditional branching using the “else if” statement. By using else if, you can evaluate multiple conditions in sequence and execute the appropriate process.

Syntax of the “else if” Statement

The else if statement is an extended syntax for evaluating multiple conditions. The basic structure is as follows.

if (condition1) {
  // Process when condition1 is true
} else if (condition2) {
  // Process when condition2 is true
} else {
  // Process when all conditions are false
}

Example: Test Score Evaluation

The following code displays an evaluation based on a test score.

let score = 75;

if (score >= 90) {
  console.log("Grade: A");
} else if (score >= 80) {
  console.log("Grade: B");
} else if (score >= 70) {
  console.log("Grade: C");
} else if (score >= 60) {
  console.log("Grade: D");
} else {
  console.log("Grade: F");
}

Summary

In this section, we explained the syntax and examples of the “else if” statement. The else if statement is a powerful feature that allows you to evaluate multiple conditions in order.

In the next section, we will introduce more advanced examples and case studies to help you build practical skills.

4. Advanced: Practical Code and Case Studies Using “else if”

In this section, we introduce practical code examples using the else if statement. By working through scenarios commonly found in real applications, you can strengthen your applied skills.

Case 1: Form Input Validation

let username = "user123";
let password = "pass123";

if (username === "") {
  console.log("Please enter a username.");
} else if (password === "") {
  console.log("Please enter a password.");
} else if (password.length < 6) {
  console.log("Password must be at least 6 characters long.");
} else {
  console.log("The input is valid.");
}

Case 2: Score Evaluation and Ranking

let score = 72;

if (score >= 90) {
  console.log("Rank: S");
} else if (score >= 80) {
  console.log("Rank: A");
} else if (score >= 70) {
  console.log("Rank: B");
} else if (score >= 60) {
  console.log("Rank: C");
} else {
  console.log("Rank: D");
}

Summary

In this section, we introduced applied examples and practical code using the else if statement.
By applying these patterns, you can handle more complex conditional logic.

In the next section, we will compare else if and switch statements in detail and explain how to choose between them.

5. “else if” vs “switch”: Which Should You Use?

In JavaScript, two commonly used approaches for conditional branching are the else if statement and the switch statement. In this section, we compare their syntax and characteristics and explain how to choose the right one for your use case.

Features of the “else if” Statement

Syntax and Example

let grade = "B";

if (grade === "A") {
  console.log("Excellent!");
} else if (grade === "B") {
  console.log("Good performance.");
} else if (grade === "C") {
  console.log("Pass level.");
} else {
  console.log("Retest required.");
}

Features of the “switch” Statement

Syntax and Example

let grade = "B";

switch (grade) {
  case "A":
    console.log("Excellent!");
    break;
  case "B":
    console.log("Good performance.");
    break;
  case "C":
    console.log("Pass level.");
    break;
  default:
    console.log("Retest required.");
}

Summary

  • “else if” statement: Suitable for complex conditional expressions and logical operators.
  • “switch” statement: Suitable for organizing conditional branching based on specific values.

6. Common Mistakes and Error Handling Tips

In this section, we explain common mistakes when using the else if statement and how to prevent them through proper error handling.

Common Mistakes and Solutions

Incorrect Condition Evaluation

let age = 20;

if (age > 18) {
  console.log("You are an adult.");
} else if (age > 20) {
  console.log("You are over 20.");
} else {
  console.log("You are a minor.");
}

Adding Error Handling

try {
  let input = prompt("Please enter a score:");
  let score = parseInt(input);

  if (isNaN(score)) {
    throw new Error("Please enter a numeric value.");
  }

  if (score >= 90) {
    console.log("Grade: A");
  } else if (score >= 80) {
    console.log("Grade: B");
  } else {
    console.log("Grade: C");
  }
} catch (error) {
  console.error(error.message);
}

Summary

By preventing mistakes in conditional expressions and using debugging and error handling techniques, you can write safer and more robust code.

7. FAQ: Answering Common Questions

In this section, we address common questions and misconceptions readers often have about the JavaScript else if statement.

Q1. What is the difference between “else if” and “if”?

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

Q2. How many times can you use “else if”?

There is no limit, but if you have many conditions, consider using a switch statement.

Q3. What is the difference between “else if” and the ternary operator?

let score = 75;
console.log(score >= 60 ? "Passed!" : "Failed.");

8. Conclusion: Master JavaScript “else if” Statements

In this article, we covered everything from the basics to advanced examples of JavaScript conditional branching using the else if statement.

Key Learning Points

  • Basic syntax: Learned how conditional expressions control program flow.
  • Practical examples: Strengthened applied skills through real-world code.
  • Comparison: Understood how to choose between else if and switch.
  • Error handling: Learned debugging techniques to prevent mistakes.

Next Steps

  • Using logical operators: Learn more complex conditional expressions.
  • Ternary operators and short-circuit evaluation: Simplify conditions.
  • Combining conditionals with functions: Improve code reusability.

Master the else if statement and start writing practical, real-world JavaScript programs!

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