JavaScript: Conditionals

JavaScript Conditional Statements

javascript
conditionals
control-flow
JavaScript Conditionals - if/else, Ternary Operator, and Switch Statements
Author

Pedro GeoGIS

Published

Monday, January 13, 2025

Modified

Sunday, November 16, 2025

1 Overview

📘 Conditionals

Conditional statements allow you to execute different code blocks based on different conditions. JavaScript provides several ways to implement conditional logic: the if statement for basic conditions, else if for multiple conditions, the ternary operator ? for concise conditional assignments, and the switch statement for handling multiple specific values. These control structures are fundamental for creating dynamic and responsive programs.

2 The “if” Statement

The if statement evaluates a condition in parentheses and, if the result is true, executes a block of code.

let year = prompt('In which year was ECMAScript-2015 specification published?', '');

if (year == 2015) {
  alert('You are right!');
}

2.1 Multiple Statements

If you want to execute more than one statement, wrap your code block inside curly braces:

if (year == 2015) {
  alert("That's correct!");
  alert("You're so smart!");
}
Tip

Best Practice: Always use curly braces {} even if there is only one statement. This improves readability and prevents errors.

// Good
if (condition) {
  doSomething();
}

// Avoid
if (condition) doSomething();

3 Boolean Conversion

The if (...) statement evaluates the expression in its parentheses and converts the result to a boolean.

Falsy values (become false): - 0 - "" (empty string) - null - undefined - NaN

Truthy values (become true): - All other values

3.1 Examples

if (0) {
  // This code never executes (0 is falsy)
}

if (1) {
  // This code always executes (1 is truthy)
}

if ("hello") {
  // This code executes (non-empty string is truthy)
}

4 The “else” Clause

The if statement may contain an optional else block. It executes when the condition is falsy.

let year = prompt('In which year was the ECMAScript-2015 specification published?', '');

if (year == 2015) {
  alert('You guessed it right!');
} else {
  alert('How can you be so wrong?');
}

5 Several Conditions: “else if”

Sometimes we need to test several variants of a condition. The else if clause lets us do that.

let year = prompt('In which year was the ECMAScript-2015 specification published?', '');

if (year < 2015) {
  alert('Too early...');
} else if (year > 2015) {
  alert('Too late');
} else {
  alert('Exactly!');
}

There can be multiple else if blocks. The final else is optional.

6 Conditional (Ternary) Operator ‘?’

Sometimes we need to assign a variable depending on a condition. The ternary operator provides a shorter and simpler way.

6.1 Syntax

let result = condition ? value1 : value2;
  • The condition is evaluated
  • If it’s truthy, value1 is returned
  • Otherwise, value2 is returned

6.2 Example

let age = prompt('How old are you?', '');
let accessAllowed = (age > 18) ? true : false;

alert(accessAllowed);

Since the comparison itself returns true/false, we can simplify:

let accessAllowed = age > 18; // same result

6.3 Multiple ‘?’

A sequence of question mark operators can return a value that depends on more than one condition:

let age = prompt('age?', 18);

let message = (age < 3) ? 'Hi, baby!' :
  (age < 18) ? 'Hello!' :
  (age < 100) ? 'Greetings!' :
  'What an unusual age!';

alert(message);

This is equivalent to:

if (age < 3) {
  message = 'Hi, baby!';
} else if (age < 18) {
  message = 'Hello!';
} else if (age < 100) {
  message = 'Greetings!';
} else {
  message = 'What an unusual age!';
}
Warning

Use ternary operator for assignment, not replacement of if

The ternary operator is great for conditional assignments but can reduce readability when used for complex logic. Use if/else for better clarity in such cases.

7 The “switch” Statement

The switch statement can replace multiple if checks. It provides a more descriptive way to compare a value with multiple variants.

7.1 Syntax

switch(x) {
  case 'value1':  // if (x === 'value1')
    ...
    break;
  case 'value2':  // if (x === 'value2')
    ...
    break;
  default:
    ...
    break;
}

7.2 How it Works

  1. The value of x is checked for strict equality (===) with the value from the first case
  2. If equality is found, switch executes the code starting from that case until the nearest break
  3. If no case is matched, the default code is executed (if it exists)

7.3 Example

let a = 2 + 2;

switch (a) {
  case 3:
    alert('Too small');
    break;
  case 4:
    alert('Exactly!');
    break;
  case 5:
    alert('Too big');
    break;
  default:
    alert("I don't know such values");
}

7.4 The Importance of break

If there is no break, execution continues with the next case without any checks (fall-through):

let a = 2 + 2;

switch (a) {
  case 3:
    alert('Too small');
  case 4:
    alert('Exactly!');
  case 5:
    alert('Too big');
  default:
    alert("I don't know such values");
}

This will output: - “Exactly!” - “Too big” - “I don’t know such values”

7.5 Grouping Cases

Several variants of case which share the same code can be grouped:

let a = 3;

switch (a) {
  case 4:
    alert('Right!');
    break;
    
  case 3:  // grouped two cases
  case 5:
    alert('Wrong!');
    alert("Why don't you take a math class?");
    break;
    
  default:
    alert('The result is strange. Really.');
}

Now both 3 and 5 show the same message.

7.6 Type Matters

The equality check in switch is always strict (===). The values must be of the same type to match.

let arg = prompt("Enter a value?");

switch (arg) {
  case '0':
  case '1':
    alert('One or zero');
    break;
    
  case '2':
    alert('Two');
    break;
    
  case 3:
    alert('Never executes!'); // prompt returns string, not number
    break;
    
  default:
    alert('An unknown value');
}

8 Comparison: if vs switch

8.1 Use if/else when:

  • You have complex conditions
  • You need to check ranges (e.g., age > 18)
  • You have boolean logic

8.2 Use switch when:

  • You have multiple specific values to check
  • You want cleaner code for many equality checks
  • You need to group similar cases

9 Examples

9.1 Example 1: Grade Calculator

let score = prompt('Enter your score (0-100):', '');
score = Number(score);

if (score >= 90) {
  alert('Grade: A');
} else if (score >= 80) {
  alert('Grade: B');
} else if (score >= 70) {
  alert('Grade: C');
} else if (score >= 60) {
  alert('Grade: D');
} else {
  alert('Grade: F');
}

9.2 Example 2: Day of Week

let day = prompt('Enter a day number (1-7):', '');
day = Number(day);

switch (day) {
  case 1:
    alert('Monday');
    break;
  case 2:
    alert('Tuesday');
    break;
  case 3:
    alert('Wednesday');
    break;
  case 4:
    alert('Thursday');
    break;
  case 5:
    alert('Friday');
    break;
  case 6:
  case 7:
    alert('Weekend!');
    break;
  default:
    alert('Invalid day number');
}

9.3 Example 3: Ternary Operator

let age = 20;

// Simple ternary
let status = (age >= 18) ? 'Adult' : 'Minor';
console.log(status); // Adult

// Nested ternary
let category = (age < 13) ? 'Child' :
               (age < 20) ? 'Teenager' :
               (age < 60) ? 'Adult' : 'Senior';
console.log(category); // Adult

9.4 Example 4: Login System

let username = prompt('Enter username:', '');
let password = prompt('Enter password:', '');

if (username === 'admin' && password === 'secret123') {
  alert('Welcome, Admin!');
} else if (username === 'admin') {
  alert('Wrong password!');
} else if (password === 'secret123') {
  alert('Wrong username!');
} else {
  alert('Invalid credentials!');
}

9.5 Example 5: Calculator with Switch

let num1 = Number(prompt('Enter first number:', ''));
let operator = prompt('Enter operator (+, -, *, /):', '');
let num2 = Number(prompt('Enter second number:', ''));

let result;

switch (operator) {
  case '+':
    result = num1 + num2;
    break;
  case '-':
    result = num1 - num2;
    break;
  case '*':
    result = num1 * num2;
    break;
  case '/':
    if (num2 !== 0) {
      result = num1 / num2;
    } else {
      alert('Cannot divide by zero!');
      result = 'Error';
    }
    break;
  default:
    alert('Invalid operator!');
    result = 'Error';
}

if (result !== 'Error') {
  alert(`Result: ${num1} ${operator} ${num2} = ${result}`);
}

9.6 Example 6: Season Checker

let month = prompt('Enter month (1-12):', '');
month = Number(month);

switch (month) {
  case 12:
  case 1:
  case 2:
    alert('Winter');
    break;
  case 3:
  case 4:
  case 5:
    alert('Spring');
    break;
  case 6:
  case 7:
  case 8:
    alert('Summer');
    break;
  case 9:
  case 10:
  case 11:
    alert('Fall');
    break;
  default:
    alert('Invalid month!');
}

10 Summary

  • Use if to test conditions and execute code blocks
  • Use else to handle the opposite condition
  • Use else if to test multiple conditions sequentially
  • The ternary operator ? : is a concise way to assign values based on conditions
  • Use switch for multiple equality checks against a single value
  • switch uses strict equality (===)
  • Always use break in switch unless you want fall-through behavior
  • Group similar case statements to share code
  • Choose if/else for complex conditions, switch for multiple specific values
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