Conditionals

What are conditionals?

Conditionals let you make decisions in your code. They allow you to execute different code based on whether conditions are true or false. Without conditionals, programs would execute line by line without any decision-making.

if / else

The if statement lets you execute code conditionally; only when a condition is true.

Basic if statement

const age = 20;

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

If the condition age >= 18 is true, the code inside the braces runs. If false, it's skipped.

if / else

Add an else clause to handle the false case:

const age = 15;

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

if / else if / else

Chain multiple conditions with else if:

const score = 85;

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: F");
}
// Output: "Grade: B"

JavaScript evaluates conditions from top to bottom and executes only the first matching block.

Truthy and falsy in conditionals

Remember from the data types guide that JavaScript uses truthy/falsy values:

const name = "Alice";

if (name) {  // "Alice" is truthy
  console.log("Name exists");
}

const count = 0;
if (count) {  // 0 is falsy, so this won't run
  console.log("Has items");
}

// Be explicit when checking for specific values
if (count > 0) {  // Better: explicit check
  console.log("Has items");
}

Multiple conditions

Combine conditions with logical operators:

const age = 25;
const hasLicense = true;

if (age >= 18 && hasLicense) {  // Both must be true
  console.log("Can drive");
}

if (age < 13 || age > 65) {  // Either can be true
  console.log("Eligible for discount");
}

if (!hasLicense) {  // NOT operator
  console.log("Cannot drive");
}

switch

Use switch when you have multiple conditions based on the same value:

const day = "Monday";

switch (day) {
  case "Monday":
    console.log("Start of the week");
    break;
  case "Friday":
    console.log("Almost weekend!");
    break;
  case "Saturday":
  case "Sunday":
    console.log("Weekend!");
    break;
  default:
    console.log("Regular day");
}

Important: Always include break statements (except when you want fall-through behavior). Without break, execution "falls through" to the next case:

const day = "Monday";

switch (day) {
  case "Monday":
    console.log("Monday");  // This runs
    // Missing break!
  case "Tuesday":
    console.log("Tuesday");  // This also runs! (fall-through)
    break;
}

The default case handles values that don't match any case.

break in switch

As mentioned above, break is used in switch statements to prevent fall-through:

switch (value) {
  case 1:
    console.log("One");
    break;  // Exit switch
  case 2:
    console.log("Two");
    break;
}

Common patterns

Conditional assignment

const age = 20;
const status = age >= 18 ? "adult" : "minor";
// Equivalent to:
// let status;
// if (age >= 18) {
//   status = "adult";
// } else {
//   status = "minor";
// }

Early returns

In functions, use early returns to handle edge cases:

function processUser(user) {
  if (!user) {
    return null;
  }
  
  if (!user.email) {
    return null;
  }
  
  // Process user...
  return processedUser;
}

Nested conditionals

You can nest conditionals inside other conditionals:

const age = 25;
const hasLicense = true;

if (age >= 18) {
  if (hasLicense) {
    console.log("Can drive");
  } else {
    console.log("Need a license");
  }
} else {
  console.log("Too young to drive");
}

Choosing the right conditional

• Use if/else for simple conditionals based on different variables
• Use switch for conditionals based on a single value with multiple cases
• Use ternary operator (? :) for simple one-line conditionals
• Use early returns in functions to handle edge cases first