If/Else & Comparison Operators

Making Decisions with if/else

Programs need to make decisions. The if statement is the most fundamental decision-making tool. It evaluates a condition and executes code based on whether that condition is truthy or falsy.

// Basic if statement
let temperature = 25;

if (temperature > 30) {
  console.log("It's hot!");
}
// Output: nothing — condition is false

// if/else
if (temperature > 30) {
  console.log("It's hot!");
} else {
  console.log("It's comfortable.");
}
// Output: "It's comfortable."

// if/else if/else — multiple conditions
if (temperature > 35) {
  console.log("Extreme heat warning!");
} else if (temperature > 28) {
  console.log("It's hot.");
} else if (temperature > 20) {
  console.log("Nice weather!");
} else if (temperature > 10) {
  console.log("A bit chilly.");
} else {
  console.log("Bundle up!");
}

Comparison Deep Dive

// Always use strict equality ===
let age = "25";  // string from user input

age === 25;   // false — different types (string vs number)
age == 25;    // true  — loose equality coerces "25" to 25

// This is why == causes bugs:
"" == false;   // true  — surprising!
0 == false;    // true  — surprising!
null == undefined; // true — sometimes useful

// Real-world gotcha — form inputs return strings:
const form = { age: "25" };
if (form.age > 17) {  // works because > coerces
  console.log("Adult");
}
// Better — explicitly convert:
if (parseInt(form.age) >= 18) {
  console.log("Adult");
}

// Comparing objects — always false (different references)
{} === {};    // false (different objects in memory)
[] === [];    // false (different arrays in memory)
const a = [1,2,3];
const b = a;
a === b;      // true (same reference!)

Practical if/else Patterns

// Guard clauses — return early instead of nesting
function processUser(user) {
  // Bad — deeply nested:
  if (user) {
    if (user.isActive) {
      if (user.hasPermission) {
        // actual code buried 3 levels deep
        return doSomething(user);
      }
    }
  }
}

// Good — guard clauses, flat code:
function processUser(user) {
  if (!user) return null;
  if (!user.isActive) return { error: "User inactive" };
  if (!user.hasPermission) return { error: "No permission" };
  
  return doSomething(user);  // happy path at end
}

// Checking multiple conditions
const isAdult = age >= 18;
const hasID = true;
const hasMoney = balance > 0;

if (isAdult && hasID && hasMoney) {
  console.log("Entry allowed");
} else if (!isAdult) {
  console.log("Must be 18+");
} else if (!hasID) {
  console.log("Need valid ID");
} else {
  console.log("Insufficient funds");
}