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en/tutorials/js/4js-if-else

# JavaScript if/else & Truthy/Falsy

Conditionals let your code make decisions based on different situations.

## if Statement

Checks a condition and runs code only if it's true:

```javascript
let temperature = 30;

if (temperature > 25) {
  console.log("It's hot outside!");
}
```

## else Statement

Provides an alternative when the condition is false:

```javascript
let temperature = 15;

if (temperature > 25) {
  console.log("It's hot!");
} else {
  console.log("It's cool!");
}
```

## else if — Multiple Conditions

```javascript
let score = 85;
let grade;

if (score >= 90) {
  grade = "A";
} else if (score >= 80) {
  grade = "B";
} else if (score >= 70) {
  grade = "C";
} else {
  grade = "F";
}
```

JavaScript checks conditions in order. It runs the first true block and skips the rest.

## Truthy and Falsy Values

JavaScript treats non-boolean values as true or false:

```javascript
// Falsy — treated as false
false, 0, "", null, undefined, NaN

// Truthy — treated as true
true, 1, -1, "hello", " ", [], {}, function(){}
```

**Remember:** An empty array `[]` is truthy, even though it has no elements.

```javascript
let name = "John";

// Direct truthy check — cleaner
if (name) {
  console.log("Name exists");
}
```

## Ternary Operator

Short if/else for simple conditions:

```javascript
let age = 18;
let status = age >= 18 ? "Adult" : "Minor";
```

Format: `condition ? valueIfTrue : valueIfFalse`

## Complete Example: Login Check

```javascript
let username = null;
let isLoggedIn = false;
let isAdmin = false;

if (!username) {
  console.log("Please log in");
} else if (!isLoggedIn) {
  console.log("Account not active");
} else if (isAdmin) {
  console.log("Welcome, Admin!");
} else {
  console.log("Welcome, User!");
}
```

## Best Practices

1. **Use strict equality** — `===` prevents type coercion bugs
2. **Use early returns** — Reduce nesting
3. **Keep conditions simple** — Break complex logic apart
4. **Use ternary for simple cases** — Don't overcomplicate
5. **Understand truthy/falsy** — Prevents unexpected behavior

## Common Mistakes

1. **Using `==` instead of `===`** — Type coercion causes bugs
2. **Forgetting `else`** — No fallback case
3. **Deep nesting** — Hard to read and maintain
4. **Confusing truthy/falsy** — `0` and `""` are falsy
5. **Assignment in condition** — Use `===` not `=`

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