# Truthy and Falsy Values

In JavaScript, a **truthy** value is a value that is considered `true` when encountered in a Boolean context. All values are truthy unless they are defined as **falsy** (i.e., except for `false`, `0`, `-0`, `0n`, `""`, `null`, `undefined`, and `NaN`).

## 1. Falsy Values

There are only 8 falsy values in JavaScript. If you memorize these, you know that **everything else** is truthy.

1.  `false`: The boolean keyword.
2.  `0`: The number zero.
3.  `-0`: The number negative zero.
4.  `0n`: BigInt zero.
5.  `""` (or `''`, ``): Empty string.
6.  `null`: The absence of any value.
7.  `undefined`: The primitive value.
8.  `NaN`: Not a Number.

```javascript
if (0) {
  // This code will NOT run
}
```

## 2. Truthy Values

Everything that is not on the list above is truthy. This includes some values that might seem "empty" or "false" in other languages.

*   `"0"` (a string containing zero)
*   `"false"` (a string containing the text "false")
*   `[]` (an empty array)
*   `{}` (an empty object)
*   `function() {}` (an empty function)

```javascript
if ([]) {
  console.log("Empty arrays are truthy!"); // This runs
}

if ("0") {
  console.log("String '0' is truthy!"); // This runs
}
```

## 3. Logical Operators and Short-Circuiting

Truthy and falsy values are crucial when using logical operators `||` (OR) and `&&` (AND).

### OR (`||`)
Returns the first **truthy** value, or the last value if all are falsy.

```javascript
const name = "";
const displayName = name || "Guest"; 
console.log(displayName); // "Guest" (because "" is falsy)
```

### AND (`&&`)
Returns the first **falsy** value, or the last value if all are truthy.

```javascript
const isLoggedIn = true;
const user = { name: "Alice" };

// If isLoggedIn is truthy, return user.name
const userName = isLoggedIn && user.name; 
console.log(userName); // "Alice"
```

## 4. Double Bang (`!!`)

You can convert any value to its boolean equivalent using the double NOT operator (`!!`).

```javascript
console.log(!!"hello"); // true
console.log(!!0);       // false
console.log(!!{});      // true
```

[[programming/javascript/vanilla/javascript]]
[[programming/javascript/vanilla/type-coercion]]