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## What it is

> Default parameters let you set a value for a function parameter **right in its definition**,
> which is used when that argument is missing at the call site or equals `undefined`.

---

### Example without default parameters

```javascript
function greet(name) {
  console.log(`Hello, ${name}!`);
}

greet("Alice"); // Hello, Alice!
greet();        // Hello, undefined!
```

If the argument is not passed, `name` equals `undefined`.

---

### Example with a default parameter

```javascript
function greet(name = "guest") {
  console.log(`Hello, ${name}!`);
}

greet("Alice"); // Hello, Alice!
greet();        // Hello, guest!
```

Now, if the argument is not passed,
the function takes the default value `"guest"`.

---

## How it works

A default parameter kicks in if the argument:

- **is not passed at all**, or
- **is explicitly equal to** `undefined`

```javascript
function test(a = 10) {
  console.log(a);
}

test();          // 10 (no argument)
test(undefined); // 10 (explicit undefined)
test(null);      // null (not undefined!)
```

---

## Expressions and functions can be used

The default value does not have to be a constant,
it can be **any expression**, even a call to another function.

```javascript
function getRandom() {
  return Math.floor(Math.random() * 10);
}

function printNumber(n = getRandom()) {
  console.log(n);
}

printNumber(); // random number from 0 to 9
printNumber(7); // 7
```

---

## Order and combining

Default parameters can be set **in any order**,
but most often go at the end of the argument list.

```javascript
function info(name, age = 18, city = "Kyiv") {
  console.log(`${name}, ${age} years old, from ${city}`);
}

info("Alice");            // Alice, 18 years old, from Kyiv
info("Bob", 25, "Lviv");  // Bob, 25 years old, from Lviv
```

If you skip a value that is not at the end, you must explicitly pass `undefined`:

```javascript
info("Oleh", undefined, "Warsaw");
// Oleh, 18 years old, from Warsaw
```

---

## Other parameters can be used as default values

```javascript
function sum(a, b = a) {
  return a + b;
}

console.log(sum(5));    // 10
console.log(sum(5, 2)); // 7
```

---

## Difference from the old approach (pre-ES6)

Previously, programmers wrote an "emulation" using `||`:

```javascript
function greet(name) {
  name = name || "guest";
  console.log(`Hello, ${name}!`);
}
```

But this had a drawback:

```javascript
greet(""); // Hello, guest! - even though the string is empty, not undefined
```

Default parameters are better because they only kick in **for** `undefined`,
not for "falsy" values like `0`, `""` or `false`.

---

## Remember in short

| Property | Description |
|---|---|
| When it kicks in | If the argument is not passed or equals `undefined` |
| Value type | Any expression, number, string, function, etc. |
| Can reference previous parameters | Yes |
| Works in arrow functions | Yes |
| Does not replace `null`, `0`, `''` | No, they count as passed values |

---

## Example to remember

```javascript
function connect(host = "localhost", port = 3000) {
  console.log(`Connecting to ${host}:${port}`);
}

connect(); // localhost:3000
connect("api.example.com"); // api.example.com:3000
connect("api.example.com", 8080); // api.example.com:8080
```

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