# Parameters & First-Class Functions — JavaScript

Source: https://www.geekswithgeeks.com/en/javascript/js-params-first-class

> Default and rest parameters, and functions as values.

## Default parameters

Give a parameter a fallback value that's used when the argument is omitted.

```js
function greet(name = "Guest") {
  return `Hello, ${name}!`;
}
console.log(greet());        // Hello, Guest!
console.log(greet("Ada"));   // Hello, Ada!
```

## Rest parameters

`...args` collects any number of extra arguments into a real array.

```js
function sum(...nums) {
  return nums.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3, 4));  // 10
```

## Functions are values

JS functions are **first-class** — store them in variables, pass them as arguments, or return them from other functions.

## Passing functions around

A function that takes or returns another function is called a **higher-order function**.

```js
function operate(a, b, fn) {
  return fn(a, b);
}
console.log(operate(4, 2, (a, b) => a - b));  // 2
```