# First-Class and Higher-Order Functions — Functional Programming Concepts

Source: https://www.geekswithgeeks.com/en/functional-programming/fn-higher

> Functions that take or return functions.

## Functions are ordinary values

A language has **first-class functions** when functions can be assigned to variables, stored in data structures, passed as arguments and returned from other functions. A **higher-order function** is one that takes a function as an argument or returns one. Familiar examples are `Array.prototype.map`, `filter`, `sort` with a comparator, `setTimeout` and event listeners. Higher-order functions let you separate the *what varies* (the function you pass) from the *what stays the same* (the iteration, retry or timing logic).

## Passing behaviour around

When functions are values, you can store them, pass them and return them, which is the basis of most FP techniques.

![Three ideas: higher-order functions, closures, and map/filter/reduce.](assets/figures/functional-programming/section-3-map.svg) — Figure 3.1 — Functions in, functions out, functions over collections.

## Taking and returning functions

A retry wrapper and a comparator factory.

```typescript
// Takes a function: generic retry logic, caller supplies the action
async function withRetry<T>(action: () => Promise<T>, attempts = 3): Promise<T> {
  let lastError: unknown;
  for (let i = 0; i < attempts; i++) {
    try { return await action(); } catch (e) { lastError = e; }
  }
  throw lastError;
}

// Returns a function: build comparators from a key
const by = <T>(key: (t: T) => number) =>
  (a: T, b: T) => key(a) - key(b);

type Item = { name: string; price: number };
const items: Item[] = [{ name: 'pen', price: 2 }, { name: 'book', price: 12 }];
const cheapestFirst = [...items].sort(by(i => i.price)); // copy, then sort
```

## A power drill and its bits

The drill (higher-order function) provides the motor; the bit you plug in (the passed function) decides whether it drills, screws or sands.

**Quiz:** Which is a higher-order function?

- [x] A function that accepts a callback as an argument
- [ ] Any function with more than three parameters
- [ ] A function declared at the top of a file
- [ ] A function that returns a number

*Answer:* A function that accepts a callback as an argument. Higher-order means it works with functions as values.