What is currying and partial application?

Question

Accepted Answer

**Currying** transforms a function that takes N arguments into a chain of N functions that each take one argument. **Partial application** fixes *some* arguments now and returns a function waiting for the rest.

```js
// normal
const add = (a, b, c) => a + b + c;

// curried — one argument at a time
const curriedAdd = a => b => c => a + b + c;
curriedAdd(1)(2)(3); // 6

// partial application — pre-fill some args
const add10 = curriedAdd(10);   // a is fixed at 10
add10(5)(1);                     // 16
```

Each step uses a **closure** to remember the earlier arguments until all are supplied.

## Why it's useful: building specialized functions

```js
const log = level => message => console.log(`[${level}] ${message}`);
const error = log("ERROR");   // reusable, pre-configured logger
const info = log("INFO");
error("disk full");           // [ERROR] disk full
```

Instead of repeating the level everywhere, you create configured functions once. This shines in functional pipelines and event handlers:

```js
const onChange = field => e => setForm(f => ({ ...f, [field]: e.target.value }));
<input onChange={onChange("email")} />  // a handler specialized to "email"
```

## Generic curry / libraries

Libraries (Ramda, lodash `curry`) auto-curry any function so you can supply arguments in flexible groups.

## Why it matters

Currying enables clean **function composition**, reusable configured callbacks, and point-free style.

It's common in functional codebases and a frequent interview topic to test understanding of closures.