13. What is the currying function?

Currying Function in JavaScript

A currying function is a function that transforms a function with multiple arguments into a sequence of functions, each taking a single argument.

Example of a Curried Function

Instead of:

javascript
CopyEdit
function add(a, b) {
  return a + b;
}

console.log(add(2, 3)); // Output: 5

A curried version:

javascript
CopyEdit
function curriedAdd(a) {
  return function (b) {
    return a + b;
  };
}

console.log(curriedAdd(2)(3)); // Output: 5

Here, curriedAdd(2) returns a new function that expects b and then performs the addition.

Currying with Arrow Functions

A more concise version using arrow functions:

javascript
CopyEdit
const curriedMultiply = a => b => a * b;

console.log(curriedMultiply(3)(4)); // Output: 12

Each function takes one argument at a time and returns another function until all arguments are received.

Why Use Currying?

1. Reusability:

   javascript
   CopyEdit
   const addFive = curriedAdd(5);
   console.log(addFive(10)); // Output: 15
   

   addFive is now a reusable function.

2. Function Composition: Used in functional programming for building small, reusable functions.

3. Partial Application: Allows fixing some arguments while leaving others open.

Auto-Currying with Lodash

Lodash provides a curry function for automatic currying:

javascript
CopyEdit
const _ = require('lodash');

const curriedSum = _.curry((a, b, c) => a + b + c);
console.log(curriedSum(1)(2)(3)); // Output: 6
console.log(curriedSum(1, 2)(3)); // Output: 6
console.log(curriedSum(1, 2, 3)); // Output: 6

It allows flexible argument passing.

Conclusion

• Currying transforms a function into a chain of single-argument functions.
• Useful for reusability, function composition, and partial application.
• Common in functional programming and libraries like Lodash.

Would you like more examples or a real-world use case? 🚀