JavaScript Concept: Currying and its implementation in Redux

🔍 What is Currying?

Currying is a functional programming technique where a function with multiple arguments is transformed into a sequence of functions, each taking one argument at a time.

Instead of:

const add = (a, b) => a + b;

You curry it as:

const curriedAdd = (a) => (b) => a + b;

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📌 Why Use Currying?

• Reusability: You can create specialized versions of functions.
• Function composition: Useful in pipelines and functional chains.
• Cleaner syntax for functional operations.

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✅ Examples

1. Normal Function

const add = (a, b) => a + b;
console.log(add(2, 3)); // 5

2. Curried Function

const curriedAdd = (a) => (b) => a + b;
console.log(curriedAdd(3)(5)); // 8

3. Multi-level Curried Function

function curriedAdd2(a) {
  return function (b) {
    return function (c) {
      return a + b + c;
    };
  };
}
console.log(curriedAdd2(4)(6)(2)); // 12

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4. Real-Life Example: Calculating Discount

Normal:

const totalPrice = (amount, discount) => amount - amount * discount;
console.log(totalPrice(100, 0.3)); // 70

Curried:

const curriedTotalPrice = (discount) => (amount) => amount - amount * discount;
const withDiscount = curriedTotalPrice(0.3);

console.log(withDiscount(100)); // 70
console.log(withDiscount(200)); // 140
console.log(withDiscount(250)); // 175

This helps you reuse the same discount logic without repeating the discount value every time.

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🛠 Currying in Redux Middleware

Redux middleware often uses currying to handle the flow of store → next → action.

const logger = (store) => (next) => (action) => {
  console.group(action.type);
  console.info("previous state", store.getState());
  const result = next(action);
  console.info("next state", store.getState());
  console.groupEnd();
  return result;
};

export default logger;

This is a triple-curried function:

• (store) gives access to the state and dispatch
• (next) passes control to the next middleware or reducer
• (action) is the actual dispatched action

Used in store.ts like this:

import logger from "./middlewares/logger";

middleware: (getDefaultMiddleware) => getDefaultMiddleware().concat(logger);

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🧩 Summary

Concept	Explanation
Currying	Transforming a function so it takes arguments one at a time
Syntax	f(a, b) → f(a)(b)
Use Case	Reusability, configuration, partial application
Redux	Currying is used in middleware to access store, next, and action

--this article is written with the help of AI.