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Recursion / Function

🚀 Frontend DSA 7 - 🧮 Currying in JavaScript: Create a Sum Function with Infinite Calls Ending with ()

Currying is a powerful and elegant technique in JavaScript used to transform functions with multiple arguments into a sequence of functions, each taking a single argument.

In this guide, we’ll build a recursive curried sum function that allows you to call:

sum(1)(2)(3)(4)(5)(); // ➞ 15

Let’s understand how this magic works. 🧙‍♂️✨

✅ Goal

We want to define a function sum() such that:

sum(0)(1)(2)(3)(4)(5)(); // ✅ Output: 15

It should support:

  • Chainable calls: sum(1)(2)(3)...
  • Termination with an empty () call
  • Recursive addition of all arguments

🧠 JavaScript Currying Function – Final Code

function sum(x) {
  // Define a nested function that continues the chain
  function next(y) {
    // If y is passed (truthy), continue summing recursively
    if (y !== undefined) {
      return sum(x + y);  // recursive accumulation
    }
    return x;  // Termination: return the total sum
  }
  
  return next;
}

// 🧪 Test
console.log(sum(0)(1)(2)(3)(4)(5)());  // Output: 15
console.log(sum(5)(5)(5)());           // Output: 15
console.log(sum(10)());                // Output: 10

Problem Statement

🔍 Explanation

Step-by-step breakdown of sum(0)(1)(2)(3)(4)(5)():

  1. sum(0) returns a function next
  2. next(1) returns sum(0 + 1) → sum(1)
  3. Then sum(1)(2) → sum(3)
  4. ...
  5. Finally, the last call is sum(15)() where y === undefined
  6. It returns 15

⚠️ JavaScript Note

if (y)

In your original code, this could be problematic if one of the inputs is 0 (because 0 is falsy). A safer condition would be:

if (y !== undefined)

Otherwise, something like sum(0)(0)(1)(2)() would not behave correctly. ✅

💡 JavaScript Interview Insight

This type of problem is popular in JavaScript interviews to test:

  • Closures
  • Currying
  • Recursion
  • Function chaining
  • Edge case handling (0, undefined, null, etc.)

🧪 More Test Cases

console.log(sum(1)(2)(3)());            // 6
console.log(sum(5)(10)(-3)());          // 12
console.log(sum(0)(0)(0)(1)());         // 1
console.log(sum(100)(200)());           // 300

📦 Bonus: ES6 Arrow Version

const sum = x => y => y !== undefined ? sum(x + y) : x;

💬 Want More?

Would you like to:

  • Extend this to multiply values? 🤔
  • Collect values in an array instead of summing? 📊
  • Build a curried logger or validator function?

I can help with all of them — just say the word! 👇

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