How to do Currying in JavaScript

Currying is a technique that transforms a function that takes multiple arguments into a sequence of functions that each take a single argument. Instead of calling a function with all its arguments at once, you call it with one argument which returns a new function expecting the next argument, and so on until all arguments are provided and the final result is computed.

A normal function add takes two numbers and returns their sum immediately when called with both. A curried version of add accepts the first number and returns a new function. That returned function accepts the second number and returns the sum. The curried version splits the original call into two separate calls, each providing one argument.

Currying works because of closures. When the outer function receives the first argument, it returns an inner function. That inner function closes over the first argument, keeping it alive in memory. When the inner function is called with the second argument, it has access to both the first argument from its closure and the second argument from the current call, and can use both to compute the result.

A generic curry utility takes any function and returns a curried version. Inside the utility, return a function called curried that collects arguments. If curried has received at least as many arguments as the original function expects, call the original function with all collected arguments. Otherwise, return a new function that when called combines the previous arguments with the new ones and calls curried again recursively.

Partial application is closely related to currying. It means calling a function with some of its arguments and getting back a new function that only needs the remaining arguments. With a curried function, partial application happens naturally every time you call with fewer arguments than required. You can create specialized functions by partially applying common arguments and saving the result.

The power of currying comes from creating specialized functions from general ones. For example, a general multiply function that is curried can be partially applied with the value two to create a double function. The double function only needs one more argument. You can also create a triple function by partially applying three. Both are derived from the same general multiply function without writing them separately.

Currying integrates naturally with function composition. When functions accept one argument and return one value, they are easy to chain together. A curried map function partially applied with a transformation can be composed with a curried filter partially applied with a predicate to create a data processing pipeline. Each step is reusable and the pipeline reads like a clear description of what it does.

All currying uses closures but not all closure usage is currying. Currying specifically refers to transforming an N-argument function into N single-argument functions. A factory function that returns a configured function uses closures but may not be currying if it does not follow the strict one-argument-at-a-time pattern. Understanding the distinction helps you communicate precisely and apply the right technique for each problem.