JavaScript: What is Currying ?

Currying is an advanced technique of working with functions. It’s used not only in JavaScript, but in other languages as well.

Currying is a transformation of functions that translates a function from callable as f(a, b, c) into callable as f(a)(b)(c).

Currying doesn’t call a function. It just transforms it.

Let’s see an example first, to better understand what we’re talking about, and then practical applications.

We’ll create a helper function curry(f) that performs currying for a two-argument f. In other words, curry(f) for two-argument f(a, b) translates it into a function that runs as f(a)(b):

function curry(f) {

// curry(f) does the currying transform

return function(a) { return function(b) { return f(a, b); }; }; }

// usage

function sum(a, b) { return a + b; }

let curriedSum = curry(sum);

alert( curriedSum(1)(2) ); // 3

More advanced implementations of currying, such as _.curry from lodash library, return a wrapper that allows a function to be called both normally and partially:
 

function sum(a, b) {
  return a + b;
}

let curriedSum = _.curry(sum); // using _.curry from lodash library

alert( curriedSum(1, 2) ); // 3, still callable normally
alert( curriedSum(1)(2) ); // 3, called partially

Advanced curry implementation

In case you’d like to get in to the details, here’s the “advanced” curry implementation for multi-argument functions that we could use above.It’s pretty short:
 

function curry(func) {

  return function curried(...args) {
    if (args.length >= func.length) {
      return func.apply(this, args);
    } else {
      return function(...args2) {
        return curried.apply(this, args.concat(args2));
      }
    }
  };
}

Usage examples:

function sum(a, b, c) {
  return a + b + c;
}

let curriedSum = curry(sum);

alert( curriedSum(1, 2, 3) ); // 6, still callable normally
alert( curriedSum(1)(2,3) ); // 6, currying of 1st arg
alert( curriedSum(1)(2)(3) ); // 6, full currying

Summary

Currying is a transform that makes f(a,b,c) callable as f(a)(b)(c). JavaScript implementations usually both keep the function callable normally and return the partial if the arguments count is not enough.

Currying allows us to easily get partials. As we’ve seen in the logging example, after currying the three argument universal function log(date, importance, message) gives us partials when called with one argument (like log(date)) or two arguments (like log(date, importance)).

Readmore: https://javascript.info/currying-partials#advanced-curry-implementation