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

As you can see, the implementation is straightforward: it’s just two wrappers.

• The result of curry(func) is a wrapper function(a).
• When it is called like curriedSum(1), the argument is saved in the Lexical Environment, and a new wrapper is returned function(b).
• Then this wrapper is called with 2 as an argument, and it passes the call to the original sum.

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

Currying? What for?

To understand the benefits we need a worthy real-life example.

For instance, we have the logging function log(date, importance, message) that formats and outputs the information. In real projects such functions have many useful features like sending logs over the network, here we’ll just use alert:

function log(date, importance, message) {
  alert(`[${date.getHours()}:${date.getMinutes()}] [${importance}] ${message}`);
}

Let’s curry it!

log = _.curry(log);

After that log works normally:

log(new Date(), "DEBUG", "some debug"); // log(a, b, c)

…But also works in the curried form:

log(new Date())("DEBUG")("some debug"); // log(a)(b)(c)

Now we can easily make a convenience function for current logs:

// logNow will be the partial of log with fixed first argument
let logNow = log(new Date());

// use it
logNow("INFO", "message"); // [HH:mm] INFO message

Now logNow is log with fixed first argument, in other words “partially applied function” or “partial” for short.

We can go further and make a convenience function for current debug logs:

let debugNow = logNow("DEBUG");

debugNow("message"); // [HH:mm] DEBUG message

1. 1. 1. We didn’t lose anything after currying: log is still callable normally.
      2. We can easily generate partial functions such as for today’s logs.

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

The new curry may look complicated, but it’s actually easy to understand.

The result of curry(func) call is the wrapper curried that looks like this:

// func is the function to transform
function curried(...args){
       if (args.length >= func.length)
           { // (1)
               return func.apply(this, args);
           } else {
               return function pass(...args2) { // (2)
                   return curried.apply(this, args.concat(args2));
                }
           }
   };

When we run it, there are two if execution branches:

1. Call now: if passed args count is the same as the original function has in its definition (func.length) or longer, then just pass the call to it.
2. Get a partial: otherwise, func is not called yet. Instead, another wrapper pass is returned, that will re-apply curried providing previous arguments together with the new ones. Then on a new call, again, we’ll get either a new partial (if not enough arguments) or, finally, the result.

For instance, let’s see what happens in the case of sum(a, b, c). Three arguments, so sum.length = 3.

For the call curried(1)(2)(3):

1. The first call curried(1) remembers 1 in its Lexical Environment, and returns a wrapper pass.
2. The wrapper pass is called with (2): it takes previous args (1), concatenates them with what it got (2) and calls curried(1, 2) with them together. As the argument count is still less than 3, curry returns pass.
3. The wrapper pass is called again with (3), for the next call pass(3) takes previous args (1, 2) and adds 3 to them, making the call curried(1, 2, 3) – there are 3 arguments at last, they are given to the original function.

If that’s still not obvious, just trace the calls sequence in your mind or on paper.

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)).