What Is A Currying Function In Javascript?

In this tutorial, we will see What Is A Currying Function In Javascript. Currying is an alteration of functions that turns a function from callable as f(a, b, c) into callable as f(a)(b)(c).

Currying Function In Javascript

A currying function is a function that takes multiple arguments and turns it into a sequence of functions having only one argument at a time.
let us see how it is done. So first let us consider the basic multiplication of 3 arguments.

// Normal definition
function multiply(a, b, c) {
    return a * b * c;
};
console.log(multiply(1, 2, 4));
// Output: 8

the above function can be converted into the curry function like:

// Simple curry function definition
function multiply(a) {
    return (b) => {
        return (c) => {
            return a * b * c;
        };
    };
};
console.log(multiply(1)(2)(4));
// Output: 8


// ***** Short Hand Code *****
const multiply = (a) => (b) => (c) => a * b * c;

In this way, an n-ary function becomes a unary function, and the last function returns the result of all the arguments together in a function.

Why should we create a currying function

To understand the benefits we need real-life example.
for example, we have a logging function log(date, importance, message) that formats and outputs the information. in a real-life project, such functions have many useful features like sending logs over the network.

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

lets curry it using lodash.

var curried = _.curry(log);

now our log can be work in two different ways.

// Normal Way : log(a, b, c)
log(new Date(), "DEBUGGING", "some message");
// Currying : log(a)(b)(c)
log(new Date())("DEBUGGING")("some message");

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

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

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

Now, logNow is a log with the 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("DEBUGGING");
debugNow("message"); // [HH:mm:yyyy] DEBUGGING message

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