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Function composition in JavaScript

Educative Answers Team

Introduction

Function composition is an approach where the result of one function is passed on to the next function, which is passed to another until the final function is executed for the final result. Function compositions can be composed of any number of functions.

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Traditional approach

Traditionally, function composition follows this format:

const double = x => x * 2
const square = x => x * x
// Tradition approach
var output1 = double(2);
var output2 = square(output1);
console.log(output2);
// variant two
var output_final = square(double(2));
console.log(output_final);

In the code above, we can see that we need to call the double function followed by the square function to square a term that has been doubled. We can do this by either assigning the values individually in variables and calling functions onto them (lines 4 to 5), or we could use a more direct approach like on line 8.

Alternate approach

Another approach is to use the compose and pipe functions.

compose function

compose function takes any number of functions and invokes them all one after the other:

// function composition of any number of functions
const compose = (...fns) => x => fns.reduceRight((y, f) => f(y), x); 
const double = x => x * 2
const square = x => x * x

// function composition
var output_final = compose(square, double)(2);
console.log(output_final);

In the code above, we can see that the compose function is implemented using a general approach on line 2, so now it can take any number of functions. The output remains the same as before even with using this implementation.

pipe function

On the other hand, we can reverse the order of the function invocation by using the pipe function:

// function composition using pipe of any number of functions
const pipe = (...fns) => x => fns.reduce((y, f) => f(y), x); 
const double = x => x * 2
const square = x => x * x

// function pipe
var output_final = pipe(square, double)(2);
console.log(output_final);

In the code above, we can see that the pipe function is implemented using a general approach on line 2, so it can now take any number of functions. This is similar to the previous compose function, except that it uses reduce instead of the reduceRight method. The output is different in this case because the square function is invoked before the double function while, ​in our compose function, it was in the opposite order.

Libraries for function composition

Given the importance of function composition, we may use libraries that have already completed the implementation of both the compose and pipe functions. The following libraries might help.

These libraries can be used to avoid implementing either function so you can focus​ on the uses of it.

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