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Codecademy Computer Science

functional programming in javascript

In this article, we will learn about declarative pattern, pure function, Immutability and side effects.

What is Functional Programming

  • In computer science, functional programming is a programming paradigm or pattern (a style of building the structure and elements of computer programs)
  • Functional Programming treats computation as the evaluation of mathematical functions.
  • Functional Programming avoids changing-state and mutable data.

The above definition is taken from Wikipedia, now let’s try to understand the value and benefits of FP(functional programming) in this article.

Pure functions



Mathematical Function or Pure Function

In mathematics, a function is a relation between a set of inputs and a set of permissible outputs with the property that each input combination is related to exactly one output.

In functional programming, these kinds of functions called pure functions which only depends on the received input data to the function and does not alter the input data except the returned data.

Math.random() is not pure function because it always returns new value on each call.

Math.min(1,2) is an example of pure function which always returns same value with same set of inputs


Why Functional Programming

  • Its pure function, provides confidence of not changing things outside of its scope.
  • Its reduces the complexity, need not to worry about how it is doing it, focus will be only on what it is doing.
  • Ease of testing, because it does not depend on state of the application and result verification also will be easy.
  • It makes the code more readable.
  • Functional programming makes code easier to understand.


Immutability is important to make sure one function does not change the original data rather than should return new copy of the data after manipulation.

Like if Array and Objects are passing around multiple functions and we are not maintaining immutability then functions might not get the original copy of the array and object.

It is really hard to debug if something goes wrong in case of mutable object and array.

Mutability is not bad but it should be when to have it.

Immutable Libraries

JavaScript by-default does not provide anything to make the object and array Immutable. There are libraries which can help us to achieve the immutability.

  • Seamless-immutable
  • Immutable JS