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How to write a function in JavaScript ?
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Pure Functions in JavaScript

Last Updated : 17 Dec, 2024
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A Pure Function is a function (a block of code) that always returns the same result if the same arguments are passed.

  • Pure functions return consistent results for identical inputs.
  • They do not modify external states or depend on mutable data.
  • Often used with immutable data structures to ensure reliability.
  • Their independence from external states makes them highly reusable.
JavaScript 
function add(a, b) {     return a + b; } console.log(add(2, 3));  console.log(add(2, 3));

Output
5 5
  • It always returns the same result for the same input.
  • It does not modify any external variables or state.

Note: Generally, we use the Pure function in JavaScript for any purpose.

Characteristics of Pure Functions

  • Deterministic Output: For a given set of inputs, the output is always the same.
  • No Side Effects: The function does not:
    • Modify variables outside its scope.
    • Interact with external systems like APIs, databases, or DOM manipulation.
    • Change mutable data.
  • Immutability: Pure functions do not change the input values; instead, they return a new value or object.

Example of a Function with Side Effects

Here, increment is not a pure function because it modifies the external variable count.

JavaScript 
let c = 0;  function inc() {     c++;     return c; } console.log(inc()); console.log(inc());

Output
1 2

Impure Functions: What to Avoid

Impure functions produce unpredictable results or affect external states, which can lead to bugs and make your code harder to test.

JavaScript 
let user = { name: "Meeta", age: 25 };  function updated(newAge) {     user.age = newAge;     return user; }  console.log(updated(26));  // Alters the global `user` object console.log(user.age);

Output
{ name: 'Meeta', age: 26 } 26

Real-World Applications of Pure Functions

  • Data Transformation: Use pure functions in map, filter, and reduce operations for processing data.
  • State Management: Libraries like Redux emphasize pure functions for state updates.
  • Unit Testing: Pure functions are ideal for unit tests because they have predictable outputs.
  • Performance Optimization: Pure functions are easily memoized, as their outputs depend solely on inputs.

Next Article
How to write a function in JavaScript ?

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Article Tags :
  • JavaScript
  • Web Technologies
  • javascript-functions

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