Pure and Impure functions

Pure function

A pure function is a function that follows these two rules:

• It does not have side effects like HTTP requests or data manipulation (when a function changes elements that are outside of its scope)
• It always returns the same output for the same input

Examples of side effects:

We could say that side effects are operations unrelated to calculating the final output:

• HTTP requests
• console.log
• DOM manipulation
• Mutation input data of a function
• Modifying data located outside of a function

Example of Pure function

const sum = (a, b) =>  { 
  return a + b; 
}

In this case, sum is a pure function because:

• It always returns the same result with the same pair of inputs (deterministic).
• It does not modify outside variables or states or affect the system in any observable way beyond returning a value (no side effects).

Impure function

An impure function is a function that has side effects. This means the function can modify data outside of its own scope, or interact with the external environment in a way that affects the program's state.

let counter = 0;

function incrementCounter() {
    counter += 1;  // Modifies the external variable 'counter'
    console.log(counter); // side effect
}

incrementCounter(); // 1
incrementCounter(); // 2

Summary

• A pure function is a function that follows these two rules: it does not have side effects, and it always returns the same output for the same input.
• Side effects are operations unrelated to calculating the final output inside the function.
• An impure function is a function that has side effects.