pure functions

A pure function is a function that adheres to these two rules:

1. Deterministic Behavior:
   • Given the same inputs, a pure function always returns the same output.
   • There are no variations or randomness in its behavior.
2. No Side Effects:
   • A pure function does not modify external state or variables.
   • It does not depend on external state that might change.

Characteristics of Pure Functions

• No reliance on global variables or mutable data.
• No modification of data outside the function’s scope.
• No input/output operations (e.g., logging, file handling) within the function.

How to Use Pure Functions in Your Code

1. Identify Side Effects: Refactor functions to avoid modifying external state.
2. Use Functional Programming Principles:
   • Favor immutability over mutability.
   • Avoid operations like logging, API calls, or DOM manipulation in pure functions.
3. Combine Pure Functions: Use tools like map, filter, and reduce to work with pure logic.

example of a pure function

let globalValue = 10;
 
// Impure function (modifies external state)
function impureMultiplyByTwo(x) {
    globalValue = globalValue * 2; // Side effect
    return x * 2;
}
 
// Pure function
function pureMultiplyByTwo(x) {
    return x * 2;
}
 
console.log(pureMultiplyByTwo(5)); // Output: 10
console.log(globalValue); // Remains unchanged at 10
 

Function Composition

Pure functions can be composed to build more complex operations.

const add5 = x => x + 5;
const multiplyBy3 = x => x * 3;
 
// Compose pure functions
const addThenMultiply = x => multiplyBy3(add5(x));
 
console.log(addThenMultiply(2)); // Output: 21 ((2 + 5) * 3)
 

Avoiding Mutability

const person = { name: "Alice", age: 25 };
 
// Pure function to update age
function updateAge(person, newAge) {
    return { ...person, age: newAge }; // Returns a new object
}
 
const updatedPerson = updateAge(person, 30);
 
console.log(updatedPerson); // Output: { name: "Alice", age: 30 }
console.log(person); // Original object remains unchanged: { name: "Alice", age: 25 }