JavaScript Tutorial ✦

What is a Pure Function?

Pure functions are the functions that follow 2 main rules:

1. It will give the same output for the same arguments always.
   Example:

   function test(a, b) {
     return a + b;  // 8 is the answer always when we pass 5 and 3
   }
   
   test(5, 3);  // always returns 8
   

2. It will not make any side effects.
   That means:

   • It will not modify any external variables (variables that are outside the function).
   • It will not access network (like calling an API).
   • It will not access file systems.
   • It will not manipulate the DOM.

   Example of a function not pure:

   let count = 0;
   
   function test() {
     count++;  // modifies external variable
     return count;
   }
   

   Example of a pure function:

   function test(count) {
     return count;  // does not change anything outside
   }
   

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How Pure Functions Become Useful:

• ✅ Predictable behavior – If you give the same input, you always get the same output. No surprises.
• ✅ Helps in functional programming – Because they are independent and don’t rely on anything outside.
• ✅ Easy to debug – Since there are no side effects, you don’t need to worry about other parts of code affecting it.

const add = (a, b) => a + b; // Pure function
let counter = 0;
const increment = () => counter++; // not a pure function