Pure function

A function has to always return the same output, given the same input and a function cannot modify outside by itself and no side effects.

Ex 1:
// side effect (data modified outside the function)
const array = [1, 2, 3, 4, 5];
function mutateArray(arr) {
    arr.pop();
}
mutateArray(array); // [1, 2, 3, 4];
mutateArray(array); // [1, 2, 3];

// no side-effect
const array = [1, 2, 3, 4, 5];
function removeLastItem(arr) {
    const newArr = [].concat(arr);
    newArr.pop();
    return newArr;
}
const newArr = removeLastItem(array);
console.log(newArr); // [1, 2, 3, 4];
console.log(array); // [1, 2, 3, 4, 5]; // original data is not affected
// immutable object

Ex 2:
// input --> output
function sum(num1, num2) {
    return num1 + num2;
}

function multiplyBy2(num) {
    return num * 2;
}
// the output is predectable

Every function can't be written as a pure function, because we have difference in data handling things.

It handles with,

• 1 Task

• Predictable

• Immutable state

• No Shared state

• Pure,

• return statement

• Composable

Idempotence:

The function return the same output how many time you call the function, even it is contact with outside environment. The code output is predictable.

Ex 1:
function notGood(num) {
    console.log(num);
}
notGood(5); // 5

Ex 2:
Math.abs(-50); // 50

How many times we going to call, it is going to return the same output.