pure-functions.js

// pure function
// depends only on its input
// and produces only its output

const pureFunction = input => input + 1;

const pureFunction2 = function(input) {
    return input + 1;
};


// impure function
// depends on external state or changes it
// like databases, cookies, files, console...

let someNumber = 0;
const impureFunction = () => {
    someNumber = someNumber + 1;
}; // mutates someNumber creating a side effect

// pure functions are easier to unit test
// because you don't have to mock, proxy, or inject
const identity = a => a;
test('identity output is the same as input.', () => {
    expect(identity('7urtle')).toBe('7urtle');
});