Pure Function

purefunction.js

// pure function
// evaluates the same result given the same args
// doesn't depend on and doesn't modify the states of variables outside it's scope
// no side effects (mutations, async reqs)

var foods = ['beans', 'rice', 'pizza', 'pineapple']

// slice (shallow modified copy of array) vs splice (modifies original array)

foods.slice(0,3) //returns ['beans', 'rice', 'pizza']
foods.slice(0,3) //returns ['beans', 'rice', 'pizza']
foods.slice(0,3) //returns ['beans', 'rice', 'pizza'] - return predictable

foods.splice(0,3) //returns ['beans', 'rice', 'pizza']
foods.splice(0,3) //returns ['pineapple']
foods.splice(0,3) //returns [] - same args, different results. impure.