lamda.js

const addNumers = (a, b) => a + b;

// Takes the values of an array and returns the total. Demonstrates simple
// recursion.
const totalForArray = (arr, currentTotal) => {
    currentTotal = addNumers(currentTotal + arr.shift());

    if (arr.length) {
        return totalForArray(currentTotal, arr);
    } else {
        return currentTotal;
    }
};

// Or you could just use reduce!
const totalForArray = arr => arr.reduce(addNumers);

// Should really be called divideTwoNumbers
const average = (total, count) => count / total;

const averageForArray = arr => average(arr.length, totalForArray(arr));

// Gets the value associated witht the property of an object. Intended for
// use with a collection method like map, hence the generator.
const getItem = propertyName => item => item[propertyName];