stateParks.js

const parks = [
    {
        name: 'Canyonlands',
        areaInSquareKm: 1366.2,
        location: { state: 'Utah' },
    },
    {
        name: 'Crater Lake',
        areaInSquareKm: 741.5,
        location: { state: 'Oregon' },
    },
    {
        name: 'Zion',
        areaInSquareKm: 595.9,
        location: { state: 'Utah' },
    },
];

//create function, parkNameAndState(parks), use reduce method
//return an object where the keys are the name of the states
// and the values will be an array of each park associated with that state
function parkNameAndState(parks) {
    return parks.reduce(function (stateParks, park) {        
        if (!stateParks[park.location.state]) { // for the first occurrence of a state we have to create a new array
            stateParks[park.location.state] = new Array();
        }        
        stateParks[park.location.state].push(park.name); // push the park name to our array with where key is state
        return stateParks;
    },
        [] // initialize stateParks as empty array
    );
}

// proof of concept
console.log(parkNameAndState(parks));

fyi @alexwhitmore