Left join

leftjoin.js

//Closed question on Stackoverflow
//https://stackoverflow.com/questions/62923537/create-new-object-using-two-objects-that-share-a-prop#

users = [
{userId: 1, name: "John", favoriteFruit: "Apple"},
{userId: 2, name: "Jerry", favoriteFruit: "Pear"},
{userId: 3, name: "Linda", favoriteFruit: "Peach"},
{userId: 4, name: "Cruz", favoriteFruit: "Banana"},
];

cars = [
{userId: 1, carModel: "GT200", color: "Blue"},
{userId: 1, carModel: "GT300", color: "Green"},
{userId: 2, carModel: "F200", color: "Black"},
{userId: 4, carModel: "F1200", color: "White"},
{userId: 4, carModel: "RT600", color: "Pink"},
{userId: 3, carModel: "GT200", color: "Yellow"},
];

function leftJoin(l, r, on) {
    const lkp = r.reduce((acc, cur) => {
        acc[cur[on]] = cur;
        return acc;
    }, {})
    return l.map((e) => {
        const from = lkp[e[on]];
        return {...e, ...from}
    })
}


const joined = leftJoin(cars, users, 'userId');
console.log (joined)