uniqBy.js

function uniqBy(iter, mapFn) {
    var map = new Map();
    for (let item of iter) {
        let key = mapFn(item);

        // Use the first given key identifier
        if (map.has(key)) {
            continue;
        }

        map.set(key, item);
    }

    return map;
}

// Tests

var obj1 = {
    id: 1,
    name: 'Leo'
};

var obj2 = {
    id: 2,
    name: 'Felipe'
};

var obj3 = {
    id: 1,
    name: 'Jaydson'
};

var obj4 =  {
    id: 3,
    name: 'Leo'
};

var arr = [obj1, obj2, obj3, obj4, obj1];

var result;

result = uniqBy(arr, x => x.id);

console.log(Array.from(result.values()));

// expects:
// [
//     { id: 1, name: 'Leo' },
//     { id: 2, name: 'Felipe' },
//     { id: 3, name: 'Leo' }
// ]