How to Count Unique Value from Object of Array in JavaScript

const organisations = [
  { "id": 1, "name": "nameOne" },
  { "id": 2, "name": "nameTwo" },
  { "id": 3, "name": "nameOne" }
];

const uniqueItems = (list, keyFn) => list.reduce((resultSet, item) =>
    resultSet.add(typeof keyFn === 'string' ? item[keyFn] : keyFn(item)),
  new Set).size;

console.log(uniqueItems(organisations, 'name'));
console.log(uniqueItems(organisations, ({ name }) => name));

How do I count the number of unique elements in an array of objects by an object property?

Something like this might work:

function countTableId(orders){
    // sets are, well, sets of unique items. no duplicates allowed
    let uniqueId = new Set();
    for(let ord of orders){
        uniqueId.add(ord.table_id);
    }
    // the size of the set is the number of unique items we added
    return uniqueId.size;
}

You could do the same with the food_id property in another function.

@Felix King has a one-line way of doing this, so props to him. It uses the set object and Array.prototype.map()

let unique_table_id_count = new Set(orders.map(x => x.table_id)).size
let unique_food_id_count = new Set(orders.map(x => x.food_id)).size

.map() will loop through the array and look at each table_id, then it will try to add that to the set. The set object will discard an element if it already exists, so we only get unique values. Finally, because of chaining, we can just get the size of the set and we're done!

Count unique elements in array without sorting

A quick way to do this is to copy the unique elements into an Object.

var counts = {};
for (var i = 0; i < arr.length; i++) {
    counts[arr[i]] = 1 + (counts[arr[i]] || 0);
}

When this loop is complete the counts object will have the count of each distinct element of the array.

How to count all unique values in an array of objects?

You can use a Map to collect the dates that go with a name (check your spelling in the example data!), and then you can iterate those map entries to both get the dates-array length, and unique count (via Set):

const arr = [{name: "Edward",date: "24 March"},{name: "Banana",date: "12 March"},{name: "Edward",date: "23 March"},{name: "Edward",date: "23 March"}];

let map = new Map(arr.map(o => [o.name, []]));
for (let {name, date} of arr) map.get(name).push(date);
let result = Array.from(map.entries(), ([name, dates]) => 
    ({name, uniqueDate: new Set(dates).size, totalSubmit: dates.length})
);
console.log(result);

var distances = {};
$.map(electrons,function(e,i) {
   distances[e.distance] = (distances[e.distance] || 0) + 1;
});

var distances = {};
$.each(electrons,function(i,e) {
   distances[this.distance] = (distances[this.distance] || 0) + 1;
});

var distances = {},e;
for (var i = 0,l=electrons.length; i < l; i++) { 
    e = electrons[i];
    distances[e.distance] = (distances[e.distance] || 0) + 1;
}

var result = array.reduce( (acc, o) => (acc[o.name] = (acc[o.name] || 0)+1, acc), {} );

var array =    [        {"name":"abc","age":20},        {"name":"abc","age":20},        {"name":"abc","age":20},        {"name":"xyz","age":21},        {"name":"xyz","age":21}    ];    var result = array.reduce( (acc, o) => (acc[o.name] = (acc[o.name] || 0)+1, acc), {} );
console.log(result);

var flags = [], output = [], l = array.length, i;
for( i=0; i<l; i++) {
    if( flags[array[i].age]) continue;
    flags[array[i].age] = true;
    output.push(array[i].age);
}

var items = [{ title: "Trumping the Horns", value: 5.95 }, { title: "Rocking about", value: 20.00 }, { title: "Trumping the Horns", value: 5.95 }],    result = items.reduce(        (map => (r, a) => (!map.has(a.title) && map.set(a.title, r[r.push({ title: a.title, amount: 0 }) - 1]), map.get(a.title).amount++, r))(new Map),        []    );
console.log(result)