Javascript/ES2015，计算出现次数并存储为数组中的对象

Question

I am extracting data from a JSON feed and am trying to count the number of times each name (name_class) occurs.

I want to return an array of objects in the following format, where count refers to the number of times the name appears:

myArray = [{name:"John", count: 5}, {name: "Sarah", count: 2}, {name: "Oliver", count: 3}];

So far I have the following, but this does not count the number of occurrences, it simply adds another object to the array;

   let namestUi = {
    renderNames(names){
        var counter2 = [];
        let namesFound = names.map((name) => {
          let {name_class} = name;

            if(counter2.name == name_class){
              counter2.count++;
            } else { counter2.push({name: name_class, count: 1}); }
        });
        console.log(counter2);
        return namesFound;
      }
    };

This is building the array of objects but not counting the number of occurrences.

Answer 1

function count(names){
    // Count occurrences using a map (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map)
    let map = new Map();
    for (let name of names) {
        map.set(name, (map.get(name) || 0) + 1);
    }

    // Transform to array of { name, count }
    let result = [];
    for (let [name, count] of map) {
        result.push({name: name, count: count});
    }

    return result;
}

count(["a", "b", "c", "a", "b", "a"])
// [{"name":"a","count":3},{"name":"b","count":2},{"name":"c","count":1}]

Answer 2

There are some problems in how you use your counter variable. You access properties as if it is a plain object, but you defined it as an array. Also your map callback function is not returning any value.

You could use this code which uses a Map to key counters by name, and then applies Array.from to convert that Map to an array of objects as you requested:

 let namestUi = { renderNames(names){ return Array.from( names.reduce( (counters, {name_class}) => counters.set(name_class, (counters.get(name_class) || 0) + 1), new Map() ), ([name, count]) => ({name, count}) ); } }; let result = namestUi.renderNames([ { name_class: "John" }, { name_class: "John" }, { name_class: "Frank" }, { name_class: "John" }, ]); console.log(result);