javascript count number values repeated in a jsonArray items

问题

i need to count the number of occurrences of values in jsonArray items in javascript. consider this as my jsonArray:

{"name":"jack","age":23},
{"name":"john","age":20},
{"name":"alison","age":23},
{"name":"steve","age":25},
.
.
.

now for example i need to know how many times each age is repeated in this array, i want to count each value for each property, i mean i need a result like this :

 name : {"jack" : 2,"james" : 10,"john" : 1,....}

 age : {"23":10,"20":15,"25":2,....}

what is the simplest way to achieve this?

EDIT : my array is very big and i don't want to call count function for each value. i want a code to count each value repeat times.

回答1:

you can have a look at Array.prototype.reduce

function mapToProp(data, prop) {
  return data
    .reduce((res, item) => Object
      .assign(res, {
        [item[prop]]: 1 + (res[item[prop]] || 0)
      }), Object.create(null))
  ;
}


const data = [
  {"name": "jack", "age": 23},
  {"name": "john", "age": 20},
  {"name": "alison", "age": 23},
  {"name": "steve", "age": 25}
];

console.log('mapAndCountByAge', mapToProp(data, 'age'))

回答2:

You could use an object, where the keys and the values are used as property for the count.

var array = [{ name: "jack", age:23 }, { 'name': "john", age: 20 }, { name: "alison", age: 23 }, { name: "steve", age: 25 }],
    result = {};

array.forEach(function (o) {
    Object.keys(o).forEach(function (k) {
        result[k] = result[k] || {};
        result[k][o[k]] = (result[k][o[k]] || 0) + 1;
    });
});

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

回答3:

Not as elegant as Nina's and Hitmands' answers, but this is easier to understand for beginners (in case any beginner is reading this).

var jsonArray = [
  {"name": "John", "age": 20},
  {"name": "John", "age": 21},
  {"name": "Doe", "age": 21},
  {"name": "Doe", "age": 23},
  {"name": "Doe", "age": 20},
];
var names = [],
    ages = [];

// separate name data and age data into respective array
// only need to access the original data once
jsonArray.forEach(function(val){
    names.push(val['name']);
    ages.push(val['age']);
});

function aggregate(array){
    var obj = {};

    array.forEach(function(val){
        if (!obj[val])
            // create new property if property is not found
            obj[val] = 1;
        else
            // increment matched property by 1
            obj[val]++;
    });
    
    return JSON.stringify(obj);
}

console.log("Names: " + aggregate(names));
console.log("Ages: " + aggregate(ages));

In my opinion, Nina has the best answer here.

EDIT

In fact, when presented with large dataset (as OP stated that "the array is very big"), this method is about 45% faster than Nina's answer and about 81% faster than Hitmand's answer according to jsben.ch. I don't know why though.

来源：https://stackoverflow.com/questions/44829459/javascript-count-number-values-repeated-in-a-jsonarray-items

标签

javascript

arrays

json