lodash: count values from array of objects

Question

I have an array of objects like this:

[
{"question":"Q1","answer":"my answer 2"},
{"question":"Q1","answer":"my answer"}
{"question":"Q1","answer":"my answer"}
{"question":"Q2","answer":"answer 2"}
]

I would like to group by the question keys and return the counts of each answer.

e.g.

{
    "Q1": [{
        "answer": "my answer",
        "count": 2
    }, {
        "answer": "my answer 2",
        "count": 1
    }],
    "Q2": [{
        "answer": "answer 2",
        "count": 1
    }]
}

,

I am able to groupBy questions using: .groupBy("question") and count occurances of values using .countBy() but I am not sure how to combine the grouping and counting functions to achieve the desired output?

Sure, although I am using lodash in this project would be interesting to compare how it could be done without. — Gids
– Gids, Commented May 16, 2016 at 11:34

filype · Accepted Answer · 2016-05-16 10:49:29Z

7

You can start with _.groupBy(array, 'question') - then use .map

For example:

var arr = [
{"question":"Q1","answer":"my answer 2"},
{"question":"Q1","answer":"my answer"},
{"question":"Q1","answer":"my answer"},
{"question":"Q2","answer":"answer 2"}
];

var result = _(arr)
  .groupBy('question')
  .map(function(item, itemId) {
    var obj = {};
    obj[itemId] = _.countBy(item, 'answer')
    return obj
  }).value();

console.log(JSON.stringify(result, null, 2));

See the working version at: http://jsbin.com/wixoronoqi/edit?js,console

answered May 16, 2016 at 10:49

filype

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3 Comments

ryeballar Over a year ago

I think it would be better to use mapValues() instead of map(), and just return the countBy() value directly.

filype Over a year ago

I wasn't aware of mapValues, but yes, it would be an improvement to use mapValues

tokland Over a year ago

With ES6: var obj = {}; obj[itemId] = _.countBy(item, 'answer'); return obj -> return {[itemId]: _.countBy(item, 'answer')}.

Gael D. · Accepted Answer · 2020-02-13 21:18:11Z

Here is a native/vanilla js solution to this problem, using Array.reduce(), with and without the spread operator.

With spread operator, immutable style :

const questions = [
  {"question":"Q1","answer":"my answer 2"},
  {"question":"Q1","answer":"my answer"},
  {"question":"Q1","answer":"my answer"},
  {"question":"Q2","answer":"answer 2"}
];

const groupedQuestions = questions.reduce( (g, q) => {
  return {
    ...g,
    [q.question]: {
      ...g[q.question],
      [q.answer] : (g[q.question] && g[q.question][q.answer] || 0) + 1
    }
  }
}, {})

document.write(JSON.stringify(groupedQuestions))

Without spread operator :

const questions = [
  {"question":"Q1","answer":"my answer 2"},
  {"question":"Q1","answer":"my answer"},
  {"question":"Q1","answer":"my answer"},
  {"question":"Q2","answer":"answer 2"}
];

const groupedQuestions = questions.reduce( (g, q) => {
  typeof g[q.question] !== "undefined" || (g[q.question] = {});
  typeof g[q.question][q.answer] !== "undefined" || (g[q.question][q.answer] = 0);
  g[q.question][q.answer] += 1;
  return g;
}, {})

document.write(JSON.stringify(groupedQuestions))

RomanPerekhrest · Accepted Answer · 2016-05-16 11:43:07Z

A "native javascript" solution using Array.forEach and Array.push functions:

var arr = [{"question":"Q1","answer":"my answer 2"},{"question":"Q1","answer":"my answer"}, {"question":"Q1","answer":"my answer"}, {"question":"Q2","answer":"answer 2"}];

var result = {};
arr.forEach(function(v){
    var key = v['question'], Q = this[key], found = false;
    if (Q) {
        var len = Q.length;
        while (len--) {
            if (Q[len]['answer'] === v['answer']) {
                Q[len]['count']++;
                found = true;
            }
        }
        if (!found) Q.push({'answer': v['answer'], 'count' : 1});
    } else {
        this[key] = [];
        this[key].push({'answer': v['answer'], 'count' : 1});
    }
}, result);

console.log(JSON.stringify(result, 0, 4));

The output:

{
    "Q1": [
        {
            "answer": "my answer 2",
            "count": 1
        },
        {
            "answer": "my answer",
            "count": 2
        }
    ],
    "Q2": [
        {
            "answer": "answer 2",
            "count": 1
        }
    ]
}

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