Javascript - Efficient way to get distinct values from an array of objects

Question

I'm trying to categorize an array of questions I've gotten from our application's API, and I've come up with a method that works, but it doesn't feel awesome. Can anyone tell me if there's a better way to do this?

Here's the code I have now. I'm not too worried about making it super optimized, but it shouldn't end up slow.

$scope.patron.categories = questions.map(function(question, i) {
    // check if question has a new CategoryId
    if(i === 0 || (i > 0 && question.CategoryId !== questions[i-1].CategoryId)) {
        // put questions with the same CategoryId together
        var catQuestions = questions.filter(function(q) {
            return q.CategoryId === question.CategoryId;
        });

        return {
            id: question.CategoryId,
            name: question.Category,
            collapsed: true,
            valid: false,
            questions: catQuestions
        };
    }
}).filter(function(category) {
    // because Array.prototype.map doesn't remove these by itself
    return category !== undefined;
});

You can use a Set to efficiently store the set of unique values used to distinguish elements. As you'd expect, Set.add() will silently not create duplicate entries, and Set.has() scales better than Array.indexOf() — Touffy
– Touffy, Commented Oct 5, 2015 at 21:42

Buzinas · Accepted Answer · 2015-10-05 21:25:50Z

1

Although it's primarily opinion-based, I prefer a simple loop.

For sure it's better for performance, and I think it's better for readability also.

var catIds = [], cats = [];

questions.forEach(function(q, i) {
  var idx = catIds.indexOf(q.CategoryId);
  if (idx === -1) {
    catIds.push(q.CategoryId);
    cats.push({
      id: q.CategoryId,
      name: q.Category,
      collapsed: true,
      valid: false,
      questions: [q]
    });
  }
  else {
    cats[idx].questions.push(q);
  }
});

$scope.patron.categories = cats;

answered Oct 5, 2015 at 21:25

Buzinas

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1 Comment

Andrux51 Over a year ago

Thanks, this is great, and pretty clean, too.

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