JavaScript - get count of object with specific value

Question

I have referred these questions listed below, but didn't helped me out.

1. Counting the occurrences / frequency of array elements
2. Count values of the inner two-dimensional array - javascript
3. Count elements in a multidimensional array

I want to get count from object with specific value. For example, I have this array list,

var testData = [
  {
    "issue_type": "Warning",
    "created_date": "2019-05-13T13:43:16.437Z",
  },
  {
    "issue_type": "Warning",
    "created_date": "2019-05-13T13:45:16.330Z",
  },
  {
    "issue_type": "Alert",
    "created_date": "2019-05-13T13:43:16.437Z",
  },
  {
    "issue_type": "Alert",
    "created_date": "2019-05-13T13:45:16.330Z",
  }
]

I want to count how many objects with key "issue_type"="Warning" are there.

I tried with this loop but it is not what I'm looking for,

var counts = {};
for (var i = 0; i < arr.length; i++) {
    var num = arr[i];
    counts[num] = counts[num] ? counts[num] + 1 : 1;
}
console.log(count['issue_type']);

Please suggest me the way out.

Answer 1

You could take the value as key for counting.

var testData = [{ issue_type: "Warning", created_date: "2019-05-13T13:43:16.437Z" }, { issue_type: "Warning", created_date: "2019-05-13T13:45:16.330Z" }, { issue_type: "Alert", created_date: "2019-05-13T13:43:16.437Z" }, { issue_type: "Alert", created_date: "2019-05-13T13:45:16.330Z" }],
    counts = testData.reduce((c, { issue_type: key }) => (c[key] = (c[key] || 0) + 1, c), {});

console.log(counts);