How to get specific key counts that are serials

Question

There is a dynamic object with the item_description_ key suffix index. In addition to these keys, there are other different keys.

const input = {
  ...
  item_description_1: "1"
  item_description_2: "2"
  item_description_3: "3"
  ...
}

How can I get the counts of the item_description_ keys? Expected result should be 3 in the above example.

[There's no such thing as a "JSON Object"](http://benalman.com/news/2010/03/theres-no-such-thing-as-a-json/) See also [What is the difference between JSON and Object Literal Notation?](https://stackoverflow.com/q/2904131) — VLAZ, Dec 08 '20 at 07:18
are there keys with different prefixes than `item_description_` and you only want to count the ones with `item_description_` prefix? — illusion, Dec 08 '20 at 07:18
that's correct, there are other keys, and i only want the count of `item_description` prefix key counts @illusion — wangdev87, Dec 08 '20 at 07:18

score 4 · Accepted Answer · answered Dec 08 '20 at 07:20

You can use Object.keys to get all the keys of the object into the array; then filter on the key starting with item_description and count the length of the resultant array:

const input = {
  another_key: 'x',
  item_description_1: "1",
  item_description_2: "2",
  item_description_3: "3",
  something_else: 4
}

const cnt = Object.keys(input)
  .filter(v => v.startsWith('item_description'))
  .length;

console.log(cnt);

If your browser doesn't support startsWith, you can always use a regex e.g.

.filter(v => v.match(/^item_description/))

DSCH · Answer 2 · 2020-12-08T07:23:13.290

2

const keyPrefixToCount = 'item_description_';

const count = Object.keys(input).reduce((count, key) => {
  if (key.startsWith(keyPrefixToCount)) {
    count++
  }
  return count;
}, 0)

console.log(count) // 3 for your input

You probably should remove the prefix into a variable.

Edit: as per VLAZ comment, startsWith would be more accurate

edited Dec 08 '20 at 07:23

answered Dec 08 '20 at 07:20

DSCH

2,146
1
18
29

@VLAZ true, thanks for spotting that. I guess startsWith or a regex would be better – DSCH Dec 08 '20 at 07:21

score 0 · Answer 3 · answered Dec 08 '20 at 07:27

i think with regular expression will also be a good choice, just two more lines:

const input = {
  item_descrip3: "22",
  item_description_1: "1",
  item_description_2: "2",
  item_description_3: "3",
  test22: "4"
}
const regex = /^item_description_[1-9][0-9]*$/
let result = Object.keys(input).filter(item => regex.test(item))

How to get specific key counts that are serials

3 Answers3