Is there a more elegant way of filtering key/value pairs based on key values?

Question

I have an array of objects like this,

value = [
  {"f1_x":1, "f1_y":3, "f2_x": 5},
  {"f1_x":2, "f2_y":4, "f2_x": 6}
]

Now, I only want to keep key/value pairs where key ends with _x. So, the above would look like this,

value = [
  {"f1_x":1, "f2_x": 5},
  {"f1_x":2, "f2_x": 6}
]

I used the following, but is there any more elegant way of doing the same?

var x = value.map(e => {
  r = {}
  for (const f in e)
    if (f.toLowerCase().endsWith("_x"))
      r[f] = e[f]
  return r
})

What's wrong with the current approach? More elegant according to whom? — evolutionxbox, May 25 '21 at 09:41
Isn't this better suited for https://codereview.stackexchange.com/? Unless there's something wrong with the current approach? — evolutionxbox, May 25 '21 at 09:43
You should check dev twitter from last 3-4 days. There was a huge discussion on this very question. https://twitter.com/dan_abramov/status/1396813335795011587 this summs it up. — The Fool, May 25 '21 at 09:47
[There's no such thing as a JSON Object](http://benalman.com/news/2010/03/theres-no-such-thing-as-a-json/). You have [plain objects expressed as object literal notation](https://stackoverflow.com/questions/2904131/what-is-the-difference-between-json-and-object-literal-notation) — VLAZ, May 25 '21 at 09:49
@TheFool Could you write an answer with the relevant points? — Mast, May 25 '21 at 09:50
Add a `const` to `r`. Otherwise, it looks fine. There are plenty of alternatives like `reduce` or `Object.fromEntries` with `filter` etc. Whether they are elegant is opinion-based. — adiga, May 25 '21 at 09:51
Can you define "more elegant"? What are the metrics to determine it? — VLAZ, May 25 '21 at 09:52
Duplicate of [From an object, extract properties with names matching a test](https://stackoverflow.com/questions/60431862) — adiga, May 25 '21 at 09:57
The only problem in your code is the missing `let` in `r = {}` — georg, May 25 '21 at 10:53

score 3 · Accepted Answer · answered May 25 '21 at 09:54

You can use Object.prototype.fromEntries and Object.prototype.entries

const value = [
  {"f1_x":1, "f1_y":3, "f2_x": 5},
  {"f1_x":2, "f2_y":4, "f2_x": 6}
];

const result = value.map((obj) => Object.fromEntries(Object.entries(obj).filter(([key]) => key.endsWith("_x"))));
console.log(result)

score 1 · Answer 2 · answered May 25 '21 at 09:55

Not quite sure if it's any better, but here's a single liner:

let value = [
    {"f1_x":1, "f1_y":3, "f2_x": 5},
    {"f1_x":2, "f2_y":4, "f2_x": 6}
];

let newValue = value.map(e => Object.fromEntries(Object.entries(e).filter(p => p[0].toLowerCase().endsWith("_x"))));

console.log(newValue);

score 1 · Answer 3 · answered May 25 '21 at 09:55

1

I can write it like this for a little better readability.

var x = value.map(e => {
let r = {};
let newKeys = Object.keys(e).filter((x) => x.toLowerCase().endsWith("_x"));
newKeys.forEach((x) => r[x] = e[x]);
return r
});

answered May 25 '21 at 09:55

Tushar Shahi

16,452
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score 1 · Answer 4 · answered May 25 '21 at 09:55

In the following method, you can use Array methods explicitly.

let value = [
  {"f1_x":1, "f1_y":3, "f2_x": 5},
  {"f1_x":2, "f2_y":4, "f2_x": 6}
]

let filtered = value.map(obj => 
  Object.keys(obj).reduce((a, c) => {  
    c[c.length - 1] === 'x' ? a[c] = obj[c] : null;    
    return a
  }, {})

)

console.log(filtered);

Is there a more elegant way of filtering key/value pairs based on key values?

4 Answers4