downsample.js

const data = '[{"x":"2016-07-03T17:33:23","y":7.891028963922976},{"x":"2016-07-03T17:33:24","y":7.82822252631146},{"x":"2016-07-03T17:33:25","y":8.142617930391477},{"x":"2016-07-03T17:33:26","y":8.433371969681007},{"x":"2016-07-03T17:33:27","y":8.746266191837195},{"x":"2016-07-03T17:33:28","y":8.759654975345702},{"x":"2016-07-03T17:33:29","y":8.610320311313277},{"x":"2016-07-03T17:33:30","y":8.958775225112122},{"x":"2016-07-03T17:33:31","y":9.0476595068776},{"x":"2016-07-03T17:33:32","y":8.884845728679654},{"x":"2016-07-03T17:33:33","y":9.124182294766484},{"x":"2016-07-03T17:33:34","y":8.85299331748572},{"x":"2016-07-03T17:33:35","y":8.11525951335481},{"x":"2016-07-03T17:33:36","y":7.6028100287391265},{"x":"2016-07-03T17:33:37","y":5.922823556564772},{"x":"2016-07-03T17:33:38","y":5.380828682728633},{"x":"2016-07-03T17:33:39","y":4.604150619714715},{"x":"2016-07-03T17:33:40","y":0.8921589081729535},{"x":"2016-07-03T17:33:41","y":0.6085054233193794},{"x":"2016-07-03T17:33:42","y":0.5122195280530762},{"x":"2016-07-03T17:33:43","y":0},{"x":"2016-07-03T17:33:44","y":0},{"x":"2016-07-03T17:33:45","y":0.328919785553777},{"x":"2016-07-03T17:33:46","y":2.1874091130266153},{"x":"2016-07-03T17:33:47","y":2.4789788133254738},{"x":"2016-07-03T17:33:48","y":2.301475774980622},{"x":"2016-07-03T17:33:49","y":2.308636792441717},{"x":"2016-07-03T17:33:50","y":2.228020312775266},{"x":"2016-07-03T17:33:51","y":1.6204685975259536},{"x":"2016-07-03T17:33:52","y":0.7524687215155168},{"x":"2016-07-03T17:33:53","y":0.5356171231357203},{"x":"2016-07-03T17:33:54","y":0.1534755603753635},{"x":"2016-07-03T17:33:55","y":0},{"x":"2016-07-03T17:33:56","y":0},{"x":"2016-07-03T17:33:57","y":0},{"x":"2016-07-03T17:33:58","y":0},{"x":"2016-07-03T17:33:59","y":0},{"x":"2016-07-03T17:34:00","y":0},{"x":"2016-07-03T17:34:01","y":0.2885925980431889},{"x":"2016-07-03T17:34:02","y":2.2393450627657723},{"x":"2016-07-03T17:34:03","y":3.2650488403176334},{"x":"2016-07-03T17:34:04","y":3.7843939342565798},{"x":"2016-07-03T17:34:05","y":4.523580006575099},{"x":"2016-07-03T17:34:06","y":5.730031214069795},{"x":"2016-07-03T17:34:07","y":5.542967922186465},{"x":"2016-07-03T17:34:08","y":5.699448252395041},{"x":"2016-07-03T17:34:09","y":5.1249400051393295},{"x":"2016-07-03T17:34:10","y":5.121792886533596},{"x":"2016-07-03T17:34:11","y":5.27289166147052},{"x":"2016-07-03T17:34:12","y":4.861154430685466}]';

const jsonData = JSON.parse(data);
console.log(jsonData[0]);
console.log(jsonData[0]);

const downsample = (sample, size) => {
  const res = [];
  return sample.filter((el, idx) => {
    return idx % 3 === 0;
  })
}
console.log(downsample(jsonData).length);

const downsample_ = (sample,size) => {
  const isSize = (el, idx) => idx % size ===  0;
  return sample.filter(isSize);
}


console.log(downsample_(jsonData, 3))
console.log(downsample_(jsonData, 3).length)