How to split/create an array on repeated duplicated key

Question

const test = [{a:1},{b:2},{c:3},{d:4},{a:5},{b:6},{c:7},{a:8},{c:9}]

Need output like below, On repeat of key "a", need to create new array till the same key repeat

[{a:1},{b:2},{c:3},{d:4}]
[{a:5},{b:6},{c:7}]
[{a:8},{c:9}]

Answer 1

For the best performance don't mutate the source and don't push by 1 element, rather slice:

const test = [{a:1},{b:2},{c:3},{d:4},{a:5},{b:6},{c:7},{a:8},{c:9}];

let result = [], prev = 0, i = 0, [sc] = Object.keys(test[0]);
while(++i < test.length) sc in test[i] && result.push(test.slice(prev, prev = i));
result.push(test.slice(prev));

result.forEach(item => console.log(JSON.stringify(item)));

And a benchmark:

` Chrome/125
-------------------------------------------------------
Alexander       ■ 1.00x | x10000000 570 579 608 626 627
Siva KV           1.93x |  x1000000 110 114 120 128 134
Roko C. Buljan    2.77x |  x1000000 158 159 166 169 170
------------------------------------------------------- `

Open in the playground

<script benchmark data-count="5000000">
const test = [{a:1},{b:2},{c:3},{d:4},{a:5},{b:6},{c:7},{a:8},{c:9}];

// @benchmark Roko C. Buljan
[...test].reduceRight((acc, ob, i, ref) => 
  (ob.hasOwnProperty("a") && acc.unshift(ref.splice(i)), acc)
, []);

// @benchmark Siva KV
{
const result = [];
let row = [];
test.forEach((item) => {
  const key = Object.keys(item)[0];
  if (key === "a") {
    if (row.length > 0) {
      result.push(row);
    }
    row = [item];
  } else {
    row.push(item);
  }
});
if (row.length > 0) {
  result.push(row);
}
result;
}

// @benchmark Alexander
let result = [], prev = 0, i = 0, sc = Object.keys(test[0])[0];
while(++i < test.length) sc in test[i] && result.push(test.slice(prev, prev = i));
result.push(test.slice(prev));
result;
</script>
<script src="https://cdn.jsdelivr.net/gh/silentmantra/benchmark/loader.js"></script>