Append data always in future - Is this a bug?

[misterbalmer]

From the built bundle:

Backward (line 5335–5336):

case 'backward': {
this._dataList = this._dataList.concat(data); // appends 'data' at the END
Forward (line 5341–5342):

case 'forward': {
this._dataList = data.concat(this._dataList); // prepends 'data' at the BEGINNING
So:

Backward = “load older bars (to the left)”. Those should be prepended: data.concat(_dataList). The code does append (_dataList.concat(data)), so older bars end up on the right.
Forward = “load newer bars (to the right)”. Those should be appended: _dataList.concat(data). The code does prepend (data.concat(_dataList)), so newer bars would end up on the left.
So it’s a bug in v10: backward and forward merge logic are swapped. Reformatting or changing the backend won’t fix it; the chart will still put backward data at the end and forward at the start.

[misterbalmer]

the solution

Summary of what was changed in frontend-v10/main.js:

1. Init

callback(bars, { backward: true, forward: false }) → callback(bars, { backward: false, forward: true }).
So the chart is told “there is more data in the forward direction” (to the left) and will call getBars('forward') when you scroll left.
2. Forward (scroll left = load older)

If oldestTimestampWeSent == null, call callback([], noMore) and return.
Otherwise fetch with endTime: oldestTimestampWeSent, set oldestTimestampWeSent = bars[0].timestamp, and call callback(bars, { backward: false, forward: bars.length === KLINE_LIMIT }).
The chart will prepend this data so older bars appear on the left.
3. Backward

Always callback([], noMore) and return (no extra newer history; live is from subscribeBar).
4. Comment

Comment for oldestTimestampWeSent updated to say it’s used for forward (older bars), not backward.
Result when you scroll left: the chart requests forward, we return bars strictly before the current oldest (endTime = oldestTimestampWeSent), and v10 prepends them so each 1000-candle chunk appears in the correct order to the left. You can run the app and scroll left to confirm.