Question
PrimeNG p-table and HttpClientTestingModule: Pagination State on Empty Datasets
Ash Byte
0 reputation · 21 Mar 2022, 15:39 UTC
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0 reputation · 21 Mar 2022, 15:39 UTC
When integrating the p-table component with Angular's HttpClientTestingModule, developers typically use the [lazy] input to trigger data fetching via the onLazyLoad event. This allows the UI to be tested against mock observables without requiring production credentials or a live backend.
A specific challenge arises when verifying the table's behavior during edge cases, such as when a mocked service returns an empty dataset. In PrimeNG versions 13.x and later, the table's internal pagination state may not automatically reset when the returned data length is zero, potentially leading to discrepancies between the current page index and the actual available data.
This creates uncertainty when writing assertions for empty states, as the component may maintain a page index that no longer corresponds to any existing records.
p-table component provide a built-in mechanism to reset pagination state when a lazy-loaded request returns an empty array?No. PrimeNG p-table does not provide a built-in automatic reset of the pagination state when a lazy-loaded request returns an empty array. The paginator is driven by the inputs first, rows and totalRecords. With [lazy]=true the component keeps the current first offset and relies on totalRecords to compute the number of pages. If totalRecords is not updated to zero, the paginator remains on the previous page index even though the data array is empty.
angular
primengonLazyLoad event, but it does not change first for you.totalRecords controls page count rendering. With lazy loading enabled, an empty rows=[] response with a stale non-zero totalRecords leaves the UI on a page that has no data.rows=[] while totalRecords stays at the previous value.The component updates the displayed data but does not reset first when totalRecords becomes zero. The paginator therefore stays on page >1 and renders an empty body with a non-zero page indicator. This is version-sensitive; first/rows semantics have changed across PrimeNG majors, and the exact behavior should be verified for the version in use.
[totalRecords] to the backend count returned with the same lazy payload, not to the length of the current page.onLazyLoad handler, set totalRecords from the response first, then update the data source.totalRecords === 0 and the returned rows are empty, set the table reference first = 0 before assigning the new data. Guard against a loop by only changing first when it is not already zero.totalRecords === 0 with *ngIf="totalRecords > 0" to avoid showing a page control for an empty set.lazy=true, rows=10. Flush a response with rows=[] and totalRecords=0, then assert first is 0 after the handler runs.first, rows and totalRecords before and after the empty response is applied.Assumption: lazy loading is enabled and totalRecords is set from the same response as rows. If lazy is false, reset logic differs. Can you confirm whether your test uses [lazy]=true and whether totalRecords is updated from the mock response? That detail changes the recommended guard.
Note: resetting first inside onLazyLoad can trigger an extra request if not guarded by a totalRecords check. Behavior requires current verification for the specific PrimeNG major in use.
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34,775 reputation · 22 Mar 2022, 01:21 UTC
While updating totalRecords and first manually resolves the state discrepancy, it is important to note that p-table updates may not be reflected immediately in the DOM during HttpClientTestingModule assertions. Because lazy loading is asynchronous, the component's internal state and the rendered paginator may lag behind the mocked service response.
To ensure assertions for empty states are reliable, verify the following in your test suite:
fixture.detectChanges() after flushing the mock observable to ensure the table's value and pagination inputs are processed.first property, verify the presence of the p-table empty message template in the DOM to confirm the UI has transitioned to the empty state.This approach prevents race conditions where a test might fail because it checks the paginator state before the Angular change detection cycle has synchronized the UI with the empty dataset.