Validate how many times a question value appears across panels

categories/expressions-and-triggers/validate-value-count-across-dynamic-panels.md

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+# Validate How Many Times a Question Value Appears Across Dynamic Panels
+
+## Problem
+
+Is there a built-in way in SurveyJS to validate how many times a specific question value appears across all panels in a Dynamic Panel (or similar repeating structure)?
+
+For example, given a Dynamic Panel with the same question repeated in each panel: can SurveyJS validate that a value appears no more than **N** times across all panels?
+
+I know [`keyName`](https://surveyjs.io/form-library/documentation/api-reference/dynamic-panel-model#keyName) can be used to prevent duplicate responses for a question inside a Dynamic Panel, but is there a way to use expression-based validation to determine how many times a specific question value appears across panels?
+
+## Solution
+
+Yes. Use an **expression validator** on the Dynamic Panel together with the built-in [`countInArray`](https://surveyjs.io/form-library/documentation/design-survey/conditional-logic#countinarray) function.
+
+`countInArray` counts items in a Dynamic Panel, Dynamic Matrix, or Multi-Select Matrix array where a specified field matches optional filter conditions. Attach a validator to the **panel dynamic question** (not to the inner template question) so the rule runs against all panels at once.
+
+### How `countInArray` works
+
+```text
+countInArray({panelDynamicQuestion}, 'innerQuestionName', optionalFilterExpression)
+```
+
+- **First argument** — the Dynamic Panel question reference (for example, `{employees}`).
+- **Second argument** — the name of a question inside the panel template whose value you want to evaluate.
+- **Third argument** (optional) — a Boolean expression evaluated per panel/row. Only panels that satisfy the filter are counted.
+
+To limit how often a specific choice appears (for example, `'JavaScript'` selected in no more than three panels), use the `allof` operator in the filter expression.
+
+### Survey JSON Schema
+
+The example below limits the `'JavaScript'` skill to at most three employee records:
+
+```json
+{
+  "pages": [
+    {
+      "name": "page1",
+      "elements": [
+        {
+          "type": "paneldynamic",
+          "name": "employees",
+          "title": "Employees",
+          "validators": [
+            {
+              "type": "expression",
+              "text": "You can select 'JavaScript' in no more than 3 employee records.",
+              "expression": "countInArray({employees}, 'skills', {skills} allof 'JavaScript') <= 3"
+            }
+          ],
+          "templateElements": [
+            {
+              "type": "checkbox",
+              "name": "skills",
+              "title": "Select skills",
+              "choices": [
+                "JavaScript",
+                "TypeScript",
+                "React",
+                "Angular",
+                "Vue"
+              ]
+            }
+          ],
+          "panelCount": 1,
+          "displayMode": "tab"
+        }
+      ]
+    }
+  ]
+}
+```
+
+Change the comparison (`<= 3`) and the filter expression to match your limit and value.
+
+
+`countInArray` also works with [Dynamic Matrix](https://surveyjs.io/form-library/documentation/api-reference/dynamic-matrix-table-model) and [Multi-Select Matrix](https://surveyjs.io/form-library/documentation/api-reference/multi-select-matrix-table-model) when you need the same kind of cross-row counting.
+
+### `keyName` vs expression validation
+
+| Approach | Use when |
+| --- | --- |
+| [`keyName`](https://surveyjs.io/form-library/documentation/api-reference/dynamic-panel-model#keyName) on the Dynamic Panel | Each panel must have a **unique** value for one template question (duplicates are blocked entirely). |
+| `countInArray` + expression validator | A value may repeat, but only up to **N** times (or you need a minimum count). |
+
+## Learn More
+
+- [Conditional Logic — Built-in Functions (`countInArray`)](https://surveyjs.io/form-library/documentation/design-survey/conditional-logic#countinarray)
+- [Data Validation](https://surveyjs.io/form-library/documentation/data-validation)
+- [Dynamic Panel Model API (`keyName`, `keyDuplicationError`)](https://surveyjs.io/form-library/documentation/api-reference/dynamic-panel-model)
+- [Expressions in Dynamic Panel (example)](https://surveyjs.io/form-library/examples/how-to-use-expressions-in-dynamic-panel/documentation)