SurveyJS: Dynamic Matrix Row Access With Variables

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SurveyJS: Dynamic Matrix Row Access with Variables

Hey guys! Let's dive into a common issue in SurveyJS where you're trying to access a specific row in a dynamic matrix using a variable as an index. It's like trying to grab the right book off a shelf, but the label is a bit wonky. Specifically, we're tackling the problem where expressions like {Subscription[{LastIndex}].Amount} don't quite work as expected.

Understanding the Problem

So, you've got a dynamic matrix question in your SurveyJS survey, and you want to pull a value from one of its rows—say, the last row. You figure you can use a variable (like LastIndex) to specify the row number. Makes sense, right? But when you try to use an expression like {Subscription[{LastIndex}].Amount}, it doesn't return the value you're after. It's like the expression engine is having trouble interpreting the variable as an index. Let's see how to deal with this.

The Scenario

Imagine a survey where users input subscription details into a dynamic matrix. Each row represents a subscription, with columns for things like "Payment date", "Amount", "Period", and "Receipt". You want to display the amount from the last subscription entered. You calculate the index of the last row using a calculated value, but plugging that index into the expression to get the amount doesn't work.

Why It Doesn't Work (and What to Do About It)

The core issue here is how SurveyJS's expression engine handles variables within array index accessors. While it seems like it should work, the engine might not be correctly interpreting the variable as a numerical index. So, what's the workaround? Let's explore some approaches to get this working.

Diving Deeper: Code Example

To illustrate, here’s a sample JSON configuration that highlights the problem:

{
  "pages": [
    {
      "name": "page1",
      "elements": [
        {
          "type": "matrixdynamic",
          "name": "Subscription",
          "defaultValue": [
            {
              "Payment date": "2025-11-14",
              "Amount": 1000,
              "Period": "2026",
              "Receipt": "1"
            },
            {
              "Payment date": "2025-11-07",
              "Amount": 2000,
              "Period": "2027",
              "Receipt": "2"
            }
          ],
          "columns": [
            {
              "name": "Payment date",
              "cellType": "text",
              "inputType": "date"
            },
            {
              "name": "Amount",
              "cellType": "text",
              "maskType": "currency"
            },
            {
              "name": "Period",
              "cellType": "dropdown",
              "choices": [
                "2026",
                "2027",
                "2028"
              ]
            },
            {
              "name": "Receipt",
              "cellType": "text"
            }
          ],
          "choices": [
            1,
            2,
            3,
            4,
            5
          ],
          "cellType": "text",
          "maxRowCount": 10
        },
        {
          "type": "expression",
          "name": "lastIndexVal",
          "expression": "{LastIndex}"
        },
        {
          "type": "expression",
          "name": "question1",
          "startWithNewLine": false,
          "title": "Last payment (fixed index)",
          "expression": "{Subscription[1].Amount}"
        },
        {
          "type": "expression",
          "name": "Last payment",
          "startWithNewLine": false,
          "expression": "{Subscription[{LastIndex}].Amount}"
        }
      ]
    }
  ],
  "calculatedValues": [
    {
      "name": "LastIndex",
      "expression": "{Subscription.length} - 1"
    }
  ]
}

In this example, {Subscription[{LastIndex}].Amount} is intended to retrieve the 'Amount' from the last row of the 'Subscription' matrix. However, it won't work directly.

Workarounds and Solutions

Here are a few ways we can solve this problem, working around the limitations of the expression engine. These solutions will allow you to access the required values from the matrix.

1. JavaScript-Based Calculation

One robust approach is to use a JavaScript-based calculated value. This gives you the full power of JavaScript to manipulate the data and extract the value you need. Here’s how you'd set it up:

  • Create a Calculated Value: Define a calculated value in your survey JSON.
  • Use JavaScript: Write JavaScript code within the calculated value's expression to access the matrix, get the last row, and extract the 'Amount'.
{
  "name": "lastPaymentAmount",
  "expression": "function() { var matrix = this.getValue('Subscription'); if (matrix && matrix.length > 0) { return matrix[matrix.length - 1].Amount; } return null; }"
}

Explanation:

  • this.getValue('Subscription') retrieves the data from the 'Subscription' matrix.
  • We check if the matrix exists and has at least one row.
  • matrix[matrix.length - 1] accesses the last row of the matrix.
  • .Amount extracts the value from the 'Amount' column.
  • The function returns null if the matrix is empty or doesn't exist, preventing errors.

Then, to display this value in your survey, you can use an expression:

{
  "type": "expression",
  "name": "displayLastPayment",
  "title": "Last Payment Amount",
  "expression": "{lastPaymentAmount}"
}

2. Leveraging SurveyJS Events

Another approach is to use SurveyJS events to calculate and store the value. This is useful if you need to perform more complex logic or calculations based on user interactions.

  • Listen to the onValueChanged Event: Attach a handler to the onValueChanged event of the 'Subscription' matrix.
  • Calculate the Value: Inside the event handler, calculate the 'Amount' from the last row and store it in a separate survey data element.
  • Access the Stored Value: Use an expression to display the stored value.

Here’s how you'd implement this in JavaScript:

survey.onValueChanged.add(function(sender, options) {
  if (options.name === 'Subscription') {
    var matrix = sender.getValue('Subscription');
    if (matrix && matrix.length > 0) {
      var lastAmount = matrix[matrix.length - 1].Amount;
      sender.setValue('lastPaymentAmount', lastAmount);
    } else {
      sender.setValue('lastPaymentAmount', null);
    }
  }
});

Explanation:

  • We listen for changes to the 'Subscription' matrix.
  • We retrieve the matrix data and check if it has rows.
  • We extract the 'Amount' from the last row.
  • We store the value in a survey data element called 'lastPaymentAmount'.

Then, define an expression to display the value:

{
  "type": "expression",
  "name": "displayLastPayment",
  "title": "Last Payment Amount",
  "expression": "{lastPaymentAmount}"
}

3. Utilizing Hidden Questions and Default Values

This method involves using a hidden question to store the calculated index and then referencing this question in another expression. This is slightly less direct but can be effective in simpler scenarios.

  • Create a Hidden Question: Add a hidden question to your survey to store the calculated LastIndex.
  • Set Default Value: Use a calculated value to set the default value of the hidden question to {Subscription.length} - 1.
  • Reference in Expression: In your expression, reference the hidden question to access the matrix row: {Subscription[hiddenQuestionName].Amount}.

Here’s the JSON configuration:

{
  "type": "text",
  "name": "LastIndexHidden",
  "visible": false,
  "defaultValueExpression": "{Subscription.length} - 1"
},
{
  "type": "expression",
  "name": "LastPayment",
  "title": "Last Payment Amount",
  "expression": "{Subscription[{LastIndexHidden}].Amount}"
}

Explanation:

  • LastIndexHidden is a hidden text question that stores the calculated index.
  • defaultValueExpression calculates the last index when the survey loads.
  • The final expression uses the value from LastIndexHidden to access the 'Amount'.

Best Practices and Considerations

  • Error Handling: Always include error handling in your JavaScript code to handle cases where the matrix is empty or the data is missing. This prevents unexpected errors and ensures a smooth user experience.
  • Performance: For very large matrices, consider optimizing your JavaScript code to minimize performance impact. Avoid unnecessary loops and calculations.
  • Readability: Use meaningful variable names and comments in your code to improve readability and maintainability.
  • Testing: Thoroughly test your solutions with different data sets to ensure they work correctly in all scenarios.

Conclusion

While directly using variables as array indices in SurveyJS expressions might not always work as expected, there are several effective workarounds. By using JavaScript-based calculated values, leveraging SurveyJS events, or utilizing hidden questions, you can successfully access and manipulate data within dynamic matrices. Choose the method that best suits your needs and complexity of your survey. Remember to test your implementation thoroughly to ensure accurate and reliable results. Happy surveying!