Use custom function in expressions

Example of using custom functions in an expression 

                        function rowsWithValue(params) {
   if (!params && params.length < 2) return false;
   var matrixValue = params[0];
   var value = params[1];
   var rowCount = 0;
   for(var key in matrixValue) {
     if(matrixValue[key] == value) rowCount++;
   }
   return rowCount;
}
Survey.FunctionFactory.Instance.register("rowsWithValue", rowsWithValue);

    
        Survey.StylesManager.applyTheme("default");
    


var json = {
    questions: [
        {
            type: "matrix",
            name: "quality",
            title: "Please indicate if you agree or disagree with the following statements",
            columns: [
                {
                    value: "agree",
                    text: "Agree"
                }, {
                    value: "disagree",
                    text: "Disagree"
                }
            ],
            rows: [
                {
                    value: "affordable",
                    text: "Product is affordable"
                }, {
                    value: "doeswhatitclaims",
                    text: "Product does what it claims"
                }, {
                    value: "betterthanother",
                    text: "Product is better than other products on the market"
                }, {
                    value: "easytouse",
                    text: "Product is easy to use"
                }
            ]
        },
        {
          type: "comment",
          name: "disappointedComment",
          title: "We are really sorry that our products doesn't suite your needs. Could provide us with some comments to help us imporove it?",
          visibleIf: "rowsWithValue({quality}, 'disagree') >= 3"
        },
        {
          type: "comment",
          name: "approvedComment",
          title: "Thank you for appreciating our product. Could provide us with some comments to make it event better?",
          visibleIf: "rowsWithValue({quality}, 'agree') >= 3"
        }
    ]
};

window.survey = new Survey.Model(json);


    survey.onComplete.add(function(result) {
        document.querySelector('#surveyResult').textContent =
            "Result JSON:\n" + JSON.stringify(result.data, null, 3);
    });


$("#surveyElement").Survey({ 
    model: survey 
});


                    
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