Conditional Logic and Dynamic Texts

This help topic describes how to implement custom conditional logic and add dynamic texts to your survey.

Survey UI texts support placeholders whose values are computed at runtime to make the texts dynamic. Placeholders can be used in the following places:

• Titles and descriptions of surveys, pages, panels, and questions
• Properties that accept HTML markup (completedHtml, loadingHtml, etc.)
• Expressions

You can use the following values as placeholders:

• Question Values
• Variables
• Calculated Values

To use a question value as a placeholder, specify the question's name in curly brackets. The name will be replaced with the question value. For instance, the following example defines two Text questions: First Name and Last Name. An Html question uses their name property to reference them and display their values:

const surveyJson = {
  "elements": [
    { "name": "firstName", "type": "text", "title": "First Name", "defaultValue": "John" },
    { "name": "lastName", "type": "text", "title": "Last Name", "defaultValue": "Smith" },
    {
      "name": "greetings",
      "type": "html",
      "html": "<p>Hello, {firstName} {lastName}!</p>"
    }
  ]
};

For questions with a specified valueName property, use its value instead of the name value.

In single- and multiple-selection question types (Dropdown, Checkbox, Radiogroup, Tag Box, Image Picker), items can contain a display value in addition to a question value. In this case, placeholders are replaced with display values. If you want to use question values instead, disable the useDisplayValuesInDynamicTexts property.

Certain question types can contain multiple values. Use a dot symbol to access a specific value (item or cell):

View Demo

In dynamic questions, use a zero-based index to access a specific value (question or cell):

You can also use prefixes, such as row, panel, and parentPanel, to access a specific question or cell relative to the question you configure:

View Demo

Variables are used to store JavaScript-calculated values. To create or change a variable, call the Survey's setVariable(name, value) method. In the following code, this method sets a currentYear variable used to display the current year in an Html question:

import { Model } from "survey-core";

const surveyJson = {
  "elements": [{
    "name": "footer",
    "type": "html",
    "html": "© 2015-{currentyear} Devsoft Baltic OÜ"
  }]
};

const survey = new Model(surveyJson);

survey.setVariable("currentyear", new Date().getFullYear());

If you need to get a variable's value, call the getVariable(name) method. For example, the following code outputs the currentyear variable's value into the browser's console:

console.log(survey.getVariable("currentyear"));

You can also call the getVariableNames() method to get a list of all available variables:

console.log(survey.getVariableNames()); // Outputs [ "currentyear" ]

Calculated values allow you to register an expression under a required name. If the expression includes questions, variables, or functions, it is recalculated each time their values are changed.

To configure a calculated value, define the calculatedValues array in the survey JSON schema. Each object in this array should contain the following fields:

• name - A name that identifies the calculated value.
• expression - An expression that returns the calculated value.
• includeIntoResult - A Boolean property that specifies whether to include the calculated value in survey results.

The following code shows how to calculate a full name value based on the first and last names:

const surveyJson = {
  "elements": [
    { "name": "firstName", "type": "text", "title": "First Name", "defaultValue": "John" },
    { "name": "lastName", "type": "text", "title": "Last Name", "defaultValue": "Smith" },
    {
      "name": "greetings",
      "type": "html",
      "html": "<p>Hello, {fullname}!</p>"
    }
  ],
  "calculatedValues": [{
    "name": "fullname",
    "expression": "{firstName} +  ' ' + {lastName}"
  }]
};

View Demo

Variables and calculated values are both used to perform custom calculations within a survey. However, they also have a number of important differences. The following table compares variables with calculated values across multiple criteria:

If you need to save a variable in survey results, create an intermediary calculated value that references the variable. Enable the calculated value's includeIntoResult property to save the value in survey results. The following code shows how to save a currentyear-var variable value in survey results via a currentyear calculated value:

import { Model } from "survey-core";

const surveyJson = {
  "elements": [{
    "name": "footer",
    "type": "html",
    "html": "© 2015-{currentyear} Devsoft Baltic OÜ"
  }],
  "calculatedValues": [{
    "name": "currentyear",
    "expression": "{currentyear-var}",
    "includeIntoResult": true
  }]
};

const survey = new Model(surveyJson);

survey.setVariable("currentyear-var", new Date().getFullYear());

Expressions allow you to add logic to your survey and perform calculations right in the survey JSON schema. Expressions are evaluated and re-evaluated at runtime.

SurveyJS supports the following expression types:

• String expression
  An expression that evaluates to a string value. The following string expression evaluates to "Adult" if the age function returns a value of 21 or higher; otherwise, the expression evaluates to "Minor":

  "expression": "iif(age({birthdate}) >= 21, 'Adult', 'Minor')"
  

• Numeric expression
  An expression that evaluates to a number. The following numeric expression evaluates to the sum of the total1 and total2 question values:

  "expression": "sum({total1}, {total2})"
  

• Boolean expression
  An expression that evaluates to true or false. Boolean expressions are widely used to implement conditional logic. Refer to the following help topic for more information: Conditional Visibility.

Expressions can include question names, variables, and calculated values (described in the Dynamic Texts section). Plus, expressions can use built-in and custom functions.

The SurveyJS expression engine is built upon the PEG.js parser generator. The following table gives a brief overview of operators that you can use within expressions. For a detailed look at the grammar rules used by the expression parser, refer to the survey-library GitHub repository.

Functions allow you to perform additional calculations within an expression. One expression can contain multiple function calls.

Functions can accept arguments. For example, the following expression shows the built-in age and iif functions. age accepts the value of a birthdate question. iif accepts three arguments: a condition, a value to return when the condition is truthy, and a value to return when the condition is falsy.

"expression": "iif(age({birthdate}) >= 21, 'Adult', 'Minor')"

The following built-in functions are available:

• iif
• isContainerReady
• isDisplayMode
• age
• currentDate
• today
• year
• month
• day
• weekday
• getDate
• diffDays
• sum
• max
• min
• avg
• sumInArray
• maxInArray
• minInArray
• avgInArray
• countInArray
• displayValue
• propertyValue

If you do not find a required function in the list above, you can implement a custom function with the required functionality.

Definition: iif(condition: expression, valueIfTrue: any, valueIfFalse: any): any

Returns the valueIfTrue value if the condition is truthy or the valueIfFalse value if the condition is falsy.

Example: "expression": "iif({question1} + {question2} > 20, 'High', 'Low')"

View Source Code

Definition: isContainerReady(nameOfPanelOrPage: string): boolean

Returns true if all questions in a given panel or page have valid input; otherwise, returns false. An empty question value is considered valid if neither validators nor required status is defined for it.

Example: "expression": "isContainerReady('page1')"

View Source Code

Definition: isDisplayMode(): boolean

Returns true if the survey is in display or preview mode.

Example: "expression": "isDisplayMode()"

View Source Code

Definition: age(birthdate: any): number

Returns age according to a given birthdate. The date argument (which is typically taken from a question) should be defined as a valid JavaScript Date.

Example: "expression": "age({birthdate})"

View Source Code

Definition: currentDate(): Date

Returns the current date and time.

Example: "expression": "currentDate()"

View Source Code

Definition: today(daysToAdd?: number): Date

Returns the current date or a date shifted from the current by a given number of days. For example, today() returns the current date, 0 hours, 0 minutes, 0 seconds; today(-1) returns yesterday's date, same time; today(1) returns tomorrow's date, same time.

Examples:

• "expression": "today()"
• "expression": "today(2)"

View Source Code

Definition: year(date?: Date): number

Returns the year of a given date.

Example: "expression": "year({birthdate})"

View Source Code

Definition: month(date?: Date): number

Returns the month of a given date as a value from 1 (January) to 12 (December).

Example: "expression": "month({birthdate})"

View Source Code

Definition: day(date?: Date): number

Returns the day of the month for a given date as a value from 1 to 31.

Example: "expression": "day({birthdate})"

View Source Code

Definition: weekday(date?: Date): number

Returns the day of the week for a given date as a value from 0 (Sunday) to 6 (Saturday).

Example: "expression": "weekday({birthdate})"

View Source Code

Definition: getDate(questionName: expression): Date

Returns a Date value converted from a given question's value.

Example: "expression": "getDate({birthdate})"

View Source Code

Definition: dateDiff(fromDate: any, toDate: any, "days" | "months" | "years"): number

Returns a difference between two given dates in full days (default), months, or years.

Example: "expression": "dateDiff({birthdate}, today(), "months")"

View Source Code

This function is obsolete. Use the dateDiff function instead.

Definition: diffDays(fromDate: any, toDate: any): number

Returns the number of days between two given dates.

Example: "expression": "diffDays({startDate}, {endDate}) < 7"

View Source Code

Definition: sum(param1: number, param2: number, ...): number

Returns the sum of passed numbers.

Example: "expression": "sum({total1}, {total2})"

View Source Code

Definition: max(param1: number, param2: number, ...): number

Returns the maximum of passed numbers.

Example: "expression": "max({total1}, {total2})"

View Source Code

Definition: min(par1: number, par2: number, ...): number

Returns the minimum of passed numbers.

Example: "expression": "min({total1}, {total2})"

View Source Code

Definition: avg(par1: number, par2: number, ...): number

Returns the average of passed numbers.

Example: "expression": "avg({total1}, {total2}, {total3})"

View Source Code

Definition: sumInArray(questionName: expression, dataFieldName: string, filter?: expression): number

Returns the sum of numbers taken from a specified data field. This data field is searched in an array that contains a user response to a Multi-Select Matrix, Dynamic Matrix, or Dynamic Panel question. The optional filter parameter defines a rule according to which values are included in the calculation.

The following code sums up values from a "total" matrix column but includes only the rows with a "categoryId" column equaling 1:

Example: "expression": "sumInArray({matrixdynamic}, 'total', {categoryId} = 1)"

View Source Code

Definition: maxInArray(questionName: expression, dataFieldName: string, filter?: expression): number

Returns the maximum of numbers taken from a specified data field. This data field is searched in an array that contains a user response to a Multi-Select Matrix, Dynamic Matrix, or Dynamic Panel question. The optional filter parameter defines a rule according to which values are included in the calculation.

The following code finds a maximum value within a "quantity" matrix column, but the value should be under 100:

Example: "expression": "maxInArray({matrixdynamic}, 'quantity', {quantity} < 100)"

View Source Code

Definition: minInArray(questionName: expression, dataFieldName: string, filter?: expression): number

Returns the minimum of numbers taken from a specified data field. This data field is searched in an array that contains a user response to a Multi-Select Matrix, Dynamic Matrix, or Dynamic Panel question. The optional filter parameter defines a rule according to which values are included in the calculation.

The following code finds a minimum value within a "quantity" matrix column but searches for it only in the rows with a "categoryId" column equaling 1 and includes only positive values:

Example: "expression": "minInArray({matrixdynamic}, 'quantity', {quantity} > 0 and {categoryId} = 1)"

View Source Code

Definition: avgInArray(questionName: expression, dataFieldName: string, filter?: expression): number

Returns the average of numbers taken from a specified data field. This data field is searched in an array that contains a user response to a Multi-Select Matrix, Dynamic Matrix, or Dynamic Panel question. The optional filter parameter defines a rule according to which values are included in the calculation.

The following code finds an average of values within a "quantity" matrix column, excluding zeroes:

Example: "expression": "avgInArray({matrixdynamic}, 'quantity', {quantity} > 0)"

View Source Code

Definition: countInArray(questionName: expression, dataFieldName: string, filter?: expression): number

Returns the total number of array items in which a specified data field has a value other than null or undefined. This data field is searched in an array that contains a user response to a Multi-Select Matrix, Dynamic Matrix, or Dynamic Panel question.

The following code finds the total number of matrix rows with a "quantity" column value greater than zero but includes only the rows with a "categoryId" column equaling 1:

Example: "expression": "countInArray({matrixdynamic}, 'quantity', {quantity} > 0 and {categoryId} = 1)"

View Source Code

Definition: displayValue(questionName: string, value?: any): any

Returns a question's display text. Supports questions nested within panels or matrices.

The second parameter allows you to get a display text associated with a specific value rather than with the current question value. For instance, the following expression references a display text that corresponds to value 5 in a Dropdown question. If you don't pass the second parameter, the displayValue function returns a display text for the currently selected question value.

Example: "expression": "displayValue('my-dropdown-question', 5)"

View Source Code

Definition: propertyValue(questionName: string, propertyName: string): any

Returns the value of a property specified for a given question. Supports questions nested within panels or matrices.

Example: "expression": "propertyValue('question1', 'visible')"

View Source Code

In addition to built-in functions, expressions can use custom functions. They allow you to extend the functionality of your survey.

Your custom function must accept only one array-like parameter that will contain all the passed arguments. For example, the following code passes two arguments to a custom function myFunc:

"expression": "myFunc({question1}, {question2})"

However, the myFunc implementation must accept only one parameter—an array that contains all arguments:

function myFunc(params) {
  let q1_value = params[0];
  let q2_value = params[1];
  // ...
  return someValue;
}

After you implement a custom function, register it in FunctionFactory:

import { FunctionFactory } from "survey-core";

FunctionFactory.Instance.register("myFunc", myFunc);

For illustrative purposes, the code below shows the built-in age function implementation:

import { FunctionFactory } from "survey-core";

// Accepts a birthdate and returns the current age in full years
function age(params: any[]): any {
  if (!params && params.length < 1) return null;
  if (!params[0]) return null;
  var birthDate = new Date(params[0]);
  var today = new Date();
  var age = today.getFullYear() - birthDate.getFullYear();
  var m = today.getMonth() - birthDate.getMonth();
  if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
    age -= age > 0 ? 1 : 0;
  }
  return age;
}
// Register the `age` function under the `age` alias
FunctionFactory.Instance.register("age", age);

View Source Code View Demo

You can access any survey element via a survey instance. Use the this.survey property to get the survey instance within your custom function implementation. This property allows you to design your function so that it accepts a survey element name as a parameter:

"expression": "myFunc('questionName')"

Within the function implementation, you can use the passed name to get the instance of the corresponding element:

function myFunc(params) {
  const questionInstance = this.survey.getQuestionByName(params[0]);
  // ...
}

If an expression requires time-consuming calculations or a request to a server, implement an asynchronous custom function. The following code shows an example of an asynchronous function:

function asyncFunc(params: any[]): any {
  setTimeout(() => {
    // Return the function result via the callback
    this.returnResult(yourValue);
  }, 100);
}

After you implement an asynchronous function, register it in FunctionFactory. The third parameter specifies if this function is asynchronous:

import { FunctionFactory } from "survey-core";

FunctionFactory.Instance.register("asyncFunc", asyncFunc, true);

View Demo

You can specify whether an individual survey element is visible, read-only, or required based on a condition. This functionality is built upon Boolean expressions. Such expressions evaluate to true or false.

A survey parses and runs all expressions on startup. If a Boolean expression evaluates to false, the corresponding element becomes invisible (or read-only, or optional); if it evaluates to true, the element becomes visible (or enabled, or required). After any value used in an expression changes, the survey re-evaluates this expression.

The table below shows examples of Boolean expressions. For a full list of operators available within expressions, refer to the Supported Operators section.

You should use different properties to specify the visibility of questions and items (choices, rows, columns).

Assign Boolean expressions to the visibleIf, enableIf, and requiredIf properties of questions, panels, and pages. In the following example, the visibleIf property is used to hide the drivers-license question for respondents under 16 years old:

const surveyJson = {
  "elements": [{
    "name": "birthdate"
  }, {
    "name": "drivers-license",
    "title": "Have you got a driver's license?",
    "visibleIf": "age({birthdate}) >= 16"
  }]
}

If you do not specify the visibleIf, enableIf, and requiredIf properties, an element's state depends on the isVisible, isReadOnly, and isRequired properties. You can specify them at design time or use them to get or set the current state at runtime. If you set one of these properties for a panel or page, all nested questions inherit the setting.

View Demo

SurveyJS allows you to control available choices, columns, and rows based on previous answers.

Individual items (choices, columns, rows) can be configured with objects. Each object can have a visibleIf property that accepts an expression. When the expression evaluates to true, the associated item becomes visible.

In the following code, the SMS and WhatsApp choices are visible only if a user has entered their phone number:

const surveyJson = {
  "elements": [{
    "name": "Contacts"
    "choices": [
      "Email",
      { "value": "SMS", "visibleIf": "{phone} notempty" },
      { "value": "WhatsApp", "visibleIf": "{phone} notempty" }
    ]
  },
  // ...
  ]
}

View Demo

This technique has one drawback: if a question contains many items, you have to copy the same expression into every item that should have dynamic visibility. If that is your case, use the technique described in the next topic.

You can specify one expression that will run against every item (choice, row, column). If the expression evaluates to true, the item becomes visible. Assign your expression to the choicesVisibleIf, rowsVisibleIf, or columnsVisibleIf property. To access the current item, use the {item} operand.

The following code shows how to specify the choicesVisibleIf property. The "default" question includes selected choices from the "installed" question. The "secondChoice" question also includes selected choices from the "installed" question, but uses the choiceVisibleIf property to filter out the choice selected in the "default" question.

const surveyJson = {
  "elements": [{
    "name": "installed",
    "choices": ["Google Chrome", "Microsoft Edge", "Firefox", "Internet Explorer", "Safari", "Opera"],
    // ...
  }, {
    "name": "default",
    "choicesFromQuestion": "installed",
    "choicesFromQuestionMode": "selected"
    // ...
  }, {
    "name": "secondChoice",
    "choicesFromQuestion": "installed",
    "choicesFromQuestionMode": "selected",
    "choicesVisibleIf": "{item} != {default}",
    // ...
  }]
}

Triggers allow you to implement additional logic that isn't related to read-only or required state or visibility. Each trigger is associated with an expression and an action. A survey re-evaluates this expression each time values used in it are changed. If the expression returns true, the survey performs the associated action.

The following triggers are available:

• complete
• setvalue
• copyvalue
• runexpression
• skip

Completes the survey. The expression is evaluated only when a user switches to the next page.

In the following code, a trigger completes the survey if the "age" question on this page has a value under 18:

const surveyJson = {
  "elements": [{
    "name": "age", 
    // ...
  }],
  "triggers": [
    { "type": "complete", "expression": "{age} < 18" }
  ]
}

View Demo

Sets a specified value to a given question. The setValue property specifies the value; the setToName property specifies the question name.

In the following code, triggers are used to set the "ageType" value to "minor" or "adult" based on the "age" question value:

const surveyJson = {
  "elements": [{
    "name": "age", 
    // ...
  }, {
    "name": "ageType",
    // ...
  }],
  "triggers": [{
    "type": "setvalue",
    "expression": "{age} < 18",
    "setToName": "ageType",
    "setValue": "minor"
  }, {
    "type": "setvalue",
    "expression": "{age} >= 18",
    "setToName": "ageType",
    "setValue": "adult"
  }]
}

You can use the setValueExpression and setValueIf properties as an alternative.

Copies a value from one question to another. The fromName property specifies the source question; the setToName property specifies the target question.

In the following code, a trigger copies the "billingAddress" question value into the "shippingAddress" question if the "sameAsBilling" question is Yes:

const surveyJson = {
  "elements": [{
    "name": "billingAddress", 
    // ...
  }, {
    "name": "shippingAddress",
    // ...
  }, {
    "name": "sameAsBilling",
    "choices": [ "Yes", "No" ]
    // ...
  }],
  "triggers": [{
    "type": "copyvalue",
    "expression": "{sameAsBilling} = 'Yes'",
    "fromName": "billingAddress",
    "setToName": "shippingAddress"
  }]
}

You can use the setValueExpression and setValueIf properties as an alternative.

If the expression is true, the trigger runs another expression specified by the runExpression property. You can also save the result of runExpression as a question value. For this, assign the question's name to the setToName property.

Switches the survey to a target question's page and focuses the question. The gotoName property specifies the target question.

In the following code, a trigger navigates to the "additionalInfoPage" page and focuses the "additionalInfo" question if the "sameAsBilling" question is Yes:

const surveyJson = {
 "pages": [{
   "name": "billingAddressPage",
   "elements": [{
     "name": "billingAddress",
     // ...
    }]
  }, {
   "name": "shippingAddressPage",
   "elements": [{
      "name": "sameAsBilling",
      "choices": [ "Yes", "No" ]
      // ...
  }, {     
     "name": "shippingAddress",
     "visibleIf": "{sameAsBilling} = 'No'",
     // ...
    }]
  }, {
   "name": "additionalInfoPage",
   "elements": [{
     "name": "additionalInfo",
     // ...
    }]
  }],
 "triggers": [{
   "type": "skip",
   "expression": "{sameAsBilling} = 'Yes'",
   "gotoName": "additionalInfo"
  }]
}