Performing Calculations

You can obtain derived numerical values in your form inputs through the use of calculateExpressions. calculateExpressions are used in form inputs of type number. You can extend your input's questionOptions definition to include a calculate property in which you would nest your calculateExpression definition.

The following is an example of a calculateExpression:

{
  "label": "BMI:Kg/M2",
  "questionOptions": {
    "rendering": "number",
    "concept": "a89c60c0-1350-11df-a1f1-0026b9348838",
    "max": "100",
    "min": "0",
    "calculate": {
      "calculateExpression": "calcBMI(height,weight)"
    }
  },
  "type": "obs",
  "validators": [],
  "hide": {
    "hideWhenExpression": "age<18"
  }
}

Here, the calculateExpression returns the value of invoking calcBMI with the given height and weight values.

More Examples

1. Performing a basic calculation

{
  "label": "Average number of sex acts per week",
  "type": "obs",
  "id": "weeklyCondomRequirements",
  "questionOptions": {
    "concept": "25852ebc-9fc6-40fa-b502-1ce90bc2bcc8",
    "rendering": "number"
  }
}

The following question computes weeklyCondomRequirements using the value provided in the previous question.

{
  "label": "Condom requirements per month",
  "type": "obs",
  "id": "condomRequirementsMonth",
  "questionOptions": {
    "concept": "7b3f4d87-17ca-4660-b909-4c53c60cdcf1",
    "rendering": "number",
    "calculate": {
      "calculateExpression": "weeklyCondomRequirements * 4"
    }
  }
}

2. Using moment (opens in a new tab) to compute the difference between two dates in years

"calculate": {
  "calculateExpression": "(moment().endOf('year')).diff(moment(dateOfBirth), 'years')"
}

3. Computing `Morisky 4 Total Score` from decimal values

"calculate": {
  "calculateExpression": "isNaN(parseFloat(feelsBetter) + parseFloat(feelsWorse) + parseFloat(notKeen) + parseFloat(forget)) ? undefined : (parseFloat(feelsBetter) + parseFloat(feelsWorse) + parseFloat(notKeen) + parseFloat(forget))"
}

4. Computing `BmiForAgeZscore` using the `calcBMIForAgeZscore` and `isEmpty` expression helpers

"calculate": {
  "calculateExpression": "!isEmpty(height) && !isEmpty(weight) ? calcBMIForAgeZscore(bmiForAgeRef,height,weight): ''"
}

5. Computing `Estimated Delivery Date` using moment (opens in a new tab)

"calculate": {
  "calculateExpression": "moment(lastMenstrualPeriodDate).isValid() ? moment(lastMenstrualPeriodDate).add(280, 'days').toDate() : ''"
}

Hiding Fields Historical Expressions