Computed Fields

Computed fields are values calculated automatically from form data. They update in real-time as users fill out the form.

Structure

Define computed fields in the computed property:

{
  "computed": {
    "fieldName": {
      "expression": "...",
      "label": "Display Label",
      "format": "currency",
      "display": true
    }
  }
}

Properties

Property	Type	Required	Description
expression	string	Yes	FEEL expression to calculate the value
label	string	No	Display label when shown in form
format	string	No	Display format (see Formats)
display	boolean	No	Whether to show in the form UI

Basic Example

{
  "schema": {
    "properties": {
      "quantity": { "type": "integer" },
      "unitPrice": { "type": "number" }
    }
  },
  "computed": {
    "total": {
      "expression": "quantity * unitPrice",
      "label": "Total",
      "format": "currency",
      "display": true
    }
  }
}

Expression Syntax

Arithmetic Operations

{
  "sum": { "expression": "a + b" },
  "difference": { "expression": "a - b" },
  "product": { "expression": "a * b" },
  "quotient": { "expression": "a / b" },
  "power": { "expression": "base ** exponent" }
}

Referencing Form Fields

Reference any field by its name:

{
  "fullName": {
    "expression": "firstName + \" \" + lastName"
  }
}

Referencing Other Computed Values

Use the computed. prefix to reference previously defined computed fields:

{
  "computed": {
    "subtotal": {
      "expression": "quantity * unitPrice"
    },
    "tax": {
      "expression": "computed.subtotal * 0.1"
    },
    "total": {
      "expression": "computed.subtotal + computed.tax"
    }
  }
}

Note: Computed fields are evaluated in dependency order (topological sort). A computed field can reference any other computed field regardless of declaration order.

Conditional Expressions

Use if-then-else for conditional logic:

{
  "discount": {
    "expression": "if quantity >= 10 then 0.1 else 0"
  },
  "tier": {
    "expression": "if total > 1000 then \"gold\" else if total > 500 then \"silver\" else \"bronze\""
  }
}

Null Handling

Handle cases where values might be missing:

{
  "safeTotal": {
    "expression": "if quantity = null then 0 else quantity * unitPrice"
  }
}

Formats

The format property controls how computed values are displayed:

Format	Description	Example Output
currency	Currency format (USD)	$1,234.56
percent	Percentage	25%
decimal(N)	N decimal places	3.14
date	Date only	1/15/2024
datetime	Date and time	1/15/24, 10:30 AM

{
  "taxRate": {
    "expression": "0.0825",
    "format": "percent",
    "display": true
  },
  "total": {
    "expression": "subtotal * (1 + taxRate)",
    "format": "currency",
    "display": true
  }
}

Common Patterns

Calculating Totals

{
  "subtotal": {
    "expression": "quantity * unitPrice"
  },
  "tax": {
    "expression": "computed.subtotal * taxRate"
  },
  "total": {
    "expression": "computed.subtotal + computed.tax",
    "label": "Order Total",
    "format": "currency",
    "display": true
  }
}

Percentage Calculations

{
  "percentComplete": {
    "expression": "completedTasks / totalTasks * 100",
    "format": "percent",
    "display": true
  }
}

Conditional Status

{
  "status": {
    "expression": "if score >= 90 then \"Excellent\" else if score >= 70 then \"Good\" else \"Needs Improvement\"",
    "display": true
  }
}

Date Calculations

{
  "daysUntilDue": {
    "expression": "date(dueDate) - date(today())"
  }
}

BMI Calculator Example

{
  "schema": {
    "properties": {
      "weight": { "type": "number", "description": "Weight in kg" },
      "height": { "type": "number", "description": "Height in cm" }
    }
  },
  "computed": {
    "heightInMeters": {
      "expression": "height / 100"
    },
    "bmi": {
      "expression": "weight / (computed.heightInMeters ** 2)",
      "label": "BMI",
      "format": "decimal(1)",
      "display": true
    },
    "category": {
      "expression": "if computed.bmi < 18.5 then \"Underweight\" else if computed.bmi < 25 then \"Normal\" else if computed.bmi < 30 then \"Overweight\" else \"Obese\"",
      "label": "Category",
      "display": true
    }
  }
}

Using Computed Values

In Visibility Conditions

{
  "discountCode": {
    "visibleWhen": "computed.total > 100"
  }
}

In Validation Rules

{
  "validations": [
    {
      "rule": "computed.total <= budget",
      "message": "Order exceeds budget"
    }
  ]
}

Display Options

Set display: true to show the computed value in the form:

{
  "total": {
    "expression": "quantity * price",
    "label": "Total Price",
    "format": "currency",
    "display": true
  }
}

Computed fields with display: false (or omitted) are calculated but not shown. They can still be used in other expressions and validation rules.