Dynamic Form Fields - Computed Values & Calculations

Computed properties let you make component settings dynamic, they recalculate automatically whenever the form state changes. In FormEngine Core, you turn a static property into a computed one by setting computeType: "function" and providing the calculation logic in fnSource. In this guide, you’ll learn:

How computed properties differ from static values
When to use computeType and fnSource
Practical patterns for dynamic labels, helper text, and validation messages
How to avoid infinite loops and performance pitfalls

Overview

A computed property is a ComponentProperty whose value is determined by a JavaScript function that runs inside the form’s execution context. The function receives the current IFormData (as the form parameter) and can read form.data, form.errors, form.state, and other form metadata. Computed properties are useful for:

Dynamic labels and hints that change based on other field values
Conditional formatting (colors, visibility hints)
Real‑time validation feedback beyond the built‑in schema validation
Derived values that you want to display but not store as form data

Computed properties are a powerful but advanced mechanism. Because they run on every form change, careless use can create infinite update loops or performance bottlenecks. Use them sparingly and always test the interaction chain between computed properties and the data they depend on.

How computed properties work

Inside a component’s props, any property can be either a static value or a computed one. The structure is the same as for conditional rendering and localization, but with computeType "function".

Property value shapes

Static property (fixed value):

{
  "props": {
    "label": {
      "value": "Email"
    }
  }
}

Computed property (dynamic function):

DynamicHelperText.json

{
  "props": {
    "helperText": {
      "computeType": "function",
      "fnSource": "return form.data.email ? 'Looks good' : 'Enter your email';"
    }
  }
}

The fnSource field contains the body of a JavaScript function that receives form (type IFormData) and returns the property value. Do not wrap the body in function(form) { ... }, write the code that would appear inside the function.

Execution context

Inside fnSource you have access to:

Variable	Type	Purpose
`form`	IFormData	The current form state, errors, and metadata.
`form.data`	`Record<string, unknown>`	Values of all fields keyed by their `dataKey` / `key`.
`form.errors`	`Record<string, unknown>`	Validation errors keyed by `dataKey` / `key`.
`form.state`	`Record<string, unknown>`	Custom state you can set via actions or event handlers.
`form.index`	`number \| undefined`	Index of the current item inside repeaters.

The function runs after the form data changes and before the component renders. If the result differs from the previous computed value, the component updates.

Examples

Dynamic helper text

Show a hint that adapts to the user’s input.

PasswordStrength.json

{
  "key": "password",
  "type": "MuiTextField",
  "props": {
    "label": {
      "value": "Password"
    },
    "type": {
      "value": "password"
    },
    "helperText": {
      "computeType": "function",
      "fnSource": "const pwd = String(form.data.password || '');\nif (pwd.length === 0) return 'Enter a password';\nif (pwd.length < 8) return 'Too short, use at least 8 characters';\nreturn 'Strong password';"
    }
  }
}

Conditional label

Change a label based on a checkbox selection.

BillingLabel.json

{
  "key": "address",
  "type": "MuiTextField",
  "props": {
    "label": {
      "computeType": "function",
      "fnSource": "const isBusiness = form.data.accountType === 'business';\nreturn isBusiness ? 'Business address' : 'Home address';"
    },
    "helperText": {
      "value": "Street, City, ZIP"
    }
  }
}

Real‑time validation feedback

Provide immediate feedback beyond schema validation.

This example is made for illustrative purposes. If you need to show validation data, use the form validation mechanisms.

EmailFeedback.json

{
  "key": "email",
  "type": "MuiTextField",
  "props": {
    "label": {
      "value": "Email"
    },
    "helperText": {
      "computeType": "function",
      "fnSource": "const email = String(form.data.email || '').trim();\nif (!email) return '';\nconst isValid = /^[^\\s@]+@[^\\s@]+\\.[^\\s@]+$/.test(email);\nif (!isValid) return 'Please enter a valid email address';\nconst isCompany = email.endsWith('@company.com');\nreturn isCompany ? 'Company email detected' : 'Personal email';"
    }
  }
}

Live examples

Dynamic character counter

Show a remaining‑character count below a text field.

Live example

live

function App() {
  const form = {
    tooltipType: 'MuiTooltip',
    errorType: 'MuiErrorWrapper',
    form: {
      key: 'Screen',
      type: 'Screen',
      children: [
        {
          key: 'bio',
          type: 'MuiTextField',
          props: {
            label: {value: 'Bio'},
            multiline: {value: true},
            maxRows: {value: 4},
            helperText: {
              computeType: 'function',
              fnSource: `const text = String(form.data.bio || '');
                         const length = text.length;
                         const remaining = 100 - length;
                         if (remaining < 0) return '❌ ' + Math.abs(remaining) + ' characters over limit';
                         if (remaining < 20) return '⚠️ ' + remaining + ' characters left';
                         return '✓ ' + remaining + ' characters left';`
            }
          }
        }
      ]
    }
  }

  const getForm = useCallback(() => JSON.stringify(form), [form])

  return <FormViewer view={muiView} getForm={getForm}/>
}

Price calculator

Compute a total price based on quantity and unit price, and display it as helper text.

Live example

live

function App() {
  const form = {
    tooltipType: 'MuiTooltip',
    errorType: 'MuiErrorWrapper',
    form: {
      key: 'Screen',
      type: 'Screen',
      children: [
        {
          key: 'quantity',
          type: 'MuiTextField',
          props: {
            label: {value: 'Quantity'},
            type: {value: 'number'},
            helperText: {value: 'Number of items'}
          }
        },
        {
          key: 'unitPrice',
          type: 'MuiTextField',
          props: {
            label: {value: 'Unit price ($)'},
            type: {value: 'number'},
            helperText: {value: 'Price per item'}
          }
        },
        {
          key: 'total',
          type: 'MuiTypography',
          props: {
            children: {
              computeType: 'function',
              fnSource: `const q = Number(form.data.quantity || 0);
                         const p = Number(form.data.unitPrice || 0);
                         const total = q * p;
                         if (total <= 0) return 'Enter quantity and price';
                         if (total > 1000) return '💵 Total: $' + total.toFixed(2) + ' (bulk discount available)';
                         return '💰 Total: $' + total.toFixed(2);`
            }
          }
        }
      ]
    }
  }

  const getForm = useCallback(() => JSON.stringify(form), [form])

  return <FormViewer view={muiView} getForm={getForm}/>
}

Conditional styling via computed property

Makes the button accessible if the checkbox is checked.

Live example

live

function App() {
  const form = {
    tooltipType: 'MuiTooltip',
    errorType: 'MuiErrorWrapper',
    form: {
      key: 'Screen',
      type: 'Screen',
      children: [
        {
          key: 'acceptTerms',
          type: 'MuiFormControlLabel',
          props: {
            control: {value: 'Checkbox'},
            label: {value: 'I accept the terms and conditions'}
          }
        },
        {
          key: 'submitButton',
          type: 'MuiButton',
          props: {
            children: {value: 'Submit'},
            disabled: {
              computeType: 'function',
              fnSource: 'return form.data.acceptTerms !== true;'
            }
          }
        }
      ]
    }
  }

  const getForm = useCallback(() => JSON.stringify(form), [form])

  return <FormViewer view={muiView} getForm={getForm}/>
}

Pitfalls and precautions

Infinite update loops

The most dangerous risk with computed properties is creating a circular dependency: a computed property writes to form.data (directly or via an action), which triggers recomputation, which writes again, and so on.

DangerousExample.json

{
  "props": {
    "value": {
      "computeType": "function",
      "fnSource": "// NEVER DO THIS: writing to the same field you read from\nform.data.someField = form.data.someField + 1;\nreturn form.data.someField;"
    }
  }
}

Never modify form.data inside a computed property. Treat form as read‑only. If you need to update the form based on a calculation, use an action or an event handler instead.Also, do not return new objects from the computed property as a result. FormEngine Core will assume that the value of the computed property changes every time and will cause an infinite loop of form rendering.

Performance impact

Computed properties run on every form change. If you have many computed properties or complex logic inside them, the form may feel sluggish. Do:

Keep the logic inside fnSource short and fast.
Use computed properties only for properties that truly need to be dynamic.

Don’t:

Perform network requests, large array operations, or synchronous heavy computations inside fnSource.
Create chains where property A depends on B, B depends on C, and C depends on A.

Error handling

If the function throws an exception, FormEngine Core catches it and logs a warning to the console. The property will keep its previous value (or fall back to the component’s default). Always test your computed functions with edge cases: null, undefined, empty strings, unexpected types.

Best practices

Keep functions pure: A computed property should be a pure function of form. No side effects, no modifications to form.
Use for display, not storage: Prefer computed properties for labels, helper text, colors, and read‑only displays. For values that should be saved as form data, use regular data‑bound fields.
Leverage form.hasErrors: When you want to reflect validation state, check form.hasErrors and form.errors rather than re‑implementing validation logic.
Test the dependency chain: Before deploying, verify that changing any field doesn’t cause unexpected recomputation loops.

Summary

Computed properties let you define component settings that recalculate automatically when the form state changes.
Set computeType: "function" and provide the JavaScript function body in fnSource.
The function receives form (type IFormData) and must return the property value.
Use with caution: Avoid modifying form.data, keep functions lightweight, and watch for circular dependencies.
Ideal for dynamic labels, real‑time feedback, conditional formatting, and derived display values.

For more information:

Next steps

Conditional Logic — Show or hide components based on form conditions
Actions & Events — Respond to user interactions and form changes

​Overview

​How computed properties work

​Property value shapes

​Execution context

​Examples

​Dynamic helper text

​Conditional label

​Real‑time validation feedback

​Live examples

​Dynamic character counter

​Live example

​Price calculator

​Live example

​Conditional styling via computed property

​Live example

​Pitfalls and precautions

​Infinite update loops

​Performance impact

​Error handling

​Best practices

​Summary

​Next steps

Overview

How computed properties work

Property value shapes

Execution context

Examples

Dynamic helper text

Conditional label

Real‑time validation feedback

Live examples

Dynamic character counter

Live example

Price calculator

Live example

Conditional styling via computed property

Live example

Pitfalls and precautions

Infinite update loops

Performance impact

Error handling

Best practices

Summary

Next steps