Validations

Validations let you define what makes a field valid or not, and how the user should be informed when it’s not. FormBuilder supports two kinds of validation:

• Simple validation rules as strings, like required or email
• Advanced object-based validations, for custom rules, messages, and cross-field logic

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Simple validations (as strings)

This is the most concise way to validate a field. You just list built-in rule names:

{
  "component": "text",
  "label": "Email",
  "validations": ["required", "email"]
}

If the value is missing or not an email, the field will show an error.

Available string validations

Rule	Description
required	Must not be empty
date_format	Must be a valid date
only_int	Only integer numbers allowed
only_letter	Only letters allowed
only_int_price	Integer price format only
email	Valid email address
required_email	Required and must be a valid email
niss	Valid NISS (national ID)
iban	Valid IBAN
required_tile	An item must be selected on the tile input
phone_number	Valid phone number
required_phone_number	Required and must be a valid phone number

You can combine several in a single array.

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Advanced validations (as objects)

Use objects if you need:

• Custom error messages
• Comparisons against another field
• Conditional validations
• Regex patterns
• Type coercion or derived values (like age)

Example: exact match

{
  "value": "MyCompany",
  "message": "The company name must be MyCompany",
  "assertion": "equal"
}

Example: number comparison

{
  "value": 17,
  "message": "Minimum 18 years",
  "assertion": "isAbove"
}

Using expectation instead of assertion

{
  "value": 17,
  "message": "Minimum 18 years",
  "expectation": "to.be.above"
}

Compare to another field

{
  "value": { "from": "age" },
  "message": "You cannot have more experience than your age",
  "expectation": "to.be.below"
}

Validate based on another property’s existence

{
  "property": { "from": "age" },
  "message": "You have to define your age first",
  "assertion": "exist"
}

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Regex-based validation

{
  "regex": {
    "pattern": "^[A-Z0-9]+$",
    "flags": "i",
    "must_match": true
  },
  "message": "Only uppercase letters and numbers allowed"
}

• If must_match is true: value must match the regex
• If false: value must not match the regex

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Combining multiple conditions with operator

You can combine multiple object validations and control how they’re evaluated:

{
  "assertion": "exists",
  "message": "Name is required",
  "operator": "or"
},
{
  "assertion": "isEmpty",
  "message": "Name is required",
  "operator": "or"
}

warning

Currently, you cannot nest condition groups — everything is evaluated flatly.

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propertyAs: Force a type or interpretation

Sometimes you want to compare a value as a number, or as an age, or as duration since a date. Use propertyAs to convert types at runtime.

Available coercion values

• string
• number
• bool
• age (years since date)
• years, months, days
• yearsFrom, monthsFrom, daysFrom

Example: compare derived age

{
  "property": "date_of_birth",
  "propertyAs": "age",
  "value": 30,
  "assertion": "isBelow",
  "message": "The age must be below 30 years"
}

Example: duration from another date

{
  "fromDate": "first_date",
  "propertyAs": "yearsFrom",
  "value": 17,
  "assertion": "isAbove",
  "message": "Min 17 years from above date"
}

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Conditional validations

A validation can also be enabled only if a condition is met:

{
  "assertion": "exists",
  "message": "Name is required",
  "conditions": [
    {
      "property": { "from": "name_is_required" },
      "assertion": "isTrue"
    }
  ]
}

This validation only runs if name_is_required is true.

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Storybook examples

Simple Condition#

Condition Of Conditions#

Required Under Conditions#

Age Based Condition#

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For assertion syntax, see Conditions