Coming from ngx-formly? The migration guide maps defaultProps, extends, and the formly extensions API to their ng-forge equivalents.

Configure global defaults for all forms at provider level, or per-form via defaultProps.

The Cascade

ng-forge applies props in priority order: more specific always wins.

Library-level
withXxxFields({...})All forms
Form-level
defaultPropsOne form
Field-level
propsOne field

Custom Adapter Config

Custom adapters define their own config interface. The cascade pattern works the same way: implement a config merger in your adapter to support library-level and form-level defaults. See Building an Adapter for a full guide.

Library-level (provider)

// Define your own config interface:
export interface MyAdapterConfig {
  variant?: 'primary' | 'secondary';
  size?: 'sm' | 'lg';
}

// Pass it to withCustomFields():
provideDynamicForm(
  ...withCustomFields({ variant: 'primary', size: 'sm' })
)

Form-level (defaultProps)

Use (your form config type) from (your adapter package) for type-safe defaultProps with autocomplete. 

// Define a typed FormConfig alias using your config:
import { FormConfig } from '@ng-forge/dynamic-forms';
import { MyAdapterConfig } from './my-adapter';

type MyFormConfig = FormConfig<any, any, MyAdapterConfig>;

const config = {
  defaultProps: {
    variant: 'secondary',
  },
  fields: [
    { type: 'my-input', key: 'name', label: 'Name' },
  ],
} as const satisfies MyFormConfig;

Field-level Props

Each field type also accepts its own adapter-specific props. See Field Types for the full per-field reference.

Nullable values

Value fields accept an optional nullable?: boolean flag. When true:

  • value accepts null in addition to the field's normal type (e.g. string | null).
  • An omitted value resolves to null instead of the type-specific empty default ('', NaN, [], …).
  • nullable stays orthogonal to required: they describe different layers. nullable declares that the model accepts null (data shape). required is a validation constraint. ng-forge maps { type: 'required' } to the Signal Forms required() validator, which treats null as invalid, so a field that is both nullable and required will fail required-validation when the value is null. The flags are independent OpenAPI concepts; combine them if that matches your schema, but understand the runtime interaction.
{
  key: 'middleName',
  type: 'input',
  label: 'Middle Name',
  nullable: true,
  value: null,       // allowed; also the resolved default when omitted
}

Read-side caveat. A user clearing a text input reads back as "", not null. This is a DOM/Web IDL contract, identical to classic Reactive Forms. nullable is a contract for accepted values, not a guarantee of emitted ones. If your backend distinguishes null from empty string, handle the coercion at submission.

Multiple forms on one page

Each field renders a DOM id derived from its key (id="email", id="email-input"), and that id is reused for the <label for>, aria-describedby, and error/hint targets. Render two forms built from the same config on one page and those ids collide: clicking the second form's label focuses the first form's input, and duplicate ids are invalid HTML that also break getByLabelText-style queries.

ng-forge scopes ids to a form instance to prevent this:

  • One form on the page: ids stay clean and unprefixed (email-input). Nothing changes, so existing selectors and data-testids are untouched.
  • Two or more forms visible at once: each form automatically gets a generated prefix (df-1, df-2, …), so its ids become df-2_email-input. Detection tracks the forms currently rendered, so a form reverts to clean, unprefixed ids once it's the only one left (and hidden/cached pages, such as a previous route still in the DOM, don't count).

Set options.idPrefix when you want stable, human-readable ids regardless of how many forms are on the page (recommended when you control both forms). An explicit prefix always wins over auto-detection:

const billing = {
  options: { idPrefix: 'billing' },
  fields: [{ key: 'email', type: 'input', label: 'Email' }],
} as const satisfies FormConfig;
// → <form id="billing">, <input id="billing_email-input">, <label for="billing_email-input">

The prefix is the outermost id segment, composing with group and array scoping:

{idPrefix}_{group}_{key}_{index}   e.g.  billing_address_street_0

Use a valid id token: letters, digits, -, _. Whitespace or punctuation is replaced with _ (with a dev-mode warning), since a space would break the aria-describedby token list and for/id matching.

Writing E2E selectors or aria-* references against a form that shares the page with another form? Set an explicit idPrefix so the ids you target are deterministic, then select #billing_email-input rather than the auto-generated #df-2_email-input.

Next Steps