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.

Large-form rendering

Paged and flat forms use separate rendering controls.

For a paged form, options.pagePreloadWindow controls how many neighbouring pages are mounted and preloaded. The default is 1. Set it to 0 when only the active page should load, or increase it when fast jump navigation matters more than initial work.

For a large flat form, options.fieldWindowing can defer leaf fields until they approach the viewport:

const config = {
  options: {
    fieldWindowing: {
      eager: 20,
      placeholderHeight: '4rem',
      park: { margin: '100%' },
    },
  },
  fields: [
    // Flat field definitions
  ],
} as const satisfies FormConfig;

• eager mounts the leading leaf fields immediately.
• placeholderHeight reserves layout space before a deferred field mounts.
• park leaves mounted offscreen DOM in place while removing its view from routine change detection. Disabled, readonly, required, and validation state stays current. Other model-to-DOM updates catch up when the field returns.
• park.margin follows IntersectionObserver.rootMargin: use one to four px or % values. Unsupported units fall back to the inherited margin.
• fieldWindowing: false disables inherited deferred mounting for this form.
• A park-only object changes parking without changing the inherited mounting mode. Use { park: false } to opt out or { park: true } to opt in.

Use withFieldWindowing() at provider level when the same defaults should apply to every form. Per-form options take precedence.

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

• Examples: Browse complete form examples
• Field Types: Per-field adapter props reference
• Building an Adapter: Build your own adapter