Rent a car: States and conditional rendering

The form from the previous chapter renders every field unconditionally. We’ll now:

1. Declare two states — differentReturn and hasDiscount — that mirror the matching toggles.
2. Gate the optional fields with include: { in: [...] }.

Validators come next — they’re covered end-to-end in the Validation errors chapter.

Declaring states

States are named boolean expressions evaluated against $form. Where they live depends on which API you picked:

Programmatic

In the programmatic API, states live under formConfig.states — a top-level property of the same config object you wired to the form component in the previous chapter, right next to formDef:

React

rent-a-car.page.tsx

const config = {
  formDef: rentACarForm,
  formConfig: {
    states: {
      differentReturn: '$form.differentReturn === true',
      hasDiscount: '$form.hasDiscountCode === true',
    },
  },
};

Angular

rent-a-car.page.component.ts

export class RentACarPage {
  protected config = {
    formDef: rentACarForm,
    formConfig: {
      states: {
        differentReturn: '$form.differentReturn === true',
        hasDiscount: '$form.hasDiscountCode === true',
      },
    },
  };
}

Lit

rent-a-car.page.element.ts

export class RentACarPage extends LitElement {
  config = {
    formDef: rentACarForm,
    formConfig: {
      states: {
        differentReturn: '$form.differentReturn === true',
        hasDiscount: '$form.hasDiscountCode === true',
      },
    },
  };
  // …rest of the class unchanged…
}

Vue

RentACarPage.vue

<script setup lang="ts">
const config = {
  formDef: rentACarForm,
  formConfig: {
    states: {
      differentReturn: '$form.differentReturn === true',
      hasDiscount: '$form.hasDiscountCode === true',
    },
  },
};
</script>

Vanilla JS

src/main.js

const config = {
  formDef: rentACarForm,
  formConfig: {
    states: {
      differentReturn: '$form.differentReturn === true',
      hasDiscount: '$form.hasDiscountCode === true',
    },
  },
};

JSON

In the JSON API, states live at the form file’s root, alongside "form":

rent-a-car.form.json

{
  "states": {
    "differentReturn": "$form.differentReturn === true",
    "hasDiscount": "$form.hasDiscountCode === true"
  },
  "form": [
    // …widgets…
  ]
}

The page component stays exactly as it was in the intro chapter — config just carries formDef:

React

rent-a-car.page.tsx

const config = { formDef: rentACarForm };

Angular

rent-a-car.page.component.ts

export class RentACarPage {
  protected config = { formDef: rentACarForm };
}

Lit

rent-a-car.page.element.ts

export class RentACarPage extends LitElement {
  config = { formDef: rentACarForm };
  // …rest of the class unchanged…
}

Vue

RentACarPage.vue

<script setup lang="ts">
const config = { formDef: rentACarForm };
</script>

Vanilla JS

src/main.js

const config = { formDef: rentACarForm };

Gating fields with include

Add include: { in: ['<stateName>'] } to each optional widget — it will only render while the named state is active. In our form, that’s two widgets: the return-office dropdown (gated by differentReturn) and the discount-code input (gated by hasDiscount).

Programmatic

gui.inputs.dropdown('returnOffice', {
  label: 'Return to office',
  include: { in: ['differentReturn'] },
  // …items, validator…
});

gui.inputs.textInput('discountCode', {
  label: 'Discount code',
  include: { in: ['hasDiscount'] },
  validator: { required: true, minLength: 4 },
});

JSON

{
  "kind": "input",
  "type": "dropdown",
  "path": "returnOffice",
  "label": "Return to office",
  "include": { "in": ["differentReturn"] }
}

{
  "kind": "input",
  "type": "textinput",
  "path": "discountCode",
  "label": "Discount code",
  "include": { "in": ["hasDiscount"] },
  "validator": { "required": true, "minLength": 4 }
}

The toggles that drive the states (differentReturn and hasDiscountCode) stay in the form unconditionally — they’re what the user interacts with to flip the state on or off.

Result

Programmatic

02-states.ts

import { gui } from '@golemui/gui-shared';

export default [
  gui.inputs.dropdown('car', {
    labelField: 'label',
    valueField: 'id',
    items: [
      {
        id: 'compact',
        label: 'Compact',
      },
      {
        id: 'suv',
        label: 'SUV',
      },
      {
        id: 'convertible',
        label: 'Convertible',
      },
      {
        id: 'luxury',
        label: 'Luxury',
      },
    ],
    label: 'Select car',
    validator: {
      type: 'string',
      required: true,
    },
  }),
  gui.layouts.grid([
    gui.inputs.dropdown('collectOffice', {
      labelField: 'label',
      valueField: 'id',
      items: [
        {
          id: 'lhr',
          label: 'London Heathrow',
        },
        {
          id: 'cdg',
          label: 'Paris CDG',
        },
        {
          id: 'fra',
          label: 'Frankfurt Main',
        },
      ],
      label: 'Collect from office',
      validator: {
        type: 'string',
        required: true,
      },
    }),
    gui.inputs.dropdown('returnOffice', {
      labelField: 'label',
      valueField: 'id',
      items: [
        {
          id: 'lhr',
          label: 'London Heathrow',
        },
        {
          id: 'cdg',
          label: 'Paris CDG',
        },
        {
          id: 'fra',
          label: 'Frankfurt Main',
        },
      ],
      label: 'Return to office',
      validator: {
        type: 'string',
        required: true,
      },
      include: {
        in: ['differentReturn'],
      },
    }),
  ], {
    direction: 'row',
    autoFit: true,
  }),
  gui.inputs.booleanInput('differentReturn', {
    label: 'Choose a different return location',
  }),
  gui.inputs.rangeCalendar('rentalDates', {
    numberOfMonths: 2,
    label: 'Rental dates',
    validator: {
      required: true,
      minItems: 1,
      maxItems: 1,
    },
  }),
  gui.inputs.radiogroup('rentalType', {
    options: [
      {
        label: 'Daily',
        value: 'daily',
      },
      {
        label: 'Weekly',
        value: 'weekly',
      },
      {
        label: 'Monthly',
        value: 'monthly',
      },
    ],
    label: 'Rental type',
    validator: {
      type: 'string',
      required: true,
    },
  }),
  gui.inputs.booleanInput('driverOver25', {
    label: 'Driver aged over 25',
    validator: {
      const: true,
      required: true,
    },
  }),
  gui.inputs.booleanInput('hasDiscountCode', {
    label: 'I have a discount code',
  }),
  gui.inputs.textInput('discountCode', {
    label: 'Discount code',
    validator: {
      required: true,
      minLength: 4,
    },
    include: {
      in: ['hasDiscount'],
    },
  }),
  gui.actions.button({
    label: 'Reserve',
    actionType: 'submit',
  }),
];

JSON

02-states.json

{
  "states": {
    "differentReturn": "$form.differentReturn === true",
    "hasDiscount": "$form.hasDiscountCode === true"
  },
  "form": [
    {
      "kind": "input",
      "type": "dropdown",
      "path": "car",
      "label": "Select car",
      "validator": { "type": "string", "required": true },
      "props": {
        "labelField": "label",
        "valueField": "id",
        "items": [
          { "id": "compact", "label": "Compact" },
          { "id": "suv", "label": "SUV" },
          { "id": "convertible", "label": "Convertible" },
          { "id": "luxury", "label": "Luxury" }
        ]
      }
    },
    {
      "kind": "layout",
      "type": "grid",
      "props": { "direction": "row", "autoFit": true },
      "children": [
        {
          "kind": "input",
          "type": "dropdown",
          "path": "collectOffice",
          "label": "Collect from office",
          "validator": { "type": "string", "required": true },
          "props": {
            "labelField": "label",
            "valueField": "id",
            "items": [
              { "id": "lhr", "label": "London Heathrow" },
              { "id": "cdg", "label": "Paris CDG" },
              { "id": "fra", "label": "Frankfurt Main" }
            ]
          }
        },
        {
          "kind": "input",
          "type": "dropdown",
          "path": "returnOffice",
          "label": "Return to office",
          "include": { "in": ["differentReturn"] },
          "validator": { "type": "string", "required": true },
          "props": {
            "labelField": "label",
            "valueField": "id",
            "items": [
              { "id": "lhr", "label": "London Heathrow" },
              { "id": "cdg", "label": "Paris CDG" },
              { "id": "fra", "label": "Frankfurt Main" }
            ]
          }
        }
      ]
    },
    {
      "kind": "input",
      "type": "toggle",
      "path": "differentReturn",
      "label": "Choose a different return location"
    },
    {
      "kind": "input",
      "type": "rangeCalendar",
      "path": "rentalDates",
      "label": "Rental dates",
      "props": { "numberOfMonths": 2 },
      "validator": { "type": "array", "required": true, "minItems": 1, "maxItems": 1 }
    },
    {
      "kind": "input",
      "type": "radiogroup",
      "path": "rentalType",
      "label": "Rental type",
      "validator": { "type": "string", "required": true },
      "props": {
        "options": [
          { "label": "Daily", "value": "daily" },
          { "label": "Weekly", "value": "weekly" },
          { "label": "Monthly", "value": "monthly" }
        ]
      }
    },
    {
      "kind": "input",
      "type": "toggle",
      "path": "driverOver25",
      "label": "Driver aged over 25",
      "validator": { "type": "boolean", "const": true, "required": true }
    },
    {
      "kind": "input",
      "type": "toggle",
      "path": "hasDiscountCode",
      "label": "I have a discount code"
    },
    {
      "kind": "input",
      "type": "textinput",
      "path": "discountCode",
      "label": "Discount code",
      "include": { "in": ["hasDiscount"] },
      "validator": { "type": "string", "required": true, "minLength": 4 }
    },
    {
      "kind": "action",
      "type": "button",
      "label": "Reserve",
      "actionType": "submit"
    }
  ]
}

• The Return to office dropdown only appears when Choose a different return location is on.
• The Discount code input only appears when I have a discount code is on.

The embedded snippet already includes the validator on every required field — those are introduced in the next chapter.

Next

Validation errors →

See also

• Features / States — gating, properties, composing.
• Form Definition / States — include / exclude / when reference.