Frontend System Design: Building a Dynamic Form Builder from Scratch

Hey everyone! I realized that many folks aren’t familiar with how frontend system design interviews work — and unfortunately, there aren’t many good resources out there either. So, I decided to tackle a common frontend system design problem and break it down into a structured solution.

Let’s dive right in!

Problem scope

Design a frontend system where users can create, modify, and interact with dynamic forms. The form builder should allow adding various field types (e.g., text, dropdown, date picker), validation rules, conditional logic, and submission handling.

Key requirements

User Interface:

• Drag-and-drop field placement.
• Real-time preview of the form being built.
• Configurable field properties (label, placeholder, validation, etc.).

Field Types:

• Text Input, Dropdown, Radio Buttons, Checkbox, Date Picker, etc.

Conditional Logic:

• Show/hide fields based on other field values.

Validation:

• Required fields, regex patterns, min/max lengths, etc.

Preview Pane (Optional):

• Real-time rendering of the form as users build it.

Now that we have the problem statement and key requirements sorted, let’s start designing the system.

1. Component Architecture

Use a modular component-based architecture for scalability.

• FormBuilder Component: Main UI to build forms.
• FieldPalette: Sidebar containing draggable field types.
• Canvas: Area to drop and arrange fields.
• FieldConfigurator: Config panel for selected field properties.
• PreviewPane(Optional): Live preview of the form.
• DynamicForm Component: UI to render the form. Handles form submission and validation dynamically.

2. State Management

Centralize the state for consistency and better performance.

• Global State: Use Redux or Context API.
• Store the structure of the form ([{ id, type, label, config }]).
• Track conditional logic (if field A === X, show field B).
• Local State: Manage UI-specific states (e.g., drag-and-drop interactions, active field selection).

3. Data Model

interface FormField {
  id: string; // Unique ID for the field
  type: string; // Field type (e.g., 'text', 'dropdown')
  label: string; // Field label
  config: {
    placeholder?: string;
    options?: string[]; // For dropdown, radio, etc.
    validation?: {
      required?: boolean;
      regex?: string; // Validation pattern
      minLength?: number;
      maxLength?: number;
    };
    conditionalLogic?: {
      showIfField: string; // Field ID
      condition: string; // E.g., 'equals', 'notEquals'
      value: string | number;
    };
  };
}

4. Drag-and-Drop Logic

Use libraries like react-dnd.

• Fields from the palette are dragged and dropped onto the canvas.
• Update the global state with the new field structure and order.

5. Conditional Logic

• Add a UI for users to configure conditions (e.g., “Show this field if Field A = ‘Yes’”).
• Store logic in the field configuration and evaluate it during rendering.

6. Form Rendering

• Iterate over the form structure and render components dynamically.
• Use a Registry Pattern for field rendering:

const fieldRegistry = {
  text: (props) => <TextField {...props} />,
  dropdown: (props) => <DropdownField {...props} />,
  // Add other field types...
};

fields.map((field) => fieldRegistry[field.type](field.config));

7. Validation Handling

• Generate validation rules dynamically based on the validation property of each field.

8. Optimization

• Virtualization: Use libraries like react-window for rendering large forms.
• Debouncing: Optimize user interactions (e.g., input changes, dragging).
• Memoization: Use React.memo or useMemo for expensive computations.

Example Workflow

1. User Action: Drag a “Text Input” field onto the canvas.
   - Add a new field object to the global state.
   - Render the new field in the canvas.
2. User Configures Field: Add a label, placeholder, and validation rules.
   - Update the corresponding field’s config in the state.
3. Save: Serialize the form state into JSON.
4. Load: Deserialize JSON to recreate the form structure.

This was a high-level design (HLD) for the problem, which can dive into low-level design (LLD) depending on where the interviewer wants to focus. Remember, system design interviews are always a collaborative conversation — there’s no single “right” or “wrong” approach.

I’ll keep sharing more insights about frontend system design, so stay tuned! If you’d like me to deep-dive into any topic from this blog or explore another problem, let me know. Your suggestions might shape the next post!