Backend: Express.js

Build a working FlowDrop backend with Express in 15 minutes.

This recipe walks you through building a complete FlowDrop backend using Express.js. By the end, you’ll have a working API that FlowDrop can talk to.

FlowDrop includes a complete example Express server at apps/example-server-express/ in the repository. This recipe is based on that implementation.

Prerequisites

• Node.js 20+
• A FlowDrop frontend (see Quick Start)

Step 1: Project Setup

mkdir flowdrop-backend && cd flowdrop-backend
npm init -y
npm install express cors
npm install -D typescript tsx @types/express @types/cors

Create tsconfig.json:

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "strict": true,
    "outDir": "dist",
    "rootDir": "src"
  }
}

Step 2: Health Endpoint

Create src/index.ts:

import express from 'express';
import cors from 'cors';

const app = express();
app.use(cors());
app.use(express.json());

const API_BASE = '/api/flowdrop';

// Health check — FlowDrop calls this on mount
app.get(`${API_BASE}/health`, (req, res) => {
  res.json({ status: 'ok', version: '1.0.0' });
});

const PORT = process.env.PORT || 3001;
app.listen(PORT, () => {
  console.log(`FlowDrop backend running on http://localhost:${PORT}`);
});

Run it:

npx tsx --watch src/index.ts

Step 3: Node Definitions

Create src/nodes.ts with your node metadata. Each node describes what appears in FlowDrop’s sidebar:

export interface NodeMetadata {
  id: string;
  name: string;
  description?: string;
  type: string;
  category: string;
  icon?: string;
  inputs: Array<{
    id: string;
    name: string;
    type: 'input';
    dataType: string;
  }>;
  outputs: Array<{
    id: string;
    name: string;
    type: 'output';
    dataType: string;
  }>;
  configSchema?: {
    type: 'object';
    properties?: Record<string, any>;
    required?: string[];
  };
}

export const nodes: NodeMetadata[] = [
  {
    id: 'text_input',
    name: 'Text Input',
    description: 'Accepts text from the user',
    type: 'simple',
    category: 'inputs',
    icon: 'mdi:text-box-outline',
    inputs: [],
    outputs: [{ id: 'output', name: 'Text', type: 'output', dataType: 'string' }],
    configSchema: {
      type: 'object',
      properties: {
        placeholder: {
          type: 'string',
          title: 'Placeholder',
          default: 'Enter text...'
        },
        multiline: {
          type: 'boolean',
          title: 'Multi-line',
          default: false
        }
      }
    }
  },
  {
    id: 'chat_model',
    name: 'Chat Model',
    description: 'Send prompts to an LLM',
    type: 'workflowNode',
    category: 'models',
    icon: 'mdi:robot-outline',
    inputs: [{ id: 'prompt', name: 'Prompt', type: 'input', dataType: 'string' }],
    outputs: [{ id: 'response', name: 'Response', type: 'output', dataType: 'string' }],
    configSchema: {
      type: 'object',
      properties: {
        model: {
          type: 'string',
          title: 'Model',
          oneOf: [
            { const: 'gpt-4', title: 'GPT-4' },
            { const: 'claude-3-sonnet', title: 'Claude 3 Sonnet' }
          ],
          default: 'gpt-4'
        },
        temperature: {
          type: 'number',
          title: 'Temperature',
          minimum: 0,
          maximum: 2,
          default: 0.7
        }
      }
    }
  },
  {
    id: 'text_output',
    name: 'Text Output',
    description: 'Display text results',
    type: 'simple',
    category: 'outputs',
    icon: 'mdi:text',
    inputs: [{ id: 'input', name: 'Text', type: 'input', dataType: 'string' }],
    outputs: [],
    configSchema: {
      type: 'object',
      properties: {
        format: {
          type: 'string',
          title: 'Format',
          enum: ['plain', 'markdown', 'json'],
          default: 'plain'
        }
      }
    }
  }
];

Add the nodes route in src/index.ts:

import { nodes } from './nodes.js';

app.get(`${API_BASE}/nodes`, (req, res) => {
  let result = nodes;

  // Filter by category
  if (req.query.category) {
    result = result.filter((n) => n.category === req.query.category);
  }

  // Search
  if (req.query.search) {
    const q = (req.query.search as string).toLowerCase();
    result = result.filter(
      (n) => n.name.toLowerCase().includes(q) || n.description?.toLowerCase().includes(q)
    );
  }

  res.json({ success: true, data: result });
});

app.get(`${API_BASE}/nodes/:id`, (req, res) => {
  const node = nodes.find((n) => n.id === req.params.id);
  if (!node) {
    return res.status(404).json({ success: false, error: 'Node not found' });
  }
  res.json({ success: true, data: node });
});

Step 4: Workflow CRUD

Create src/workflows.ts:

import crypto from 'crypto';

export interface Workflow {
  id: string;
  name: string;
  description?: string;
  nodes: any[];
  edges: any[];
  metadata?: {
    schemaVersion: string;
    createdAt: string;
    updatedAt: string;
  };
}

// In-memory storage (use a database in production)
const workflows = new Map<string, Workflow>();

export function getAllWorkflows() {
  return Array.from(workflows.values());
}

export function getWorkflowById(id: string) {
  return workflows.get(id);
}

export function createWorkflow(input: Partial<Workflow>): Workflow {
  const now = new Date().toISOString();
  const workflow: Workflow = {
    id: crypto.randomUUID(),
    name: input.name || 'Untitled Workflow',
    description: input.description,
    nodes: input.nodes || [],
    edges: input.edges || [],
    metadata: { schemaVersion: '1.0.0', createdAt: now, updatedAt: now }
  };
  workflows.set(workflow.id, workflow);
  return workflow;
}

export function updateWorkflow(id: string, updates: Partial<Workflow>): Workflow | null {
  const existing = workflows.get(id);
  if (!existing) return null;

  const updated = {
    ...existing,
    ...updates,
    id, // prevent ID change
    metadata: {
      ...existing.metadata,
      updatedAt: new Date().toISOString()
    }
  };
  workflows.set(id, updated as Workflow);
  return updated as Workflow;
}

export function deleteWorkflow(id: string): boolean {
  return workflows.delete(id);
}

Add the workflow routes in src/index.ts:

import {
  getAllWorkflows,
  getWorkflowById,
  createWorkflow,
  updateWorkflow,
  deleteWorkflow
} from './workflows.js';

app.get(`${API_BASE}/workflows`, (req, res) => {
  res.json({ success: true, data: getAllWorkflows() });
});

app.get(`${API_BASE}/workflows/:id`, (req, res) => {
  const workflow = getWorkflowById(req.params.id);
  if (!workflow) {
    return res.status(404).json({ success: false, error: 'Workflow not found' });
  }
  res.json({ success: true, data: workflow });
});

app.post(`${API_BASE}/workflows`, (req, res) => {
  const workflow = createWorkflow(req.body);
  res.status(201).json({ success: true, data: workflow });
});

app.put(`${API_BASE}/workflows/:id`, (req, res) => {
  const workflow = updateWorkflow(req.params.id, req.body);
  if (!workflow) {
    return res.status(404).json({ success: false, error: 'Workflow not found' });
  }
  res.json({ success: true, data: workflow });
});

app.delete(`${API_BASE}/workflows/:id`, (req, res) => {
  const deleted = deleteWorkflow(req.params.id);
  if (!deleted) {
    return res.status(404).json({ success: false, error: 'Workflow not found' });
  }
  res.json({ success: true, data: { id: req.params.id } });
});

Step 5: Categories & Port Config

Add these routes for the full sidebar and connection validation experience:

app.get(`${API_BASE}/categories`, (req, res) => {
  res.json({
    success: true,
    data: [
      {
        id: 'inputs',
        name: 'Inputs',
        icon: 'mdi:import',
        color: 'var(--fd-node-emerald)',
        weight: 10
      },
      {
        id: 'models',
        name: 'Models',
        icon: 'mdi:robot',
        color: 'var(--fd-node-purple)',
        weight: 20
      },
      {
        id: 'outputs',
        name: 'Outputs',
        icon: 'mdi:export',
        color: 'var(--fd-node-blue)',
        weight: 30
      },
      {
        id: 'processing',
        name: 'Processing',
        icon: 'mdi:cog',
        color: 'var(--fd-node-amber)',
        weight: 40
      },
      {
        id: 'logic',
        name: 'Logic',
        icon: 'mdi:sitemap',
        color: 'var(--fd-node-indigo)',
        weight: 50
      }
    ]
  });
});

app.get(`${API_BASE}/port-config`, (req, res) => {
  res.json({
    success: true,
    data: {
      version: '1.0.0',
      defaultDataType: 'string',
      dataTypes: [
        { id: 'string', name: 'String', color: '#10b981', category: 'basic' },
        { id: 'number', name: 'Number', color: '#3b82f6', category: 'basic' },
        { id: 'boolean', name: 'Boolean', color: '#8b5cf6', category: 'basic' },
        { id: 'json', name: 'JSON', color: '#f59e0b', category: 'complex' },
        { id: 'trigger', name: 'Trigger', color: '#ef4444', category: 'special' }
      ],
      compatibilityRules: [
        { from: 'string', to: 'json' },
        { from: 'number', to: 'string' },
        { from: 'json', to: 'string' }
      ]
    }
  });
});

Step 6: Connect to FlowDrop

In your frontend, point FlowDrop to your backend:

import { mountFlowDropApp } from '@flowdrop/flowdrop/editor';
import { createEndpointConfig } from '@flowdrop/flowdrop/core';

const app = await mountFlowDropApp(document.getElementById('editor'), {
  endpointConfig: createEndpointConfig('http://localhost:3001/api/flowdrop'),
  eventHandlers: {
    onAfterSave: async (workflow) => {
      console.log('Saved:', workflow.id);
    }
  }
});

Start both servers and you should see nodes in the sidebar, be able to drag them onto the canvas, connect them, and save workflows.

Production Considerations

This recipe uses in-memory storage for simplicity. For production:

• Database: Replace the Map with PostgreSQL, MongoDB, or any persistent store
• Validation: Add request body validation (zod, joi, etc.)
• Authentication: Add auth middleware and configure FlowDrop’s authProvider
• Rate limiting: Protect endpoints from abuse
• Error handling: Add proper error middleware

Next Steps

• Backend Implementation Guide — full endpoint reference
• Framework Integration — advanced frontend configuration
• Authentication Patterns — secure your API