JavaScript Samples

The JavaScript samples use the @modelcontextprotocol/sdk package and Zod for schema validation. Both samples run with Node.js and use ES module syntax.

Basic (stdio)
Advanced

Basic calculator server

Located at 03-GettingStarted/samples/javascript, this is the simplest possible MCP server — four arithmetic tools over a stdio transport.

Clone and navigate

git clone https://github.com/microsoft/mcp-for-beginners.git
cd mcp-for-beginners/03-GettingStarted/samples/javascript

Install dependencies

npm install

Run the server

npm start

package.json

{
  "name": "tutorial-mcp",
  "version": "1.0.0",
  "type": "module",
  "scripts": {
    "start": "node index.js"
  },
  "dependencies": {
    "@modelcontextprotocol/sdk": ">=1.26.0",
    "zod": "^3.24.2"
  }
}

Server code

index.js

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";

// Create an MCP server
const server = new McpServer({
  name: "Calculator MCP Server",
  version: "1.0.0"
});

// Add tool
server.tool(
  "add",
  { a: z.number(), b: z.number() },
  async ({ a, b }) => ({
    content: [{ type: "text", text: String(a + b) }]
  })
);

// Subtract tool
server.tool(
  "subtract",
  { a: z.number(), b: z.number() },
  async ({ a, b }) => ({
    content: [{ type: "text", text: String(a - b) }]
  })
);

// Multiply tool
server.tool(
  "multiply",
  { a: z.number(), b: z.number() },
  async ({ a, b }) => ({
    content: [{ type: "text", text: String(a * b) }]
  })
);

// Divide tool — includes divide-by-zero guard
server.tool(
  "divide",
  { a: z.number(), b: z.number() },
  async ({ a, b }) => {
    if (b === 0) {
      return {
        content: [{ type: "text", text: "Error: Cannot divide by zero" }],
        isError: true
      };
    }
    return {
      content: [{ type: "text", text: String(a / b) }]
    };
  }
);

// Connect via stdio
const transport = new StdioServerTransport();
server.connect(transport).catch(console.error);

console.log("Calculator MCP Server started");

Each tool uses a Zod schema to declare its input types. The MCP SDK validates incoming arguments automatically before calling the handler.

Advanced server with resources and event emitters

Located at 04-PracticalImplementation/samples/javascript, this sample shows a class-based server architecture with a completion tool and a search resource.

Navigate to the advanced sample

cd 04-PracticalImplementation/samples/javascript

Install and run

npm install
npm start

Server code

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import { z } from 'zod';
import EventEmitter from 'events';

class ExtendedMcpServer {
  constructor(options = {}) {
    this.serverName = options.serverName || 'JavaScript MCP Server';
    this.models = options.models || ['gpt-4', 'llama-3-70b', 'claude-3-sonnet'];
    this.events = new EventEmitter();
    this.mcpServer = new McpServer({
      name: this.serverName,
      version: options.version || '1.0.0'
    });
    this.registerCompletionTool();
    this.registerSearchResource();
  }

  async connect() {
    const transport = new StdioServerTransport();
    await this.mcpServer.connect(transport);
  }

  on(event, listener) {
    this.events.on(event, listener);
  }
}

Key patterns demonstrated

Pattern	Detail
Class-based server	Encapsulates server state and tool registration in a class
Event emitters	Emits `request` and `completion` events for monitoring/metrics
Resources with URI templates	`search://{query}` — clients read dynamic data via URI patterns
Model validation	Checks model name against a supported list before processing

Basic Examples

Basic calculator server

package.json

Server code

Advanced server with resources and event emitters

Server code

Key patterns demonstrated

Build docs developers (and LLMs) love

Basic Examples

Documentation Index

​Basic calculator server

​package.json

​Server code

​Advanced server with resources and event emitters

​Server code

​Key patterns demonstrated

Build docs developers (and LLMs) love

Basic calculator server

package.json

Server code

Advanced server with resources and event emitters

Server code

Key patterns demonstrated