dvlp is a no-configuration, no-conditionals, no-middleware, no-nonsense (no-vowels!) dev server toolkit to help you develop quickly and easily for the web. You shouldn't have to jump through hoops to get a development environment up and running, and you definitely shouldn't have to include development-only stuff in your high-quality production code! dvlp is full of hacks so your code doesn't have to be!

• No bundling: write JS modules and load them directly in the browser
• No middleware: write application servers without special dev/build/bundle middleware
• No infrastructure: mock external JSON/EventSource/WebSocket resources
• No waiting: restart application servers in the blink of an eye
• No refreshing: automatically reload clients on file change

dvlp allows you to easily serve resources from one or more project directories (static mode), from your custom application server (app mode), or from your Electron desktop application (electron mode). In all cases, dvlp creates a proxy server in front of your content, automatically injecting the necessary reload script into HTML responses to enable reloading, and watches all files for changes, restarts the app server/ electron application if necessary, and reloads all connected clients.

In addition, when working with JS modules, dvlp will ensure that so-called bare imports (import "lodash"), which are not natively supported by browsers, work by re-writing all import paths to valid urls. Since some node_modules packages are still published as CommonJS modules, non-ESM packages are bundled and converted to an ESM module using esbuild. These bundles are versioned and cached for efficient reuse in the .dvlp directory under your project root.

dvlp also includes a testServer for handling various network request scenarios (mocking, latency, errors, offline, etc.) during testing.

Install globally or locally in your project with npm/yarn:

$ npm install dvlp

Usage: dvlp [options] [path...]

Start a development server, restarting and reloading connected clients on file changes.
    Serves static files from one or more "path" directories, or a custom application
    server if "path" is a single application server file.

Options:
  -p, --port <port>   port number
  -m, --mock <path>   path to mock files (directory, file, glob pattern)
  -k, --hooks <path>  path to optional hooks registration file
  -e, --electron      run "path" file as electron.js entry file
  --ssl <path>        enable https mode by specifying path to directory containing ".crt" and ".key" files (directory,
                      glob pattern)
  -s, --silent        suppress all logging
  --verbose           enable verbose logging
  --no-reload         disable reloading connected clients on file change
  -v, --version       output the current version
  -h, --help          display help for command

Add a script to your package.json scripts:

{
  "scripts": {
    "dev": "dvlp --port 8000 src/app.js"
  }
}

...and launch:

$ npm run dev

In some cases, source code may need to be transformed into a valid format before it is executed, or a response body modified before sending it to the browser. In these cases, you can register hooks to convert file contents on the fly when imported by an application server or requested by the browser.

// scripts/hooks.js
import sass from 'sass';

const RE_SASS = /\.s[ac]ss$/;

export default {
  /**
   * Bundle non-esm node_modules dependency requested by the browser.
   * This hook is run after file read.
   *
   * @param { string } id
   * @param { string } filePath
   * @param { string } fileContents
   * @param { { esbuild: Pick<import("esbuild"), 'build'> } } context
   */
  async onDependencyBundle(id, filePath, fileContents, context) {
    if (id === 'some/package') {
      // Transform
    }
  }

  /**
   * Transform file contents for file requested by the browser.
   * This hook is run after file read, and before any modifications by dvlp.
   *
   * @param { string } filePath
   * @param { string } fileContents
   * @param { { client: { manufacturer: string, name: string, ua: string, version: string }, esbuild: Pick<import("esbuild"), 'build', 'transform'> } } context
   */
  async onTransform(filePath, fileContents, context) {
    // Note: .ts, .tsx, .jsx files are transformed by default

    if (RE_SASS.test(filePath)) {
      return sass.renderSync({
        file: filePath,
      }).css;
    }
  },

  /**
   * Manually resolve import specifier.
   * This hook is run for each import statement.
   * If returns "false", import re-writing is skipped.
   * If returns "undefined", import specifier is re-written using default resolver.
   * If "context.isDynamic", also possible to return replacement for whole expression.
   *
   * @param { string } specifier
   * @param { { importer: string, isDynamic: boolean } } context
   * @param { (specifier: string, importer: string) => string | undefined } defaultResolve
   */
  onResolveImport(specifier, context, defaultResolve) {
    if (context.isDynamic) {
      return `dynamicImport('./some-path-prefix/${specifier}.js', '${context.importer}')`;
    }
  },

  /**
   * Manually handle response for incoming server request.
   * If returns "true", further processing by dvlp will be aborted.
   *
   * @param { IncomingMessage | Http2ServerRequest } request
   * @param { ServerResponse | Http2ServerResponse } response
   * @returns { Promise<boolean> | boolean | undefined }
   */
  onRequest(request, response) {
    if (request.url === '/something') {
      response.writeHead(200);
      response.end('handled');
      return true;
    }
  },

  /**
   * Modify response body before sending to the browser.
   * This hook is run after all modifications by dvlp, and before sending to the browser.
   *
   * @param { string } filePath
   * @param { string } responseBody
   */
  onSend(filePath, responseBody) {
    if (RE_JS.test(filePath)) {
      return responseBody.replace('__VERSION__', '1.0.0');
    }
  },

  /**
   * Transform file contents for application server.
   *
   * @param { string } filePath
   * @param { { format?: string } } context
   * @param { NodeLoadLoaderHook } defaultTransform
   * @returns { { format: string; source: string | SharedArrayBuffer | Uint8Array } }
   */
  onServerTransform(filePath, context, defaultTransform) {
    // Note: .ts, .tsx, .jsx files are transformed by default
    // @see https://nodejs.org/api/esm.html#loadurl-context-defaultload
  },

  /**
   * Manually resolve import specifiers for application server.
   *
   * @param { string } specifier
   * @param { { conditions: Array<string>; parentURL?: string } } context
   * @param { NodeResolveLoaderHook } defaultResolve
   * @returns { { format?: string; url: string } }
   */
  onServerResolve(specifier, context, defaultResolve){
    // @see https://nodejs.org/api/esm.html#resolvespecifier-context-defaultresolve
  }
};

<link rel="stylesheet" href="src/index.sass" />

{
  "scripts": {
    "dev": "dvlp --hooks scripts/hooks.js --port 8000 src/app.js"
  }
}

When developing locally, it's often useful to mock responses for requests made by your server or browser application, especially when working with an external API. dvlp lets you quickly and easily mock endpoints by intercepting requests that match those registered with the -m, --mock option.

{
  "request": {
    "url": "http://www.someapi.com/v1/id/101010",
    "ignoreSearch": true
  },
  "response": {
    "headers": {
      "x-custom": "custom header"
    },
    "body": {
      "user": {
        "name": "Nancy",
        "id": "101010"
      }
    }
  }
}

{
  "request": {
    "url": "http://www.someapi.com/v1/id/101010"
  },
  "response": {
    "error": true,
    "body": {}
  }
}

[
  {
    "request": {
      "url": "http://www.someapi.com/v1/id/101010"
    },
    "response": {
      "body": {}
    }
  },
  {
    "request": {
      "url": "http://www.someapi.com/v1/id/202020"
    },
    "response": {
      "body": {}
    }
  }
]

{
  "request": {
    "url": "http://www.someplace.com/images/avatar.jpg"
  },
  "response": {
    "body": "../assets/avatar.jpg"
  }
}

{
  "scripts": {
    "dev": "dvlp --mock path/to/mock/files --port 8000 src/app.js"
  }
}

{
  "stream": {
    "url": "ws://www.somesocket.com/stream",
    "ignoreSearch": true,
    "protocol": "socket.io"
  },
  "events": [
    {
      "name": "hello Bob",
      "connect": true,
      "message": {
        "people": ["Bob Builder"]
      },
      "options": {
        "event": "update",
        "namespace": "/people"
      }
    },
    {
      "name": "hello Ernie",
      "message": {
        "people": ["Bob Builder", "Ernie Engineer"]
      },
      "options": {
        "event": "update",
        "namespace": "/people"
      }
    }
  ]
}

{
  "stream": {
    "url": "http://www.someeventsource.com/stream"
  },
  "events": [
    {
      "name": "a sequence of unfortunate events",
      "sequence": [
        {
          "message": "oh",
          "options": {
            "event": "update"
          }
        },
        {
          "message": "no",
          "options": {
            "event": "update",
            "delay": 100
          }
        },
        {
          "message": "not",
          "options": {
            "event": "update",
            "delay": 50
          }
        },
        {
          "message": "again!",
          "options": {
            "event": "update",
            "delay": 10
          }
        }
      ]
    }
  ]
}

{
  "scripts": {
    "dev": "dvlp --mock path/to/mock/files --port 8000 src/app.js"
  }
}

dvlp.pushEvent('ws://www.somesocket.com/stream', 'hello Ernie');

import { testBrowser } from 'dvlp/test-browser';

describe('some test', () => {
  before(() => {
    testBrowser.disableNetwork();
  });
  after(() => {
    testBrowser.enableNetwork();
  });

  it('should fetch mock data', async () => {
    const href = 'https://www.google.com';
    testBrowser.mockResponse(
      href,
      (req, res) => {
        res.writeHead(500);
        res.end('error');
      },
      true,
    );
    const res = await fetch(href);
    assert.equal(res.status, 500);
  });
});

As mentioned in How it works, dvlp will bundle CommonJS packages imported from node_modules in order to convert them to es6 modules. esbuild is used to create these bundles, and they are then cached on disk inside the .dvlp directory under your project root.

In the (rare) case you need to customise bundling to work with the packages you're importing, you can register a onDependencyBundle hook.

Enable development against a secure http2 server by passing the path or glob pattern to your .crt and .key files with the --ssl option.

Follow the directions here to generate a self-signed certificate for local development

dvlp uses the debug.js debugging utility internally. Set the following environment variable before running to see detailed debug messages:

$ DEBUG=dvlp* npm run dev

Serve files at filePath, starting static file server if one or more directories, or app server if a single file or function (which starts an application server when imported/called).

options include:

• certsPath: string|[string]: the path or glob pattern containing ".crt" and ".key" files (default '')
• directories: [string]: additional directories to use for resolving file requests (default [])
• hooksPath: string: the path to a hooks registration file (default '')
• mockPath: string|[string] the path(s) to load mock files from (default '')
• port: number: port to expose on localhost. Will use process.env.PORT if not specified here (default 8080)
• reload: boolean: enable/disable browser reloading (default true)
• silent: boolean: disable/enable default logging (default false)

import { server } from 'dvlp';
const appServer = await server('path/to/app.js', { port: 8080 });

Create a server for handling network requests during testing.

options include:

• autorespond: boolean enable/disable automatic dummy responses. If unable to resolve a request to a local file or mock, the server will respond with a dummy response of the appropriate type (default false)
• latency: number the amount of artificial latency to introduce (in ms) for responses (default 50)
• port: number the port to expose on localhost. Will use process.env.PORT if not specified here (default 8080)
• webroot: String the subpath from process.cwd() to prepend to relative paths (default '')

import { testServer } from 'dvlp/test';
const mockApi = await testServer({ port: 8080, latency: 20, webroot: 'src' });

Returns a TestServer instance with the following methods:

• loadMockFiles(filePath: string|[string]): void load and register mock response files (see mocking)

{
  "request": {
    "url": "http://www.someapi.com/v1/id/101010"
  },
  "response": {
    "body": {
      "user": {
        "name": "Nancy",
        "id": "101010"
      }
    }
  }
}

mockApi.loadMockFiles('path/to/mock/101010.json');
const res = await fetch('http://www.someapi.com/v1/id/101010');
console.log(await res.json()); // => { user: { name: "nancy", id: "101010" } }

• mockResponse(request: string|object, response: object|(req, res) => void, once: boolean, onMockCallback: () => void): () => void add a mock response for request, optionally removing it after first use, and/or triggering a callback when successfully mocked (see mocking). Returns a function that may be called to remove the added mock at any time.

mockApi.mockResponse(
  '/api/user/1234',
  {
    body: {
      id: '1234',
      name: 'bob',
    },
  },
  true,
);
const res = await fetch('http://localhost:8080/api/user/1234');
console.log(await res.json()); // => { id: "1234", name: "bob" }

Or pass a response handler:

const removeMock = mockApi.mockResponse(
  '/api/user/1234',
  (req, res) => {
    res.writeHead(200, {
      'Content-Type': 'application/json',
    });
    res.end(JSON.stringify({ id: '1234', name: 'bob' }));
  },
  true,
);
const res = await fetch('http://localhost:8080/api/user/1234');
console.log(await res.json()); // => { id: "1234", name: "bob" }
removeMock();

• mockPushEvents(stream: string|object, events: object|[object]): () => void add one or more mock events for a WebSocket/EventSource stream (see mocking). Returns a function that may be called to remove the added mock at any time.

const removeMock = mockApi.mockPushEvents('ws://www.somesocket.com/stream', [
  {
    name: 'hi',
    message: 'hi!',
  },
  {
    name: 'so scary',
    message: 'boo!',
  },
]);
ws = new WebSocket('ws://www.somesocket.com/stream');
ws.addEventListener('message', (event) => {
  console.log(event.data); // => hi!
  removeMock();
});

• pushEvent(stream: string|object, event: string|object’):void push data to WebSocket/EventSource clients. A string passed as 'event' will be handled as a named mock push event (see mocking)

mockApi.pushEvent('ws://www.somesocket.com/stream', 'so scary');

• ref(): void prevent process from exiting while this server is active

• unref(): void allow process to exit if this is the only active

• destroy(): Promise<void> stop and clean up running server

In addition, testServer supports the following special query parameters:

• offline simulate an offline state by terminating the request (fetch('http://localhost:3333/foo.js?offline'))
• error return a 500 server error response (fetch('http://localhost:3333/foo.js?error'))
• missing return a 404 not found response (fetch('http://localhost:3333/foo.js?missing'))
• maxage=value configure Cache-Control: public, max-age={value} cache header (fetch('http://localhost:3333/foo.js?maxage=10'))
• hang hold connection open without responding (fetch('http://localhost:3333/foo.js?hang'))

Disable all network requests with origin that is not localhost. Prevents all external network requests for the current Node.js process. If rerouteAllRequests is set to true, all external requests will be re-routed to the current running server.

testServer.disableNetwork();
await fetch('https://github.com/popeindustries/dvlp');
// => Error "network connections disabled"

Re-enables all previously disabled external network requests for the current Node.js process.

Add a mock response for request, optionally removing it after first use, and/or triggering a callback when successfully mocked (see mocking). Returns a function that may be called to remove the added mock at any time.

// Also available as "window.dvlp"
import { testBrowser } from 'dvlp/test-browser';

testBrowser.mockResponse(
  'http://localhost:8080/api/user/1234',
  {
    body: {
      id: '1234',
      name: 'bob',
    },
  },
  true,
);

Push data to WebSocket/EventSource clients. A string passed as 'event' will be handled as a named mock push event (see mocking).

testBrowser.pushEvent('ws://www.somesocket.com/stream', 'so scary');

Disable all network requests with origin that is not localhost. Prevents all external AJAX/Fetch/EventSource/WebSocket requests originating from the current browser window. If rerouteAllRequests is set to true, all external requests will be re-routed to the running dvlp service.

testBrowser.disableNetwork();
await fetch('https://github.com/popeindustries/dvlp');
// => Error "network connections disabled"

Re-enables all previously disabled requests originating from the current browser window.