@metamask/json-rpc-engine

A tool for processing JSON-RPC requests and responses.

Usage

const { JsonRpcEngine } = require('@metamask/json-rpc-engine');

const engine = new JsonRpcEngine();

Build a stack of JSON-RPC processors by pushing middleware to the engine.

engine.push(function (req, res, next, end) {
  res.result = 42;
  end();
});

Requests are handled asynchronously, stepping down the stack until complete.

const request = { id: 1, jsonrpc: '2.0', method: 'hello' };

engine.handle(request, function (err, response) {
  // Do something with response.result, or handle response.error
});

// There is also a Promise signature
const response = await engine.handle(request);

Middleware have direct access to the request and response objects. They can let processing continue down the stack with next(), or complete the request with end().

engine.push(function (req, res, next, end) {
  if (req.skipCache) return next();
  res.result = getResultFromCache(req);
  end();
});

By passing a return handler to the next function, you can get a peek at the result before it returns.

engine.push(function (req, res, next, end) {
  next(function (cb) {
    insertIntoCache(res, cb);
  });
});

If you specify a notificationHandler when constructing the engine, JSON-RPC notifications passed to handle() will be handed off directly to this function without touching the middleware stack:

const engine = new JsonRpcEngine({ notificationHandler });

// A notification is defined as a JSON-RPC request without an `id` property.
const notification = { jsonrpc: '2.0', method: 'hello' };

const response = await engine.handle(notification);
console.log(typeof response); // 'undefined'

Engines can be nested by converting them to middleware using JsonRpcEngine.asMiddleware():

const engine = new JsonRpcEngine();
const subengine = new JsonRpcEngine();
engine.push(subengine.asMiddleware());

async Middleware

If you require your middleware function to be async, use createAsyncMiddleware:

const { createAsyncMiddleware } = require('@metamask/json-rpc-engine');

let engine = new RpcEngine();
engine.push(
  createAsyncMiddleware(async (req, res, next) => {
    res.result = 42;
    next();
  }),
);

async middleware do not take an end callback. Instead, the request ends if the middleware returns without calling next():

engine.push(
  createAsyncMiddleware(async (req, res, next) => {
    res.result = 42;
    /* The request will end when this returns */
  }),
);

The next callback of async middleware also don't take return handlers. Instead, you can await next(). When the execution of the middleware resumes, you can work with the response again.

engine.push(
  createAsyncMiddleware(async (req, res, next) => {
    res.result = 42;
    await next();
    /* Your return handler logic goes here */
    addToMetrics(res);
  }),
);

You can freely mix callback-based and async middleware:

engine.push(function (req, res, next, end) {
  if (!isCached(req)) {
    return next((cb) => {
      insertIntoCache(res, cb);
    });
  }
  res.result = getResultFromCache(req);
  end();
});

engine.push(
  createAsyncMiddleware(async (req, res, next) => {
    res.result = 42;
    await next();
    addToMetrics(res);
  }),
);

Teardown

If your middleware has teardown to perform, you can assign a method destroy() to your middleware function(s), and calling JsonRpcEngine.destroy() will call this method on each middleware that has it. A destroyed engine can no longer be used.

const middleware = (req, res, next, end) => {
  /* do something */
};
middleware.destroy = () => {
  /* perform teardown */
};

const engine = new JsonRpcEngine();
engine.push(middleware);

/* perform work */

// This will call middleware.destroy() and destroy the engine itself.
engine.destroy();

// Calling any public method on the middleware other than `destroy()` itself
// will throw an error.
engine.handle(req);

Gotchas

Handle errors via end(err), NOT next(err).

/* INCORRECT */
engine.push(function (req, res, next, end) {
  next(new Error());
});

/* CORRECT */
engine.push(function (req, res, next, end) {
  end(new Error());
});

However, next() will detect errors on the response object, and cause end(res.error) to be called.

engine.push(function (req, res, next, end) {
  res.error = new Error();
  next(); /* This will cause end(res.error) to be called. */
});

Running tests

Build the project if not already built:

yarn build

yarn test