Rowan

A lightweight async middleware library.

Usage

Rowan can be used to build asynchronous middleware-style control-flow and error-handling, with particular focus on providing a rich typescript experience.

Create an instance of the Rowan class (or derivation) and call use with a middleware function

import {Rowan} from 'rowan';
 
// Create a (derived) app
const app = new Rowan();
 
// Add middleware and handlers
app.use(async (ctx) => {
  console.log(`foo: ${ctx.foo}`);
});
 

Once the middleware is all setup you call process and pass along the context.

// Use it 
await app.execute({ foo: "bar!" });

... which in this example would output to console:

foo: bar!

Processors

Processors are either a Handler<Ctx>, AutoHandler<Ctx> or Middleware<Ctx> type signature.

• Handler is a two-parameter function that will be given the ctx and next callback and should return a Promise. You are required to call next if you wish processing to continue to the next middleware processors in the chain.

app.use(async (ctx, next) => {
  ctx["start"] = Date.now();
  await next();
  ctx["finish"] = Date.now();
});

• AutoHandler is a one-parameter function that will be given the ctx object. The next processor in the chain will automatically be called for you, unless you throw an Error.

app.use(async (ctx) => {
  ctx.data = JSON.parse(ctx.raw);
});

• Middleware is a object containing a method process that will be called with two-parameters: ctx and next. It is expected that process will return a Promise<void>.

app.use({
  async process(ctx, next){
    await next();
    consol.log("Complete");
  }
});

Helpers

after-if

calls next and if the predicate returns true executes its middleware

let foo = new Rowan();
 
foo.use(new AfterIf(
  async (ctx) => ctx.valid, [
  async (ctx) => {
    console.log("valid message: ", ctx.msg);
  }
]));
 
foo.use(async (ctx) => {
  console.log("validate...")
  if (ctx.msg && ctx.msg.length > 5) {
    ctx.valid = true
  }
})
 
async function main() {
  await foo.process({ msg: "hello" });
  await foo.process({ msg: "hello world" });
}
 
main().catch(console.log);

outputs:

validate...
validate...
valid message:  hello world

after

calls next first, then executes its own middleware afterwards

let foo = new Rowan();
 
foo.use(new After([
  async (ctx) => {
    console.log(ctx.output);
  }
]));
 
foo.use(async (ctx) => {
  console.log("processing...")
  ctx.output = ctx.msg;
});
 
async function main() {
  await foo.process({ msg: "hello" });
  await foo.process({ msg: "hello world" });
}
 
main().catch(console.log);

outputs:

processing...
hello
processing...
hello world

catch

wraps its own middleware with a try...catch

foo.use(
  new Catch(
    async (err, ctx) => {
      console.log("caught: ", err.message);
    },
    new Rowan()
      .use(
        async (ctx) => {
          if (ctx != "foo") {
            throw Error("ctx must be 'foo'");
          }
        })
      .use({
        meta: { name: "Moo" },
        async process(ctx, next) {
          console.log("Moo!");
          return next();
        }
      })
  ));
 
async function main() {
  await foo.process('foo');
  await foo.process('bar');
}
 
main().catch(console.log);

outputs:

Moo!
caught: ctx must be 'foo'

if

let foo = new Rowan<string>();
 
foo.use(
  new If(
    async (ctx: string) => {
      return ctx.startsWith("foo");
    },
    [async (ctx) => {
      console.log("IF...", ctx);
    }],
    /** terminate if predicate() == true */
    true, 
  )
);
 
foo.use(async (ctx) => {
  console.log("Else...", ctx);
})
 
async function main() {  
  await foo.process('foo');
  await foo.process('foobar');
  await foo.process('bar');
}
 
main().catch(console.log);

outputs:

IF... foo
IF... foobar
Else... bar

Tools

Rowan.hierarchy()

used to build a meta hierarchy from processors that have a middleware field defined.

let foo = new Rowan(undefined, { name: "FOO" });
let bar = new Rowan();
 
bar.meta.name = "Bar";
 
bar.use((ctx, next) => {
  console.log("boo1:", ctx);
  return next();
}, { name: "Boo1" });
 
bar.use(Object.assign((ctx, next) => {
  console.log("boo2:", ctx);
  return next();
}, { meta: { name: "Boo2" } }));
 
bar.use({
  meta: { name: "Boo3" },
  middleware: [{
    meta: { name: "Custom" },
    process(x, n) { console.log("Custom:", x); return n() }
  }],
  process: function (ctx, next) {
    console.log("Boo3:", ctx);
    return Rowan.process(this.middleware, ctx, next);
  }
});
 
foo.use(bar);
 
console.log(JSON.stringify(Rowan.hierarchy(foo), null, 2));

outputs:

{
  "meta": {
    "name": "FOO"
  },
  "children": [
    {
      "meta": {
        "name": "Bar"
      },
      "children": [
        {
          "meta": {
            "name": "Boo1"
          }
        },
        {
          "meta": {
            "name": "Boo2"
          }
        },
        {
          "meta": {
            "name": "Boo3"
          },
          "children": [
            {
              "meta": {
                "name": "Custom"
              }
            }
          ]
        }
      ]
    }
  ]
}

Advanced

Rowan.process()

executes and chains a sequence of Middleware, setting up the next callback for each.

 
async function main(next: ()=>Promise<void>) {
  Rowan.process(
    [{
      async process(ctx, next) {
        console.log("first");
        return next();
      }
    },
    {
      async process(ctx, next) {
        console.log("second");
        return next();
      }
    }],
    {
      msg: "hello"
    },
    //... optional next
    next
  )
}
main(async () => {
  console.log("END")
}).catch(console.log);

outputs:

first
second
END

Rowan.convertToMiddleware()

used interally to convert supported Handler types into valid Middleware.

Rowan.convertToMiddleware(async (ctx)=>{}, {name: "foo"});

results in:

{ 
  meta: { name: 'foo' }, 
  process: [Function] 
}

Build

npm install
npm test

there is an example.ts that you can run with ts-node

ts-node example

Credits

"Rowan" Icon courtesy of The Noun Project, by ludmil, under CC 3.0