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JSR Score
3 weeks ago

Metaflow: library-level augmentation of JavaScript data flow and control flow

This is a small library of utility functions, just like fp-ts, but focusing on those things:

  • data flow patterns
  • control flow patterns


  • Node.js: npx jsr add @qnighy/metaflow but check Using JSR with Node.js for details, especially with .npmrc things.
  • Deno: deno add @qnighy/metaflow

exception.ts: expression-based exception handling

This library is about exception handling, but with two policies:

  • Honor the native JavaScript exception handling mechanism, namely throw and try-catch, rather than sticking to the "Result" pattern.
  • Turn statements into expressions, especially method chains.

Why not the "Result" pattern? I have two major reasons:

  • There is no equivalent to try! or ? as in Rust. I think you get the idea unless you are a big fan of Go's if x, err := foo(); err != nil { return nil, errors.Wrap(err); }; if y, err := bar(x); err != nil { return nil, errors.Wrap(err); }; if z, err := baz(x); err != nil { return nil, errors.Wrap(err); }; if w, err := quux(x); err != nil { return errors.Wrap(err); }, where I forgot adding nil, in the last branch when refactoring. (I'm not blaming the Wrapping part; this is a good practice)
  • It is easy to forget error handling if the result value of the operation is not used. Typical example is write() on an I/O stream. In JavaScript, this is already witnessed in the form of Promise rejection handling. So you need equivalent of @typescript-eslint/no-floating-promises whenever you invent a "Result" pattern.

Let's get to the examples:

import { Throw, Try } from "jsr:@qnighy/metaflow/exception";

// Catch all and default to null
const url = Try(() => new URL(input)).done(() => null);

// Catch a specific error and default to a special value
const url = Try(() => new URL(input)).pick(SyntaxError).done(() =>
  new URL("")

// Wrap the error
const url = Try(() => new URL(input)).pick(SyntaxError).done((e) =>
  Throw(new URLError(e))

// Use raw result value (advanced!)
const urlResult = Try(() => new URL(input)).result;
if (urlResult.type === "Ok") {
  // ...
} else {
  // ...

do.ts: pipeline alternative

This is mostly an alternative to pipelines and call-this, which unfortunately are stuck in the early stage of the TC39 process.

So it's roughly equivalent to many existing library functions named pipe, but differs in one thing: mine uses method chaining. The reason is clear: the meaning that the operator |> would convey is very similar to method chaining. I decided to (probably re-)invent a wheel because I couldn't find one, although the idea is simple.

import { Do } from "jsr:@qnighy/metaflow/do";

const result = Do(42)
  .pipe((it) => it + 1) // Equivalent to: |> % + 1
  .pipe((it) => it * 2) // Equivalent to: |> % * 2
console.log(result); // => 86

I recommend using it as the parameter name as this is the most standard way to describe the most local topic (although the name conflicts the it() DSL in test harnesses).

As a typical method chain, extending it with other operators is easy. One such example is equivalent to |> await:

import { Do } from "jsr:@qnighy/metaflow/do";

const result = await Do(42)
  .pipeAwait(async (it) => it + 1) // Equivalent to: |> await Promise.resolve(% + 1)
  .pipeAwait(async (it) => it * 2) // Equivalent to: |> await Promise.resolve(% * 2)
console.log(result); // => 86

There's also an alternative to the ~> operator:

import { Do } from "jsr:@qnighy/metaflow/do";

function double(this: number): number {
  return this * 2;
const result = Do(42)
  .pipe((it) => it + 1) // Equivalent to: |> % + 1
  .rcall(double) // Equivalent to: ~>double
console.log(result); // => 86

tap.ts: Ruby's beloved Object#tap

Utilities that would be convenient once ~> is available.

import { tapMe } from "jsr:@qnighy/metaflow/tap";

const result = 42~>tapMe((x) => console.log(x)) + 1;
console.log(result); // => 43

It also has tap(x, f) which can be used as |> tap(%, (x) => console.log(x)). However, this is more easily written as |> (console.log(%), %).

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