Tools for type-safe handling of expected failure conditions through standard control flow.

import { Fail, mkFail, isFail } from 'failerr';

const division = (num: number, div: number) => {
  if (div === 0) {
    return mkFail('division by zero', { code: 42 });
  }
  return num / div;
};

const res = division(7, 0);

if (isFail(res)) {
  res.message;    // The TypeScript compiler infers type "string"
  res.data.code;  // The TypeScript compiler infers type "number"
} else {
  res.message;    // The TypeScript compiler reports an error
}

The use of Error-based exceptions as a strategy for dealing with failure conditions in the form of throw statements and try/catch blocks has a few issues:

1. It's very slow
2. It completely bypasses TypeScript's compile-time type checking
3. It is a source of non-determinism and brittleness
4. It conflates expected and unexpected failure conditions

Modern languages, such as Rust, have addressed these concerns through their native support of monadic types such as Option and Result. However, it is the author's opinion that userland JavaScript / TypeScript implementations of these structures are not a good solution due to their implications on code structure, performance and inspectability.

A better solution for handling expected failure conditions is to represent them as first-class values independent of the Error class and provide the necessary helper functions to instantiate them and and tell them apart from other values, implementing error handling through standard control flow.

An excellent piece of writing on this topic and a primary source of inspiration for this library can be found in Austral's approach to error handling.

The Fail interface represents expected failure conditions that a program should be able to handle while guaranteeing consistent internal state.

Fail objects are created using the mkFail() functions. The isFail() function helps with discriminating whether a value is a Fail object or not.

mkFail<D = {}>(message: string, data: D): Fail<D>

The mkFail() function can be used to create Fail objects whose .message and .data properties will be set to the provided arguments.

Simple Fail with empty data:

const fail: Fail = mkFail('some message');
fail.message;     // => 'some message'
fail.data;        // => empty, frozen object

Fail with specific data shape:

const fail: Fail<{ code: number }> = mkFail('some message', { code: 42 });
fail.message;     // => 'some message'
fail.data.code;   // => 42

isFail(val: any): val is Fail<any>

The isFail() is a user-defined type guard that helps with identifying whether a value - normally the return value of a function - is a Fail object or not:

const value = (72 as Fail | number);

if (isFail(value)) {
  value.message;  // => TypeScript infers type "string"
}

When a function returns as the union of Fail and non-Fail

The Fail interface can be easily extended using types or interfaces:

type FailWithCode = Fail<{ code: number }>;

const fooBar = (): number | FailWithCode => {
  return Math.random() > 0.5 ? 42 : mkFail({ code: 42 });
};

Performance, mostly. Modeling Fail as an interface while instantiating implementations using simple objects seems to deliver between 1.5x and 1.75x greater throughput in high-performance applications (such as parsers) relative to using a class-based approach, though the difference decreases as code gets JIT-compiled over many iterations.

MIT