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README.md

Incident

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Errors with superpowers.

Installation

npm install --save incident

Features

  • Node and browser support.
  • Supports instanceof Error tests.
  • Built-in error causality tracking.
  • Compatible with Typescript discriminated unions. If you type the name, you can then use it as a discriminant property to resolve the type of the data and cause. See the example below.
  • Compatible with prototypal inheritance and ES6 class inheritance
  • Distributed with type definitions for Typescript
  • Lazy stack capture and support for lazy message formatter: never called if not needed
  • Minimal dependencies: has a single dependency on object-inspect
  • Tiny: 3kB minized, 1kB gziped

Migration from version 2

Ensure that each of you always provide a name. You should mainly look for new Incident(message) and new Incident(cause, message) which would be interpreted as new Incident(name) and new Incident(cause, name) in version 3. Also note that the order of the generic parameters changed from <Name, Data, Cause> to <Data, Name, Cause>.

Why

My goal with this library was to simplify the automatic handling of errors and provide better error messages. Javascript errors are not very helpful because extending them and then extracting data is tedious because you need a declaration statement, multiple assignations, and then match on the name that is usually the generic "Error" or rely on a brittle instance of.

To achieve automatic error handling, the information describing the error should be easily accessible programmatically. That's why you can directly pass a data object to the Incident constructor. Being able to pass data on the fly means that you do not need a declaration statement for the error separate property assignations. A key identifying the error is the second requirement for automatic error handling, Incident simply uses the name. As explained in the features, it allows to unambiguously identify the error (I'd recommend to always use a name). Using a name (as a string enum variant) is also more reliable than using instance of (prototype chain lookup) to check for the type of custom errors. The two main advantages are that it's easier to serialize / deserialize and is not affected by the module duplication of the Node module resolution algorithm.

To provide better error messages, especially for asynchronous operations, I made the cause of the error a first-class information. It allows to deal with deferred or wrapped errors more pleasant: the stack trace at each step is preserved.

The library is already pretty verbose (I am waiting for generic defaults in TS 2.3) so I'd like to keep the usage simple. For example, I have experimented with a arrays of causes (when an error has multiple simultaneous causes). The displayed messages were pretty good but it made the code to handle the errors automatically pretty complex because the type for the cause was Error | Error[] | undefined. This feature did not even make it to the version 1. If you need it, but your multiple reasons in the data object.

Finally, performance is also a goal. That's why the library performs late / lazy stack capture and allows for message formatters that are called only when needed.

Usage

Exports

function Incident; // The Incident constructor
interface StaticIncident; // Interface of the constructor
interface Incident; // Interface of the instance

Incident

An Incident has the following interface:

interface Incident<
  Data extends object,
  Name extends string = string,
  Cause extends (Error | undefined) = (Error | undefined)
> extends Error {
  name: Name;
  message: string;
  data: Data;
  cause: Cause;
  stack: string;
  toString(): string;
}

Generic parameters

  • Name: The type of the name, use a literal string type to be able to switch ont the name and benefit from Typescript's type discrimination.
  • Data: The interface of the data associated to this error. Must respect typeof data === "object"
  • Cause: The type of the error cause. Use undefined if there is no cause.

Attributes

name

A name uniquely identifying the error. Displayed at the top of the stack.

message

A debug message for developers describing the error. Displayed at the top of the stack. If you provide data but no message, the message will be generate from the data using object-inspect with default options.

data

A data object associated to the error. This should describe the error enough to handle it programmatically.

cause

A previous error that cause this error. This usually has more detail about what happened.

stack

The error stack of the incident. Contains the standard stack frames, and the stack of the cause if there is any.

Constructor

new Incident<Data, Name, Cause>([cause,] name, [data,] [message]);

You can pass almost any combination of parameters you want as long as it is in the right order (see table below for the details) and a name is specified. If you want to explicitly define the generic parameters, you don't have to define the generic parameter for a function parameter you do not use. Instanciating a new Incident instance will perform a lazy capture of the current call stack (only resolved when reading .stack or throwing the error).

  • cause

    • Type: Cause
    • Type constraint: If you provide a cause, Cause extends Error else Cause is undefined.
    • Default type: undefined
    • Default value: undefined
    • Description: A previous error that caused this Incident.
  • name

    • Type: Name
    • Type constraint: Name extends string
    • Default type: string
    • Required
    • Description: A name allowing to discriminate the error data and cause.
  • data

    • Type: Data
    • Type constraint: Data extends object
    • Default type: object
    • Default value: {}
    • Description: A data object completely describing the error.
  • message

    • Type: string | ((data?: typeof data) => string)
    • Default value: ""
    • Description: A debug message for developers. If the message is a formatter function, it will be lazily evaluated (only once) when:
      • the message property is accessed
      • the stack property is accessed
      • the error is thrown

new operator signatures table

cause name data message Comment
new<Name>(...): Incident<Name, object, undefined>
new<Name>(...): Incident<Name, object, undefined>
new<Data, Name>(...): Incident<Name, Data, undefined>
new<Data, Name>(...): Incident<Name, Data, undefined>
new<Name, Cause>(...): Incident<Name, object, Cause>
new<Name, Cause>(...): Incident<Name, object, Cause>
new<Data, Name, Cause>(...): Incident<Name, Data, Cause>
new<Data, Name, Cause>(...): Incident<Name, Data, Cause>

Call

You can call Incident as a simple function. It has the same signature as the new operator but does not capture the stack. You may want to use it for higher order errors, but I recommend to capture the stack for root causes.

The simple call also supports the additional signature (cause: Error). This will perform a conversion from any error to an instance of the currently called Incident. You can use it normalize simple errors to Incident or mitigate module duplication if you rely on instanceof. The resulting incident will have the same name, message, stack and data. If the input error was an Incident with a lazy message or stack, it will remain non-evaluated. If the argument is already an instance of the current Incident, a copy will be created. If you added extra properties, they will be lost.

cause name data message Comment
Converts to an instance of this Incident constructor
<Name>(...): Incident<Name, object, undefined>
<Name>(...): Incident<Name, object, undefined>
<Data, Name>(...): Incident<Name, Data, undefined>
<Data, Name>(...): Incident<Name, Data, undefined>
<Name, Cause>(...): Incident<Name, object, Cause>
<Name, Cause>(...): Incident<Name, object, Cause>
<Data, Name, Cause>(...): Incident<Name, Data, Cause>
<Data, Name, Cause>(...): Incident<Name, Data, Cause>

Discriminated union

// Example of type resolution on a discriminated type
import Incident from "incident";

// Associate the name "SyntaxError" to {index: number}
type SyntaxError = Incident<"SyntaxError", {index: number}, undefined>;
// Associate the name "TypeError" to {typeName: string}
type TypeError = Incident<"TypeError", {typeName: string}, undefined>;
// Created a discriminated type
type BaseError = SyntaxError | TypeError;

// Example usage accepting the discriminated type
function printError(error: BaseError): void {
  // Switch on the discriminant
  switch (error.name) {
    case "SyntaxError":
      // No need to cast: successfully resolved to SyntaxError
      const index: number = error.data.index;
      console.log(`Received a syntax error with index: ${index}`);
      break;
    case "TypeError":
      // Successfully resolved to TypeError
      const typename: string = error.data.typeName;
      console.log(`Received a type error with typename: ${typename}`);
      break;
  }
}

License

MIT License

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