README.md

chain-error: rich JavaScript errors

This is a fork of VError.

The ChainError support:

• chains of causes
• properties to provide extra information about the error
• creating your own subclasses that support all of these

ChainError chaining errors while preserving each one's error message. This is useful in servers and command-line utilities when you want to propagate an error up a call stack, but allow various levels to add their own context. See examples below.

You can hiding the lower-level messages from the top-level error. This is useful for API endpoints where you don't want to expose internal error messages, but you still want to preserve the error chain for logging and debugging.

Quick start

First, install the package:

npm install @ts-stack/chain-error

If nothing else, you can use ChainError as a drop-in replacement for the built-in JavaScript Error class:

import { ChainError } from '@ts-stack/chain-error';

const path = '/etc/passw';
const err = new ChainError(`missing file: "${path}"`);
console.log(err.message);

This prints:

missing file: "/etc/passwd"

You can also pass a cause argument, which is any other Error instance:

const err1 = new Error('No such file or directory');
const err2 = new ChainError('failed to stat "/junk"', err1);
const err3 = new ChainError('request failed', err2);
console.error(err3.message);

This prints:

request failed: failed to stat "/junk": No such file or directory

The idea is that each layer in the stack annotates the error with a description of what it was doing. The end result is a message that explains what happened at each level.

You can also decorate Error objects with additional information so that callers can not only handle each kind of error differently, but also construct their own error messages (e.g., to localize them, format them, group them by type, and so on). See the example below.

Deeper dive

The two main goals for ChainError are:

• Make it easy to construct clear, complete error messages intended for people. Clear error messages greatly improve both user experience and debuggability, so we wanted to make it easy to build them.
• Make it easy to construct objects with programmatically-accessible metadata (which we call informational properties). Instead of just saying "connection refused while connecting to 192.168.1.2:80", you can add properties like "ip": "192.168.1.2" and "tcpPort": 80. This can be used for feeding into monitoring systems, analyzing large numbers of errors (as from a log file), or localizing error messages.

To really make this useful, it also needs to be easy to compose errors: higher-level code should be able to augment the errors reported by lower-level code to provide a more complete description of what happened. Instead of saying "connection refused", you can say "operation X failed: connection refused". That's why ChainError supports causes.

In order for all this to work, programmers need to know that it's generally safe to wrap lower-level errors with higher-level ones. If you have existing code that handles errors produced by a library, you should be able to wrap those errors with a ChainError to add information without breaking the error handling code. There are two obvious ways that this could break such consumers:

• The error's name might change. People typically use name to determine what kind of Error they've got. To ensure compatibility, you can create ChainError with custom names, but this approach isn't great because it prevents you from representing complex failures. For this reason, ChainError provides findCauseByName, which essentially asks: does this Error or any of its causes have this specific name? If error handling code uses findCauseByName, then subsystems can construct very specific causal chains for debuggability and still let people handle simple cases easily. There's an example below.
• The error's properties might change. People often hang additional properties off of Error objects. If we wrap an existing Error in a new Error, those properties would be lost unless we copied them. But there are a variety of both standard and non-standard Error properties that should not be copied in this way: most obviously name, message, and stack, but also fileName, lineNumber, and a few others. Plus, it's useful for some Error subclasses to have their own private properties -- and there'd be no way to know whether these should be copied. For these reasons, ChainError first-classes these information properties. You have to provide them in the constructor, you can only fetch them with the getInfo() function, and ChainError takes care of making sure properties from causes wind up in the getInfo() output.

Let's put this all together with an example from the node-fast RPC library. node-fast implements a simple RPC protocol for Node programs. There's a server and client interface, and clients make RPC requests to servers. Let's say the server fails with an UnauthorizedError with message user 'bob' is not authorized. The client wraps all server errors with a FastServerError. The client also wraps all request errors with a FastRequestError that includes the name of the RPC call being made. The result of this failed RPC might look like this:

name: FastRequestError
message: "request failed: server error: user 'bob' is not authorized"
rpcMsgid: <unique identifier for this request>
rpcMethod: GetObject
cause:
  name: FastServerError
  message: "server error: user 'bob' is not authorized"
  cause:
  name: UnauthorizedError
  message: "user 'bob' is not authorized"
  rpcUser: "bob"

When the caller uses ChainError.getInfo(), the information properties are collapsed so that it looks like this:

message: "request failed: server error: user 'bob' is not authorized"
rpcMsgid: <unique identifier for this request>
rpcMethod: GetObject
rpcUser: "bob"

Taking this apart:

• The error's message is a complete description of the problem. The caller can report this directly to its caller, which can potentially make its way back to an end user (if appropriate). It can also be logged.
• The caller can tell that the request failed on the server, rather than as a result of a client problem (e.g., failure to serialize the request), a transport problem (e.g., failure to connect to the server), or something else (e.g., a timeout). They do this using findCauseByName('FastServerError') rather than checking the name field directly.
• If the caller logs this error, the logs can be analyzed to aggregate errors by cause, by RPC method name, by user, or whatever. Or the error can be correlated with other events for the same rpcMsgid.
• It wasn't very hard for any part of the code to contribute to this error. Each part of the stack has just a few lines to provide exactly what it knows, with very little boilerplate.

It's not expected that you'd use these complex forms all the time. Despite supporting the complex case above, you can still just do:

new ChainError(`my service isn't working`);

for the simple cases.

API

Constructor

The ChainError constructor:

constructor(message?: string, optsOrError?: ChainErrorOptions | Error, skipCauseMessage?: boolean);

Where ChainErrorOptions is a plain object with any of the following optional properties:

interface ChainErrorOptions {
  /**
   * Describes what kind of error this is. This is intended for programmatic use
   * to distinguish between different kinds of errors. Note that in modern versions of Node.js,
   * this name is ignored in the `stack` property value, but callers can still use the `name`
   * property to get at it.
   */
  name?: string;
  /**
   * Indicates that the new error was caused by `cause`. See `getCause()` below.
   * If unspecified, the cause will be `null`.
   */
  cause?: Error;
  /**
   * If specified, then the stack trace for this error ends at function `constructorOpt`.
   * Functions called by `constructorOpt` will not show up in the stack.
   * This is useful when this class is subclassed.
   */
  constructorOpt?: (...args: any[]) => any;
  /**
   * Specifies arbitrary informational properties that
   * are available through the `ChainError.getInfo(err)` static class method.
   * See that method for details.
   */
  info?: { [key: string]: any };
}

Public properties

ChainError provide the public properties as JavaScript's built-in Error objects.

Property name	Type	Meaning
name	string	Programmatically-usable name of the error.
message	string	Human-readable summary of the failure. Programmatically-accessible details are provided through ChainError.getInfo(err) class method.
stack	string	Human-readable stack trace where the Error was constructed.

The stack property is managed entirely by the underlying JavaScript implementation. It's generally implemented using a getter function because constructing the human-readable stack trace is somewhat expensive.

Class methods

The following static methods are defined on the ChainError class.

class ChainError extends Error {
  /**
   * Returns the next `Error` in the cause chain for given `err`,
   * or `null` if there is no next `ChainError`. See the `cause`
   * argument to the constructor. Errors can have arbitrarily long cause chains.
   * You can walk the `cause` chain by invoking `ChainError.getCause(err)`
   * on each subsequent return value.
   */
  static getCause(err: Error): Error | null;

  /**
   * Returns an object with all of the extra error information that's been associated
   * with this `Error` and all of its causes. These are the properties passed in using
   * the `info` option to the constructor. Properties not specified in the
   * constructor for this `Error` are implicitly inherited from this error's cause.
   *
   * These properties are intended to provide programmatically-accessible metadata
   * about the error. For an error that indicates a failure to resolve a DNS name,
   * informational properties might include the DNS name to be resolved, or even the
   * list of resolvers used to resolve it. The values of these properties should
   * generally be plain objects (i.e., consisting only of `null`, `undefined`, `numbers`,
   * `booleans`, `strings`, `objects` and arrays containing only other plain objects).
   */
  static getInfo(err: Error): ObjectAny;

  /**
   * Returns a string containing the full stack trace, with all nested errors recursively
   * reported as `'caused by:' + err.stack`.
   */
  static getFullStack(err: Error): string;

  /**
   * The `findCauseByName()` method traverses the cause chain for given `err`, looking
   * for an error whose `name` property matches the passed in `name` value. If no
   * match is found, `null` is returned.
   *
   * If all you want is to know _whether_ there's a cause (and you don't care what it is),
   * you can use `ChainError.hasCauseWithName(err, name)`.
   *
   * If a vanilla error or a non-ChainError error is passed in, then there is no cause
   * chain to traverse. In this scenario, the method will check the `name`
   * property of only `err`.
   */
  static findCauseByName(err: Error, name: string): Error | null;

  /**
   * Returns `true` if and only if `ChainError.findCauseByName(err, name)` would return
   * a non-null value. This essentially determines whether `err` has any cause in
   * its cause chain that has name `name`.
   */
  static hasCauseWithName(err: Error, name: string): boolean;
}

Examples

More examples:

const err1 = new ChainError('something bad happened');
/* ... */
const err2 = new ChainError(
  'failed to connect to "127.0.0.1:215"',
  {
    name: 'ConnectionError',
    cause: err1,
    info: {
      errno: 'ECONNREFUSED',
      remote_ip: '127.0.0.1',
      port: 215
    }
  }
);

console.log(err2.message);
console.log(err2.name);
console.log(ChainError.getInfo(err2));
console.log(err2.stack);

This outputs:

failed to connect to "127.0.0.1:215": something bad happened
ConnectionError
{ errno: 'ECONNREFUSED', remote_ip: '127.0.0.1', port: 215 }
ConnectionError: failed to connect to "127.0.0.1:215": something bad happened
  at Object.<anonymous> (/home/dap/node-chain-error/examples/info.js:5:12)
  at Module._compile (module.js:456:26)
  at Object.Module._extensions..js (module.js:474:10)
  at Module.load (module.js:356:32)
  at Function.Module._load (module.js:312:12)
  at Function.Module.runMain (module.js:497:10)
  at startup (node.js:119:16)
  at node.js:935:3

Information properties are inherited up the cause chain, with values at the top of the chain overriding same-named values lower in the chain. To continue that example:

const err3 = new ChainError(
  'request failed',
  {
    name: 'RequestError',
    cause: err2,
    info: { errno: 'EBADREQUEST' }
  }
);

console.log(err3.message);
console.log(err3.name);
console.log(ChainError.getInfo(err3));
console.log(err3.stack);

This outputs:

request failed: failed to connect to "127.0.0.1:215": something bad happened
RequestError
{ errno: 'EBADREQUEST', remote_ip: '127.0.0.1', port: 215 }
RequestError: request failed: failed to connect to "127.0.0.1:215": something bad happened
  at Object.<anonymous> (/home/dap/node-chain-error/examples/info.js:20:12)
  at Module._compile (module.js:456:26)
  at Object.Module._extensions..js (module.js:474:10)
  at Module.load (module.js:356:32)
  at Function.Module._load (module.js:312:12)
  at Function.Module.runMain (module.js:497:10)
  at startup (node.js:119:16)
  at node.js:935:3

You can also print the complete stack trace of combined Errors by using ChainError.getFullStack(err).

const err1 = new ChainError('something bad happened');
/* ... */
const err2 = new ChainError('something really bad happened here', err1);

console.log(ChainError.getFullStack(err2));

This outputs:

ChainError: something really bad happened here: something bad happened
  at Object.<anonymous> (/home/dap/node-chain-error/examples/getFullStack.js:5:12)
  at Module._compile (module.js:409:26)
  at Object.Module._extensions..js (module.js:416:10)
  at Module.load (module.js:343:32)
  at Function.Module._load (module.js:300:12)
  at Function.Module.runMain (module.js:441:10)
  at startup (node.js:139:18)
  at node.js:968:3
caused by: ChainError: something bad happened
  at Object.<anonymous> (/home/dap/node-chain-error/examples/getFullStack.js:3:12)
  at Module._compile (module.js:409:26)
  at Object.Module._extensions..js (module.js:416:10)
  at Module.load (module.js:343:32)
  at Function.Module._load (module.js:300:12)
  at Function.Module.runMain (module.js:441:10)
  at startup (node.js:139:18)
  at node.js:968:3

ChainError.getFullStack(err) is also safe to use on regular Errors, so feel free to use it whenever you need to extract the stack trace from an Error, regardless if it's a ChainError or not.