Custom Error

When creating errors with Result.fail(), we strongly recommend using custom errors instead of plain strings or generic Error objects. This guide explains what custom errors are, why they're beneficial, and how to create them effectively.

What are Custom Errors?

Custom errors are specialized error classes that provide more structure and context than generic Error objects. They come in two main forms:

1. Custom Error Classes (Recommended)

Custom error classes inherit from the built-in Error class and provide additional functionality:

import { import ResultResult } from '@praha/byethrow';
import { import zz } from 'zod';

class class ValidationErrorValidationError extends var Error: ErrorConstructorError {
  public override readonly ValidationError.name: "ValidationError"name = 'ValidationError';

  constructor(message: stringmessage: string, options: ErrorOptions | undefinedoptions?: ErrorOptions) {
    super(message: stringmessage, options: ErrorOptions | undefinedoptions);
  }
}

// Usage with Result.fail()
const const validateEmail: (email: string) => Result.Result<string, ValidationError>validateEmail = (email: stringemail: string): import ResultResult.type Result<T, E> = Result.Success<T> | Result.Failure<E>A union type representing either a success or a failure.
@typeParamT  - The type of the Success value.@typeParamE  - The type of the Failure value.@exampleimport { Result } from '@praha/byethrow';

const doSomething = (): Result.Result<number, string> => {
  return Math.random() > 0.5
    ? { type: 'Success', value: 10 }
    : { type: 'Failure', error: 'Oops' };
};
@categoryCore  TypesResult<string, class ValidationErrorValidationError> => {
  return import ResultResult.const pipe: <string, Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]>, Result.Result<string, ValidationError>>(a: string, ab: (a: string) => Result.Result<string, readonly StandardSchemaV1.Issue[]>, bc: (b: Result.Result<string, readonly StandardSchemaV1.Issue[]>) => Result.Result<string, ValidationError>) => Result.Result<string, ValidationError> (+25 overloads)pipe(
    email: stringemail,
    import ResultResult.const parse: <z.ZodString>(schema: z.ZodString) => (value: unknown) => Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]> (+1 overload)parse(import zz.function string(params?: string | z.core.$ZodStringParams): z.ZodString (+1 overload)string().ZodString.email(params?: string | z.core.$ZodCheckEmailParams): z.ZodString@deprecatedUse  z.email() instead.email()),
    import ResultResult.const mapError: <Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]>, ValidationError>(fn: (a: readonly StandardSchemaV1.Issue[]) => ValidationError) => (result: Result.Result<string, readonly StandardSchemaV1.Issue[]>) => Result.Result<string, ValidationError> (+1 overload)mapError((error: readonly StandardSchemaV1.Issue[]error) => new constructor ValidationError(message: string, options?: ErrorOptions): ValidationErrorValidationError('Invalid email format', { ErrorOptions.cause?: unknowncause: error: readonly StandardSchemaV1.Issue[]error })),
  );
};

2. Objects with Identifiable Tags

You can also use plain objects with distinguishable properties as error types:

import { import ResultResult } from '@praha/byethrow';
import { import zz } from 'zod';

type type ValidationError = {
    type: "ValidationError";
    message: string;
    value: string;
}
ValidationError = {
  type: "ValidationError"type: 'ValidationError';
  message: stringmessage: string;
  value: stringvalue: string;
};

const const validateEmail: (email: string) => Result.Result<string, ValidationError>validateEmail = (email: stringemail: string): import ResultResult.type Result<T, E> = Result.Success<T> | Result.Failure<E>A union type representing either a success or a failure.
@typeParamT  - The type of the Success value.@typeParamE  - The type of the Failure value.@exampleimport { Result } from '@praha/byethrow';

const doSomething = (): Result.Result<number, string> => {
  return Math.random() > 0.5
    ? { type: 'Success', value: 10 }
    : { type: 'Failure', error: 'Oops' };
};
@categoryCore  TypesResult<string, type ValidationError = {
    type: "ValidationError";
    message: string;
    value: string;
}
ValidationError> => {
  return import ResultResult.const pipe: <string, Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]>, Result.Result<string, {
    readonly type: "ValidationError";
    readonly message: "Invalid email format";
    readonly value: string;
}>>(a: string, ab: (a: string) => Result.Result<string, readonly StandardSchemaV1.Issue[]>, bc: (b: Result.Result<string, readonly StandardSchemaV1.Issue[]>) => Result.Result<string, {
    readonly type: "ValidationError";
    readonly message: "Invalid email format";
    readonly value: string;
}>) => Result.Result<...> (+25 overloads)
pipe(
    email: stringemail,
    import ResultResult.const parse: <z.ZodString>(schema: z.ZodString) => (value: unknown) => Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]> (+1 overload)parse(import zz.function string(params?: string | z.core.$ZodStringParams): z.ZodString (+1 overload)string().ZodString.email(params?: string | z.core.$ZodCheckEmailParams): z.ZodString@deprecatedUse  z.email() instead.email()),
    import ResultResult.const mapError: <Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]>, {
    readonly type: "ValidationError";
    readonly message: "Invalid email format";
    readonly value: string;
}>(fn: (a: readonly StandardSchemaV1.Issue[]) => {
    readonly type: "ValidationError";
    readonly message: "Invalid email format";
    readonly value: string;
}) => (result: Result.Result<string, readonly StandardSchemaV1.Issue[]>) => Result.Result<string, {
    readonly type: "ValidationError";
    readonly message: "Invalid email format";
    readonly value: string;
}> (+1 overload)
mapError((error: readonly StandardSchemaV1.Issue[]error) => ({
      type: "ValidationError"type: 'ValidationError',
      message: "Invalid email format"message: 'Invalid email format',
      value: stringvalue: email: stringemail,
    })),
  );
};

Why Use Custom Error Classes?

While both approaches work, we recommend using custom Error classes for the following reasons:

Stack Trace Availability

Custom Error classes automatically capture stack traces, making debugging much easier:

import { import ResultResult } from '@praha/byethrow';

class class DatabaseErrorDatabaseError extends var Error: ErrorConstructorError {
  public override readonly DatabaseError.name: "DatabaseError"name = 'DatabaseError';

  constructor(message: stringmessage: string, options: ErrorOptions | undefinedoptions?: ErrorOptions) {
    super(message: stringmessage, options: ErrorOptions | undefinedoptions);
  }
}

const const fetchUser: (id: string) => Result.Result<User, DatabaseError>fetchUser = (id: stringid: string): import ResultResult.type Result<T, E> = Result.Success<T> | Result.Failure<E>A union type representing either a success or a failure.
@typeParamT  - The type of the Success value.@typeParamE  - The type of the Failure value.@exampleimport { Result } from '@praha/byethrow';

const doSomething = (): Result.Result<number, string> => {
  return Math.random() > 0.5
    ? { type: 'Success', value: 10 }
    : { type: 'Failure', error: 'Oops' };
};
@categoryCore  TypesResult<type User = {
    id: string;
    name: string;
}
User, class DatabaseErrorDatabaseError> => {
  try {
    // Database operation...
  } catch (function (local var) error: unknownerror) {
    // The stack trace will show exactly where the error occurred
    return import ResultResult.const fail: <DatabaseError>(error: DatabaseError) => Result.Result<never, DatabaseError> (+1 overload)fail(new constructor DatabaseError(message: string, options?: ErrorOptions): DatabaseErrorDatabaseError('Failed to fetch user'));
  }
};

Error Chaining with Cause Option

Custom Error classes support the cause option, allowing you to preserve the original error context:

import { import ResultResult } from '@praha/byethrow';

class class DatabaseErrorDatabaseError extends var Error: ErrorConstructorError {
  public override readonly DatabaseError.name: "DatabaseError"name = 'DatabaseError';

  constructor(message: stringmessage: string, options: ErrorOptions | undefinedoptions?: ErrorOptions) {
    super(message: stringmessage, options: ErrorOptions | undefinedoptions);
  }
}

const const fetchUser: (id: string) => Result.Result<User, DatabaseError>fetchUser = (id: stringid: string): import ResultResult.type Result<T, E> = Result.Success<T> | Result.Failure<E>A union type representing either a success or a failure.
@typeParamT  - The type of the Success value.@typeParamE  - The type of the Failure value.@exampleimport { Result } from '@praha/byethrow';

const doSomething = (): Result.Result<number, string> => {
  return Math.random() > 0.5
    ? { type: 'Success', value: 10 }
    : { type: 'Failure', error: 'Oops' };
};
@categoryCore  TypesResult<type User = {
    id: string;
    name: string;
}
User, class DatabaseErrorDatabaseError> => {
  try {
    // Database operation...
  } catch (function (local var) error: unknownerror) {
    // Preserve the original error as cause
    return import ResultResult.const fail: <DatabaseError>(error: DatabaseError) => Result.Result<never, DatabaseError> (+1 overload)fail(new constructor DatabaseError(message: string, options?: ErrorOptions): DatabaseErrorDatabaseError('Failed to fetch user', { 
      ErrorOptions.cause?: unknowncause: function (local var) error: unknownerror 
    }));
  }
};

Recommended: Using @praha/error-factory

For creating custom error classes efficiently, we recommend using @praha/error-factory. This library reduces boilerplate code and ensures consistent error structures.

Installation

npm

yarn

pnpm

bun

deno

npm install @praha/error-factory

Basic Usage with Result

First, define the necessary custom error.

import { const ErrorFactory: {
    <Name extends string, Message extends string, Fields extends ErrorFields>(props: {
        name: Name;
        message: Message | ((fields: Fields) => Message);
        fields?: Fields;
    }): ErrorConstructor<Name, Message, Fields>;
    fields<Fields extends ErrorFields>(): Fields;
}
ErrorFactory } from '@praha/error-factory';

class class ValidationErrorValidationError extends ErrorFactory<"ValidationError", "Invalid input provided", ErrorFields>(props: {
    name: "ValidationError";
    message: "Invalid input provided" | ((fields: ErrorFields) => "Invalid input provided");
    fields?: ErrorFields | undefined;
}): (new (options?: ErrorOptions) => Error & Readonly<{
    name: "ValidationError";
    message: "Invalid input provided";
}>) & {
    name: "ValidationError";
}
ErrorFactory({
  name: "ValidationError"name: 'ValidationError',
  message: "Invalid input provided" | ((fields: ErrorFields) => "Invalid input provided")message: 'Invalid input provided',
}) {}

class class QueryErrorQueryError extends ErrorFactory<"QueryError", "An error occurred while executing a query", {
    query: string;
}>(props: {
    name: "QueryError";
    message: "An error occurred while executing a query" | ((fields: {
        query: string;
    }) => "An error occurred while executing a query");
    fields?: {
        query: string;
    } | undefined;
}): (new (options: ErrorOptions & {
    query: string;
}) => Error & Readonly<{
    name: "QueryError";
    message: "An error occurred while executing a query";
}> & Readonly<{
    query: string;
}>) & {
    name: "QueryError";
}
ErrorFactory({
  name: "QueryError"name: 'QueryError',
  message: "An error occurred while executing a query" | ((fields: {
    query: string;
}) => "An error occurred while executing a query")
message: 'An error occurred while executing a query',
  fields?: {
    query: string;
} | undefined
fields: const ErrorFactory: {
    <Name extends string, Message extends string, Fields extends ErrorFields>(props: {
        name: Name;
        message: Message | ((fields: Fields) => Message);
        fields?: Fields;
    }): ErrorConstructor<Name, Message, Fields>;
    fields<Fields extends ErrorFields>(): Fields;
}
ErrorFactory.fields<{
    query: string;
}>(): {
    query: string;
}
fields<{ query: stringquery: string }>(),
}) {}

class class NotFoundErrorNotFoundError extends ErrorFactory<"NotFoundError", "Resource not found", ErrorFields>(props: {
    name: "NotFoundError";
    message: "Resource not found" | ((fields: ErrorFields) => "Resource not found");
    fields?: ErrorFields | undefined;
}): (new (options?: ErrorOptions) => Error & Readonly<{
    name: "NotFoundError";
    message: "Resource not found";
}>) & {
    name: "NotFoundError";
}
ErrorFactory({
  name: "NotFoundError"name: 'NotFoundError',
  message: "Resource not found" | ((fields: ErrorFields) => "Resource not found")message: 'Resource not found',
}) {}

Next, create a function that returns the previously defined custom error together with a Result.

import { import ResultResult } from '@praha/byethrow';
import { import zz } from 'zod';

// Use custom errors in Result operations
const const validateId: (id: string) => Result.Result<string, ValidationError>validateId = (id: stringid: string) => {
  return import ResultResult.const pipe: <string, Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]>, Result.Result<string, ValidationError>>(a: string, ab: (a: string) => Result.Result<string, readonly StandardSchemaV1.Issue[]>, bc: (b: Result.Result<string, readonly StandardSchemaV1.Issue[]>) => Result.Result<string, ValidationError>) => Result.Result<string, ValidationError> (+25 overloads)pipe(
    id: stringid,
    import ResultResult.const parse: <z.ZodString>(schema: z.ZodString) => (value: unknown) => Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]> (+1 overload)parse(import zz.function string(params?: string | z.core.$ZodStringParams): z.ZodString (+1 overload)string()._ZodString<$ZodStringInternals<string>>.startsWith(value: string, params?: string | z.core.$ZodCheckStartsWithParams): z.ZodStringstartsWith('u')),
    import ResultResult.const mapError: <Result.Result<string, readonly StandardSchemaV1<Input = unknown, Output = Input>.Issue[]>, ValidationError>(fn: (a: readonly StandardSchemaV1.Issue[]) => ValidationError) => (result: Result.Result<string, readonly StandardSchemaV1.Issue[]>) => Result.Result<string, ValidationError> (+1 overload)mapError((error: readonly StandardSchemaV1.Issue[]error) => new constructor ValidationError(options?: ErrorOptions): ValidationErrorValidationError({ ErrorOptions.cause?: unknowncause: error: readonly StandardSchemaV1.Issue[]error })),
  );
};

const const executeQuery: (sql: string) => Result.ResultAsync<QueryResult, QueryError>executeQuery = (sql: stringsql: string) => {
  return import ResultResult.try<() => Promise<QueryResult>, QueryError>(options: {
    immediate: true;
    try: () => Promise<QueryResult>;
    catch: (error: unknown) => QueryError;
}): Result.ResultAsync<QueryResult, QueryError> (+7 overloads)
export try
Wraps a function execution (sync or async) or a Promise in a
Result
or
ResultAsync
type,
capturing errors and returning them in a structured way.
You can use either a custom catch handler or rely on the safe: true option
to assume the function cannot throw.
@function@typeParamT  - The function type to execute (sync or async) or a Promise type.@typeParamE  - The error type to return if catch is used.@exampleSync  try-catch
import { Result } from '@praha/byethrow';

const fn = Result.try({
try: (x: number) => {
if (x < 0) throw new Error('Negative!');
return x * 2;
},
catch: (error) => new Error('Oops!', { cause: error }),
});

const result = fn(5); // Result.Result<number, Error>
@exampleSync  try-catch with immediate execution
import { Result } from '@praha/byethrow';

const result = Result.try({
immediate: true,
try: () => {
const x = Math.random() * 10 - 5;
if (x < 0) throw new Error('Negative!');
return x * 2;
},
catch: (error) => new Error('Oops!', { cause: error }),
});

// result is Result<number, Error>
@exampleSync  safe
import { Result } from '@praha/byethrow';

const fn = Result.try({
safe: true,
try: (x: number) => x + 1,
});

const result = fn(1); // Result.Result<number, never>
@exampleSync  safe with immediate execution
import { Result } from '@praha/byethrow';

const result = Result.try({
safe: true,
immediate: true,
try: () => Math.random() + 1,
});

// result is Result<number, never>
@exampleAsync  try-catch
import { Result } from '@praha/byethrow';

const fn = Result.try({
try: async (id: string) => await fetch(`/api/data/${id}`),
catch: (error) => new Error('Oops!', { cause: error }),
});

const result = await fn('abc'); // Result.ResultAsync<Response, Error>
@exampleAsync  try-catch with immediate execution
import { Result } from '@praha/byethrow';

const result = Result.try({
immediate: true,
try: () => fetch('/api/data'),
catch: (error) => new Error('Fetch failed', { cause: error }),
});

// result is ResultAsync<Response, Error>
@exampleAsync  safe
import { Result } from '@praha/byethrow';

const fn = Result.try({
safe: true,
try: async () => await Promise.resolve('ok'),
});

const result = await fn(); // Result.ResultAsync<string, never>
@exampleAsync  safe with immediate execution
import { Result } from '@praha/byethrow';

const result = Result.try({
safe: true,
immediate: true,
try: () => Promise.resolve('ok'),
});

// result is ResultAsync<string, never>
@categoryCreatorstry({
    immediate: trueimmediate: true,
    try: () => Promise<QueryResult>try: () => const database: {
    query: (sql: string) => Promise<QueryResult>;
}
database.query: (sql: string) => Promise<QueryResult>query(sql: stringsql),
    catch: (error: unknown) => QueryErrorcatch: (error: unknownerror) => new constructor QueryError(options: ErrorOptions & {
    query: string;
}): QueryError
QueryError({ query: stringquery: sql: stringsql, ErrorOptions.cause?: unknowncause: error: unknownerror }),
  });
};

// Combine everything
const const findUser: (id: string) => Result.ResultAsync<{
    readonly id: string;
    readonly name: string;
}, ValidationError | QueryError | NotFoundError>
findUser = (id: stringid: string) => {
  return import ResultResult.const pipe: <Result.Result<string, ValidationError>, Result.ResultAsync<QueryResult, ValidationError | QueryError>, Result.ResultAsync<{
    readonly id: string;
    readonly name: string;
}, ValidationError | QueryError | NotFoundError>>(a: Result.Result<string, ValidationError>, ab: (a: Result.Result<string, ValidationError>) => Result.ResultAsync<QueryResult, ValidationError | QueryError>, bc: (b: Result.ResultAsync<...>) => Result.ResultAsync<...>) => Result.ResultAsync<...> (+25 overloads)
pipe(
    const validateId: (id: string) => Result.Result<string, ValidationError>validateId(id: stringid),
    import ResultResult.const andThen: <Result.Result<string, ValidationError>, Result.ResultAsync<QueryResult, QueryError>>(fn: (a: string) => Result.ResultAsync<QueryResult, QueryError>) => (result: Result.Result<string, ValidationError>) => Result.ResultAsync<QueryResult, ValidationError | QueryError> (+1 overload)andThen((id: stringid) => const executeQuery: (sql: string) => Result.ResultAsync<QueryResult, QueryError>executeQuery(`SELECT * FROM users WHERE id = '${id: stringid}'`)),
    import ResultResult.const andThen: <Result.ResultAsync<QueryResult, ValidationError | QueryError>, Result.Failure<NotFoundError> | Result.Success<{
    readonly id: string;
    readonly name: string;
}>>(fn: (a: QueryResult) => Result.Failure<NotFoundError> | Result.Success<{
    readonly id: string;
    readonly name: string;
}>) => (result: Result.ResultAsync<QueryResult, ValidationError | QueryError>) => Result.ResultAsync<...> (+1 overload)
andThen((row: QueryResultrow) => {
      if (!row: QueryResultrow) {
        return import ResultResult.const fail: <NotFoundError>(error: NotFoundError) => Result.Result<never, NotFoundError> (+1 overload)fail(new constructor NotFoundError(options?: ErrorOptions): NotFoundErrorNotFoundError());
      }
      return import ResultResult.const succeed: <{
    readonly id: string;
    readonly name: string;
}>(value: {
    readonly id: string;
    readonly name: string;
}) => Result.Result<{
    readonly id: string;
    readonly name: string;
}, never> (+1 overload)
succeed({ id: stringid: row: QueryResultrow.stringid, name: stringname: row: QueryResultrow.stringname });
    }),
  );
};

Finally, execute the function and handle it appropriately.

// Execute and handle errors
const const result: Result.Result<{
    readonly id: string;
    readonly name: string;
}, ValidationError | QueryError | NotFoundError>
result = await const findUser: (id: string) => Result.ResultAsync<{
    readonly id: string;
    readonly name: string;
}, ValidationError | QueryError | NotFoundError>
findUser('u123');
if (import ResultResult.const isSuccess: <{
    readonly id: string;
    readonly name: string;
}>(result: Result.Result<{
    readonly id: string;
    readonly name: string;
}, unknown>) => result is Result.Success<{
    readonly id: string;
    readonly name: string;
}>
Type guard to check if a
Result
is a
Success
.
@function@typeParamT  - The type of the success value.@paramresult  - The Result to check.@returnstrue if the result is a Success, otherwise false.@exampleimport { Result } from '@praha/byethrow';

const result: Result.Result<number, string> = { type: 'Success', value: 10 };
if (Result.isSuccess(result)) {
  console.log(result.value); // Safe access to value
}
@categoryType  GuardsisSuccess(const result: Result.Result<{
    readonly id: string;
    readonly name: string;
}, ValidationError | QueryError | NotFoundError>
result)) {
  var console: ConsoleThe console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.

Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@seesourceconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to stdout with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.
@sincev0 .1.100log(const result: Result.Success<{
    readonly id: string;
    readonly name: string;
}>
result.value: {
    readonly id: string;
    readonly name: string;
}
value);
} else {
  // Handle each error type separately.
  switch (const result: Result.Failure<ValidationError | QueryError | NotFoundError>result.error: ValidationError | QueryError | NotFoundErrorerror.Error.name: "ValidationError" | "QueryError" | "NotFoundError"name) {
    case 'ValidationError':
      var console: ConsoleThe console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.

Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@seesourceconsole.Console.error(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to stderr with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then
util.inspect() is called on each argument and the
resulting string values are concatenated. See util.format()
for more information.
@sincev0 .1.100error('Validation error:', const result: Result.Failure<ValidationError | QueryError | NotFoundError>result.error: ValidationErrorerror.Error.message: "Invalid input provided"message);
      break;
    case 'QueryError':
      var console: ConsoleThe console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.

Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@seesourceconsole.Console.error(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to stderr with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then
util.inspect() is called on each argument and the
resulting string values are concatenated. See util.format()
for more information.
@sincev0 .1.100error('Query error:', const result: Result.Failure<ValidationError | QueryError | NotFoundError>result.error: QueryErrorerror.Error.message: "An error occurred while executing a query"message, 'Query:', const result: Result.Failure<ValidationError | QueryError | NotFoundError>result.error: QueryErrorerror.query: stringquery);
      break;
    case 'NotFoundError':
      var console: ConsoleThe console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.

Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@seesourceconsole.Console.error(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to stderr with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then
util.inspect() is called on each argument and the
resulting string values are concatenated. See util.format()
for more information.
@sincev0 .1.100error('Not found error:', const result: Result.Failure<ValidationError | QueryError | NotFoundError>result.error: NotFoundErrorerror.Error.message: "Resource not found"message);
      break;
  }
}

#Custom Error

#What are Custom Errors?

#1. Custom Error Classes (Recommended)

#2. Objects with Identifiable Tags

#Why Use Custom Error Classes?

#Stack Trace Availability

#Error Chaining with Cause Option

#Recommended: Using @praha/error-factory

#Installation

#Basic Usage with Result

#Custom Error

#What are Custom Errors?

#1. Custom Error Classes (Recommended)

#2. Objects with Identifiable Tags

#Why Use Custom Error Classes?

#Stack Trace Availability

#Error Chaining with Cause Option

#Recommended: Using @praha/error-factory

#Installation

#Basic Usage with Result

Custom Error

What are Custom Errors?

1. Custom Error Classes (Recommended)

2. Objects with Identifiable Tags

Why Use Custom Error Classes?

Stack Trace Availability

Error Chaining with Cause Option

Recommended: Using @praha/error-factory

Installation

Basic Usage with Result

Custom Error

What are Custom Errors?

1. Custom Error Classes (Recommended)

2. Objects with Identifiable Tags

Why Use Custom Error Classes?

Stack Trace Availability

Error Chaining with Cause Option

Recommended: Using @praha/error-factory

Installation

Basic Usage with Result