do-try-tuple

do-try-tuple

Catches errors and rejected promises, returns tuple with error and value.

• Installation
• Usage
• Async Usage
• API
  • doTry function
  • ErrValueTuple type
  • UnknownError type
  • DoTryError class
  • DoTryErrorCode type and constants
• Important! Discriminating ErrValueTuple
  • Do not use if (error) or if (!error) to discriminate the tuple
• Using doTry().then()

Installation

npm install do-try-tuple

Usage

import doTry from 'do-try-tuple';

function div(a: number, b: number): number {
  if (b !== 0) return a / b;
  if (a !== 0) throw new Error(`Division by Zero`);
  throw new Error('Indeterminate Form');
}

const [errX, x] = doTry(() => div(4, 2));

if (errX == null) {
  const doubleX = x * 2;
  console.log('doubleX:', doubleX);
}

Async Usage

import doTry from 'do-try-tuple';

const [error, users] = await doTry(() => fetchUsers());

if (error != null) {
  console.error('Failed to fetch users:', error);
} else {
  console.log('Users:', users);
}

API

The library exports:

• doTry function (default export)
• ErrValueTuple type
• UnknownError type
• DoTryError class
• DoTryErrorCode type and constants

doTry function

takes a function that may throw an error or return a promise that may be rejected.

function doTry(fn: () => never): [UnknownError, never];
function doTry(fn: () => Promise<never>): Promise<[UnknownError, never]>;
function doTry<T>(fn: () => T): ErrValueTuple<T>;
function doTry<T>(fn: () => Promise<T>): Promise<ErrValueTuple<T>>;

ErrValueTuple type

is a union of two tuples: one representing the error case and the other representing the success case.

export type ErrValueTuple<T> =
  | [UnknownError, undefined] // error case
  | [undefined, T]; // success case

UnknownError type

type represents an unknown, non-nullish value caught by doTry function.

export type UnknownError = NonNullable<unknown>; // actually it is a {} type

In case when doTry catches null or undefined value, it returns a DoTryError as the first item of the tuple.

The library respects the same motivation as caused introduction useUnknownInCatchVariables compiler option in TypeScript:

• we cannot be sure that all thrown errors are instances of Error class

DoTryError class

is an error class that is returned when doTry cannot comply to the ErrValueTuple contract:

• when the fn argument is not a function
• when the thrown and caught error is null or undefined
• when the promise is rejected with null or undefined value

export class DoTryError extends Error {
  constructor(code: DoTryErrorCode, cause: unknown);
}

Fields:

Field	Type	Description
message	string	error message
code	DoTryErrorCode	error code
cause	unknown	caught error (null or undefined) or fn argument

DoTryErrorCode type and constants

is an union of string literal error codes that DoTryError class uses.

Constant	Description	Cause
ERR_NOT_A_FUNCTION	fn argument is not a function	fn
ERR_NULLISH_VALUE_REJECTED	promise is rejected with null or undefined value	caught value
ERR_NULLISH_VALUE_THROWN	fn throws null or undefined value	caught value

Important! Discriminating ErrValueTuple

To discriminate the ErrValueTuple type, you should compare the first element of the tuple to undefined. The most concise way to do that is to use the != null expression:

const [err, value] = doTry(() => someFn(...someArgs));

if (err != null) {
  // handle error
  return;
}

// handle value

The most performant way is to use strict equality comparison:

const [err, value] = doTry(() => someFn(...someArgs));

if (err !== undefined) {
  // handle error
  return;
}

// handle value

Do not use if (error) or if (!error) to discriminate the tuple

The if (error) expression will not work as expected in else block, because if casts err to boolean type and narrows its type in then-branch correctly, but in else-branch the type of err is still UnknownError | undefined, so TypeScript cannot discriminate the ErrValueTuple type correctly:

const [error, value] = doTry(() => someFn(...someArgs));

if (error) {
  // handle error
  return;
}

// value is still of type `T | undefined`
// error is still of type `UnknownError | undefined`

Using doTry().then()

You can map the result of doTry applied to function returning a promise using then method:

import doTry from 'do-try-tuple';

const [error, users] = await doTry(() => fetchUsers()).then(
  ([err, users]) => [err && new SomeCustomError(err), users] as const,
);

However, consider that functions returning promises can throw error synchronously:

const fetchUsers = (): Promise<string[]> => {
  if (Math.random() < 0.5) throw new Error('Failed to fetch users');
  return Promise.resolve(['Alice', 'Bob', 'Charlie']);
};

So, the doTry in this case returns an ErrValueTuple synchronously, and the attempt to call then method on it will throw an error: TypeError: doTry(...).then is not a function.

To handle this case, just add async keyword before fn argument:

const [error, users] = await doTry(async () => fetchUsers()).then(
  ([err, users]) => [err && new SomeCustomError(err), users] as const,
);

So, use

// CORRECT                       _____
const [err, value] = await doTry(async () => someFn(...))
  .then(([err, value]) => {
    // handle err and value
  });

instead of

// WRONG                         ___________
const [err, value] = await doTry(/* async */() => someFn(...))
  .then(([err, value]) => {
    // handle err and value
  });

The same is relevant for any other method of Promise class, like catch, finally, etc.