Perfective Common for TypeScript

Installation

npm install @perfective/common

After the installation you can read the full compiled documentation in the node_modules/@perfective/common/docs.html.

Key Features

The @perfective/common package facilitates writing highly readable functional code. It focuses on providing functions to handle ECMAScript types and to compose functions together easily.

Maybe monad

The @perfective/common/maybe package provides a Maybe monad implementation.

It allows you to write and compose functions that accept only present (defined and non-null) values. It helps avoid additional complexity and noise when handling null and undefined values.

For example, consider you have the User and Name types below and want to output a user’s full name.

interface User {
    name?: Name;
}

interface Name {
    first: string;
    last: string;
}

If you write functions that have to handle null and undefined values, then you would have to write something like this:

function userNameOutput(user: User | null | undefined): string {
    if (isPresent(user)) {
        const name = fullName(user.name);
        if (isPresent(name)) {
            return name;
        }
    }
    throw new Error('User name is unknown');
}

function fullName(name: Name | null | undefined): string | null {
    if (isPresent(name)) {
        const trimmed = `${name.first} ${name.last}`.trim();
        if (isNotEmpty(trimmed)) {
            return trimmed;
        }
    }
    return null;
}

Using the Maybe monad, you can write simpler and more readable functions:

import { panic } from '@perfective/common/error';
import { just, Maybe, maybe } from '@perfective/common/maybe';
import { isNotEmpty, trim } from '@perfective/common/string';

function userNameOutput(user: User | null | undefined): string {
    return maybe(user)
        .pick('name') // <1>
        .onto(fullName) // <2>
        .or(panic('User name is unknown')); // <3>
}

function fullName(name: Name): Maybe<string> {
    return just(`${name.first} ${name.last}`)
        .to(trim) // <4>
        .that(isNotEmpty); // <5>
}

1. Maybe.pick() provides a strictly typed "optional chaining" of the Maybe.value properties.
2. Maybe.onto() (flat) maps a Maybe.value to another Maybe.
3. Maybe.or() extracts a value from the Maybe with a given fallback. (or allows to throw an error).
4. Maybe.that() filters a value inside Maybe.
5. Maybe.to() maps a value inside Maybe using a given callback.

In addition to these methods, the Maybe monad also has: Maybe.into(), Maybe.which(), Maybe.when(), Maybe.otherwise(), and Maybe.through() methods.

Read more about the Maybe monad and other @perfective/common/maybe functions in the package documentation.

Result monad

The @perfective/common/result package provides the Result monad implementation (as a concrete case of the Either monad). It allows developers to increase the reliability of their code by treating errors as valid part of a function output.

A Result instance can be either a Success or a Failure. If a Result is a Success, a computation proceeds to the next step. In case of a Failure, all further computations are skipped until the recovery or exit from the computation.

The Result monad is similar to the Maybe monad, but unlike Maybe, a Result contains a reason for its Failure.

The Result monad is also similar to the Promise (as a Promise can either be "resolved" or "rejected"). But, unlike Promise, the Result chain is synchronous.

For example, consider you have an HTTP endpoint to return user data stored in the database. The purpose of the endpoint is to map a given (unsafe) user ID input to a User.

Assume you have the following functions

export interface User {
    // User data
}

/** Returns `true` if the active user has admin access. */
declare function hasAdminAccess(): boolean;

/** Builds an SQL query to load a user with a given `id`. */
declare function userByIdQuery(id: number): string;

/** Sends a given `sql` to the database and returns a User. */
declare function userQueryResult(sql: string): Promise<User>;

/** Logs a given error */
declare function logError(error: Error): void;

If you write regular imperative code you may have something like this:

/** @throws Error if a given id is invalid. */
function validUserId(id: unknown): number {
    if (typeof id !== 'string') {
        throw new TypeError('Input must be a string');
    }

    const userId = decimal(id);
    if (userId === null) {
        throw new Error('Failed to parse user id');
    }
    if (!Number.isInteger(userId) || userId <= 0) {
        throw new Error('Invalid user id');
    }
    return userId;
}

async function userResponseById(id: unknown): Promise<User> {
    try {
        return userForQuery(
            userByIdQuery(
                validUserId(id), // (1)
            ),
        );
    } catch (error: unknown) {
        logError(error as Error);
        throw error as Error;
    }
}

1. Note that validUserId() indicates that it throws an error only as a JSDoc. TypeScript compiler does not check that the code should be wrapped into the try-catch block.

Using the Result monad and functions from the @perfective/common subpackages you can write the same code as:

import { isNotNull } from '@perfective/common';
import { typeError } from '@perfective/common/error';
import { naught } from '@perfective/common/function';
import { decimal, isNonNegativeInteger } from '@perfective/common/number';
import { rejected } from '@perfective/common/promise';
import { Result, success } from '@perfective/common/result';
import { isString } from '@perfective/common/string';

function validUserId(id: unknown): Result<number> {
    return success(id)
        .which(isString, typeError('Input must be a string')) // (1)
        .to(decimal)
        .which(isNotNull, 'Failed to parse user ID') // (2)
        .that(isNonNegativeInteger, 'Invalid user ID'); // (3)
}

async function userResponseById(id: unknown): Promise<User> {
    return success(id)
        .when(hasAdminAccess, 'Access Denied') // (4)
        .onto(validUserId) // (5)
        .to(userByIdQuery)
        .through(naught, logError) // (6)
        .into(userForQuery, rejected); // (7)
}

1. Result.which() applies a type guard and narrows the Result.value type.
2. decimal() returns number | null, so another type guard is required.
3. Result.that() checks if the Success.value satisfies a given predicate.
4. Result.when() checks an external condition.
5. Result.onto() allows a different Result object to be returned (in this case, the Result of the validUserId() function).
6. Result.through() runs a given procedure (a no-op naught() function for the Success).
7. Result.into() allows the completion (folding) of the Result chain computation and switch to a different type.

In addition to the methods used in the example above, the Result monad also provides Result.or() and Result.otherwise() methods.

Read more about the Result monad and other @perfective/common/result functions in the package documentation.

Chained Exceptions

The ECMA Error class does not store a previous error. This is inconvenient, as it requires either manually adding a previous error message to a new error. Or worse, skip providing the previous error altogether.

Chaining previous errors is helpful for debugging. Especially in async environments, when most of the stack trace is full of useless function calls like next() or on the frontend with packed code and renamed functions.

The @perfective/common/error package provides the Exception class to make logging and debugging of production code easier. It supports three features:

• providing a previous error (allows to stack errors);
• using a message template with string tokens (allows to localize and format messages);
• storing additional context (simplifies logging and debugging).

.Using the Exception class and its constructors.

import { caughtError, causedBy, chained, exception } from '@perfective/common/error';

interface FetchRequest {
    method: string;
    url: string;
}

interface User {}

function numberInput(input: string): number {
    const id = Number(input);
    if (Number.isNaN(id)) {
        throw exception('Input {{value}} is not a number', { // <1>
            value: input,
        });
    }
    return id;
}

function userRequest(id: string): FetchRequest {
    try {
        const userId = numberInput(id);
        return {
            method: 'GET',
            url: `user/${userId}`,
        };
    } catch (error: unknown) { // <2>
        throw causedBy(caughtError(error), 'Invalid user id {{id}}', { // <3>
            id,
        });
    }
}

async function userResponse(request: FetchRequest): Promise<User> {
    return fetch(request.url, {
        method: request.method,
    });
}

async function user(id: string): Promise<User> {
    return Promise.resolve(id).then(userRequest).then(userResponse).catch(
        chained('Failed to load user {{id}}', { // <4>
            id,
        }),
    );
}

1. Use the exception() function to instantiate an initial Exception without previous errors.
2. Use the caughtError() function to wrap a possible non-Error value.
3. When you use a try-catch block, use the causedBy() function to create an Exception with a previous error.
4. Use the chained() function to create a callback to chain an Error (for example, in Promise or a Result).

When you want to output a chained Exception, you can use the Exception.toString() method. For the example above, the output may look like this:

Exception: Failed to load user `A`
    - Exception: Invalid user id `A`
    - Exception: Input `A` is not a number

If you want to log an Exception for debugging purposes, use the chainedStack() function. It will return a similar chain of messages as above, but each message will also contain a stack trace for each error.

Read more about the functions to handle the built-in JS errors and the Exception class in the @perfective/common/error package documentation.

Packages

Packages are organized and named around their primary type:

• @perfective/common — functions and types to handle types (e.g., TypeGuard interface), null, undefined, and void values.
• @perfective/common/array — functions and types for handling arrays.
• @perfective/common/boolean — functions and types to handle boolean values.
• @perfective/common/date — functions and types to handle Date values.
• @perfective/common/error — functions and types to handle Error and related classes.
• @perfective/common/function — functions and types for functional programming.
• @perfective/common/match — functions and types for a functional style switch-case.
• @perfective/common/maybe — a Maybe monad (Option type) implementation.
• @perfective/common/number — functions and types to handle numbers.
• @perfective/common/object — functions and types for handling the Object class.
• @perfective/common/promise — functions and types to handle the Promise class.
• @perfective/common/result — a Result monad (Result type) implementation.
• @perfective/common/string — functions and types to handle strings.

The packages have full unit test coverage.

Roadmap

The ROADMAP.adoc file describes how built-in JavaScript objects and methods are covered by the @perfective/common package.

Versioning

This package starts its versioning from 0.7.0, as it continues versioning after the deprecated packages:

• @perfective/array (v0.4.0);
• @perfective/error (v0.3.0);
• @perfective/fp (v0.6.0);
• @perfective/identity (v0.2.0);
• @perfective/match (v0.3.0);
• @perfective/maybe (v0.6.0);
• @perfective/object (v0.4.0);
• @perfective/real (v0.6.0);
• @perfective/string (v0.3.0);
• @perfective/value (v0.3.0).