Package Exports

@csvenke/compose-rules

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Readme

@csvenke/compose-rules

Simple helpers for writing composable business rules

@csvenke/compose-rules provides dead simple helpers for writing composable business rules. Attack complex problems by dividing them into smaller easier problems.

"Nothing is particularly hard if you divide it into small jobs" - Henry Ford

Install

Using npm

npm install --save @csvenke/compose-rules

Using yarn

yarn add @csvenke/compose-rules

Example

Let's say you need to verify that some value is compliant with several requirements.

It must be a number
It must be larger than 1
It must be less than 10
It must be an odd number
It must be a prime number

You could just write a function that verifies all those requirements, but requirements tend to change and changes cause regression in your code. What if you instead wrote pure functions that verifies each requirement and then wrote unit tests for each function. Then you compose all those functions into a single function that verifies all requirements.

The benefit of this approach is when requirements change you simply add/remove/edit specific functions from the composer without affecting the other functions.

Try it out online with repl.it!

import { and, not } from "@csvenke/compose-rules";

/**
 * Returns true if n is number
 */
const isNumber = n => typeof n === "number";

/**
 * Returns true if n is larger than one
 */
const isNumberLargerThanOne = n => n > 1;

/**
 * Returns true if n is less than ten
 */
const isNumberLessThanTen = n => n < 10;

/**
 * Returns true if n is even
 */
const isNumberEven = n => n % 2 === 0;

/**
 * Returns true if n is odd
 */
const isNumberOdd = not(isNumberEven);

/**
 * Returns true if n is prime
 */
const isNumberPrime = n => {
  for (let i = 2; i < n; i++) if (n % i === 0) return false;
  return n > 1;
};

/**
 * Returns true if all rules returns true
 */
const isValidValue = and(
  isNumber,
  isNumberLargerThanOne,
  isNumberLessThanTen,
  isNumberOdd,
  isNumberPrime
);

console.log(isValidValue(5)); // true
console.log(isValidValue(8)); // false
console.log(isValidValue("Hello")); // false
console.log(isValidValue(undefined)); // false

API

and

Returns a rule function that returns true if all rules are true.

Example

import { and } from "@csvenke/compose-rules";

/**
 * Returns true if n is larger than one
 */
const isNumberLargerThanOne = n => n > 1;

/**
 * Returns true if n is less than ten
 */
const isNumberLessThanTen = n => n < 10;

/**
 * Returns true if all rules returns true
 */
const hasValidValue = and(isNumberLargerThanOne, isNumberLessThanTen);

console.log(hasValidValue(11)); // false
console.log(hasValidValue(5)); // true

or

Returns a rule function that returns true if some rules are true.

Example

import { or } from "@csvenke/compose-rules";

/**
 * Returns true if name equal 'John'
 */
const isNamedJohn = name => name === "John";

/**
 * Returns true if name equals 'Jane'
 */
const isNamedJane = name => name === "Jane";

/**
 * Returns true if name equals 'John' or 'Jane'
 */
const hasValidName = or(isNamedJohn, isNamedJane);

console.log(hasValidName("Billy")); // false
console.log(hasValidName("John")); // true
console.log(hasValidName("Jane")); // true

not

Returns a rule function that returns true if all rules are false.

Example

import { not } from "@csvenke/compose-rules";

/**
 * Returns true if name is 'John'
 */
const isNamedJohn = name => name === "John";

/**
 * Returns true if name is 'Jane'
 */
const isNamedJane = name => name === "Jane";

/**
 * Returns true if name is not 'John' or 'Jane'
 */
const hasValidName = not(isNamedJohn, isNamedJane);

console.log(hasValidName("John")); // false
console.log(hasValidName("Jane")); // false
console.log(hasValidName("Billy")); // true

nand

Returns a rule function that returns false if all rules are true.

Example

import { nand } from "@csvenke/compose-rules";

/**
 * Returns true if n is larger than one
 */
const isNumberLargerThanOne = n => n > 1;

/**
 * Returns true if n is less than ten
 */
const isNumberLessThanTen = n => n < 10;

/**
 * Returns false if all rules returns true
 */
const hasValidValue = nand(isNumberLargerThanOne, isNumberLessThanTen);

console.log(hasValidValue(11)); // true
console.log(hasValidValue(5)); // false

nor

Returns a rule function that returns false if some rules are true.

Example

import { nor } from "@csvenke/compose-rules";

/**
 * Returns true if name equal 'John'
 */
const isNamedJohn = name => name === "John";

/**
 * Returns true if name equals 'Jane'
 */
const isNamedJane = name => name === "Jane";

/**
 * Returns true if name equals 'John' or 'Jane'
 */
const hasValidName = nor(isNamedJohn, isNamedJane);

console.log(hasValidName("Billy")); // true
console.log(hasValidName("John")); // false
console.log(hasValidName("Jane")); // false

Development

Installing dependencies

yarn install

Running tests

yarn test

Contributing

In lieu of a formal styleguide, take care to maintain the existing coding style.

License

MIT