pattern-matcher-es v1.0.2

Pattern Matching

A declarative pattern-matching library.

License: MIT

Table of contents

• Introduction
• Installation
• Usage

Introduction

There are limited ways in Javascript to match expressions with the exception of strings. The switch is severely limiting. To avoid accidental fallthrough an explicit break is required in each case and the only comparison that it can do is ===. This library aim is to solve these problems by providing a declarative version of the if and switch conditionals. Where the shape of the shape of the data is described by "patterns".

Installation

npm install pattern-matcher-es

Usage

// ESM
import { patternMatch, when, otherwise } from 'pattern-matcher-es'

patternMatch(data)(
    when(pattern, handler),
    otherwise(handler)
)

data can be anything. The pattern is a description of how you expect the data to look like. patternMatch executes each line by line, top to bottom. Be mindful of how you write your code, the order of the when functions matter. If no when function applies truthfully patternMatch will default to otherWise if provided, otherwise the expression evaluates to undefined.

    const example = 'This is a string'
    const res = patternMatch(example)(
        when(isString, () => 'First case'),
        when(isNumber, () => 'Second case'),
        otherWise(() => 'Third case')
    )
    console.log(res) // Will print out 'First case'

    const example = 5
    const res = patternMatch(example)(
        when(inRange(1, 1000), () => 'First case'),
        when(inRange(1, 10), () => 'Second case'),
        when(inRange(1, 5), () => 'Third case'),
        otherWise(() => 'Fourth case')
    )
    console.log(res) // Will print 'First case' since order matter.

An example with an advanced object structure and arrays.

    const example = {
        index: 5,
        ob : {
            sampleArray: ['123', '213','32132']
        }
    }
    const validator = {
        index: is(5),
        ob: {
            sampleArray: allOf(isString)
        } 
    }
    const res = patternMatch(example)(
        when(validator, () => 'First case'),
        otherWise(() => 'Second case')
    )
    console.log(res) // Will print 'First case'

Before and after example

return patternMatch(res)(
    when({ statusCode: inRange(200, 299) }, () => res.body),
    otherWise(() => {})
)

1. patternMatch will check that statusCode is a key in res.
2. The inRange guard check if the value is a number and if it is within the passed ranged.

    if (typeof res?.statusCode === number && res.statusCode >= 200 && res.statusCode < 300) {
        return res.body
    } else {
        return {}
    }