Overview

x-match-expression is a javascript pattern matching library developed in typescript that can be used standalone in the browser, in node or as ES module.

Installation

• Install it with npm install --save x-match-expression or reference it from CDN in the browser

Usage

In the browser (with a script tag)

• Add a script tag <script src="https://unpkg.com/x-match-expression/dist/browser/index.js" type="application/javascript"></script>
• And later on, use the global match function

 const financialStatus = match(42)
    .caseGreaterEqualThan(1000000, "I'm rich!")
    .caseLessThan(0, "I'm ruined")
    .default("I keep trying");
 console.log(financialStatus); // outputs I keep trying

In Node

const {match} = require("x-match-expression");

const isEven = match(2)
    .case(n => n % 2 === 0, true)
    .default(false);
 console.log(isEven); // outputs true

as ES Module

import {match} from "x-match-expression";

const areWeInTheFuture = match(new Date())
    .caseNewerThan(new Date(2050, 0, 1), true)
    .default(false);
 console.log(areWeInTheFuture); // outputs false

API

If you are using typescript or an IDE with autocompletion, just write match. and explore all the options available. Basically you have cases for every javascript primitive type + instance checks. Almost every case method has a companion method ended in "If"; This allows to put an extra predicate if the case is not enough. To end the match expression use default method.

Advanced Usage

Instance check example

import {match} from "x-match-expression";

class FatalError {
    constructor(readonly id: string, readonly date: Date) {}
}

class Warning {
    constructor(readonly title: string, readonly message: string) {}
}

class MailMessage {
    constructor(readonly sender: string, readonly subject: string, readonly message: string) {}
}

type Whatever =
    FatalError |
    Warning |
    MailMessage;

const info = getInfo(); // returns type Whatever

const infoDetails = match(info)
    .caseInstanceIf(FatalError, _ => _.id === 404 => `No content found at ${e.date.toISOString()}`)
    .caseInstance(FatalError, _ => `Error #${_.id} received`)
    .caseInstance(Warning, _ => `Warning ${_.title}: ${_.message}`)
    .caseInstance(MailMessage, _ => `You received a message from ${_.sender}`)
    .default("Unknown information received");

Float comparison

import {match} from "x-match-expression";

const a = 0.2;
const b = 0.4;
const c = 0.6;
const whatHappens = match(a+b)
    .caseEqual(c, "This number is 0.6")
    .caseAlmostEqual(c, "The number is not exactly 0.6, but it is very close") // <-- this will match
    .default("This number is not 0.6 at all");

Custom test that fails

import {match} from "x-match-expression";

function isSomething(x) {
    throw "Error!"; // <-- look at this
}

const value = match(x)
    .case(isSomething, "This is something") // <-- custom tests doesn't raise exceptions, they simple return false
    .default("This is nothing");

With union type as return

import {match} from "x-match-expression";

const twice: number | string = match("hello world")
    .caseString(s => `${s}${s}`)
    .caseNumber(n => n*2)
    .default(0);