# match-when
**Repository Path**: mirrors_FGRibreau/match-when
## Basic Information
- **Project Name**: match-when
- **Description**: :shell: Pattern matching for modern JavaScript
- **Primary Language**: Unknown
- **License**: Not specified
- **Default Branch**: master
- **Homepage**: None
- **GVP Project**: No
## Statistics
- **Stars**: 0
- **Forks**: 0
- **Created**: 2020-09-24
- **Last Updated**: 2026-05-17
## Categories & Tags
**Categories**: Uncategorized
**Tags**: None
## README
### match-when - Pattern matching for modern JavaScript
[](https://circleci.com/gh/FGRibreau/match-when/tree/master) [](https://coveralls.io/github/FGRibreau/match-when?branch=master)    [](https://twitter.com/FGRibreau) [](https://www.codementor.io/francois-guillaume-ribreau?utm_source=github&utm_medium=button&utm_term=francois-guillaume-ribreau&utm_campaign=github) [](https://join.slack.com/t/fgribreau/shared_invite/zt-edpjwt2t-Zh39mDUMNQ0QOr9qOj~jrg)
> Finally a **clear**, **succinct** and *safe* syntax to do Pattern Matching in modern JavaScript. [(backstory)](http://blog.fgribreau.com/2015/12/match-when-pattern-matching-for-modern.html)
#### Shameless plug
- [Open-Source Webhook as a Service](https://www.hook0.com/)
- [Looking for a managed Keycloak IAM ?](https://www.cloud-iam.com/)
- [**Charts, simple as a URL**. No more server-side rendering pain, 1 url = 1 chart](http://bit.ly/2dPZfbn)
#### Usage
The setup is pretty simple, simply require the library with `match` and `when` and you are ready to go!
```js
const {match, when} = require('match-when');
```
or globally
```js
require('match-when/register'); // `match` and `when` are now globally available
```
Now let's see how we would write a factorial function:
```js
const fact = match({
[when(0)]: 1,
[when()]: (n) => n * fact(n-1)
});
fact(10); // 3628800
```
Clear and simple right?
Alternatively, `match(, patternSpecification)` can be used to instantly perform a match:
```js
function fact(n){
return match(n, {
[when(0)]: 1,
[when()]: (n) => n * fact(n-1)
});
}
fact(10); // 3628800
```
Note that `when()` is a catch-all pattern and, if used, should always be the last condition. If you forget it `match()` will throw a `MissingCatchAllPattern` exception if nothing was matched.
##### Setup
```
npm i match-when -S
```
##### High Order Functions
`match` works well with high order functions like `map`, `filter` (and so on) too:
```js
[2, 4, 1, 2].map(match({
[when(1)]: "one",
[when(2)]: "two",
[when()]: "many"
}));
// [ 'two', 'many', 'one', 'two' ]
```
##### Arrays
It also works with **arrays**:
```js
function length(list){
return match({
[when([])]: 0,
[when()]: ([head, ...tail]) => 1 + length(tail)
})(list);
}
length([1, 1, 1]); // 3
```
##### OR
Sadly JavaScript does not offer us a way to overload operators so we're stuck with `when.or`:
```js
function parseArgument(arg){
return match({
[when.or("-h", "--help")]: () => displayHelp,
[when.or("-v", "--version")]: () => displayVersion,
[when()]: (whatever) => unknownArgument.bind(null, whatever)
})(arg);
}
parseArgument(process.argv.slice(1)); // displayHelp || displayVersion || (binded)unknownArgument
```
##### AND
```js
const protocols = repositories.map(match({
[when.and({useGit:true}, {useSSH: true})]: 'git+ssh:',
[when.and({useGit:true}, {useHTTP: true})]: 'git+http:',
[when.and({useGit:true}, {useHTTPS: true})]: 'git+https:',
[when()]: 'unsupported:'
}))
```
##### Regular Expressions
match-when supports [regular expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp) as well:
```js
['hey.com', 'fg@plop.com', 'fg+plop@plop.com', 'wat'].filter(match({
[when(/\S+@\S+\.\S+/)]: false, // **seems** to be a valid email (unsafe regex for doc purpose only)
[when()]: true // the email could be invalid, return it
}));
// ['hey.com', 'wat']
```
##### Range
```js
[12, 42, 99, 101].map(match({
[when.range(0, 41)]: '< answer',
[when.range(43, 100)]: '> answer',
[when(42)]: 'answer',
[when()]: '< 0, or > 100'
}));
// ['< answer', 'answer', '> answer', '< 0, or > 100']
```
### Supported patterns:
- `{ x1: pattern1, ..., xn: patternn }` - matches any object with property names `x1` to `xn` matching patterns `pattern1` to `patternn`, respectively. Only the own properties of the pattern are used.
- `[pattern0, ..., patternn]` - matches any object with property names 0 to n matching patterns `pattern0` to `patternn`, respectively.
- `/pattern/flags` - matches any values than pass the regular expression test
- `when.range(low, high)` matches any number value in the range [low, high], `low` and `high` included.
- `when.or(pattern0, ..., patternn)` - matches if at [least one](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some) `pattern` matches.
- `when.and(pattern0, ..., patternn)` - matches if [every](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every) `pattern` matches.
### Todo:
I will accept PR with their associated tests for the following features:
- define and implement some syntax to support wildcards
[todo-list inspired by pattern-match from dherman](https://github.com/dherman/pattern-match#patterns).
\* *well, of course, they are not keywords but simple functions*
#### Why/How? - the slides
#### Why/how? - the talk in **french (sorry)**
## Development sponsored by iAdvize
I work at [iAdvize](http://iadvize.com) as a Lead Developer and Architect. iAdvize is the **leading real-time customer engagement platform in Europe** and is used in 40 different countries. We are one of the french startup with the [fastest growth](http://www.iadvize.com/fr/wp-content/uploads/sites/2/2014/11/CP-Fast-50.pdf) and one of [the **greatest place to work** in **France**](https://vimeo.com/122438055).
We are looking for a [**NodeJS backend developer**](http://smrtr.io/FqP79g), a [Scala backend developer](http://smrtr.io/FqP79g), a [**JavaScript frontend developer**](http://smrtr.io/wR-y4Q), a [Full-stack Developer](http://smrtr.io/SGhrew) and a [DevOps System Engineer](http://smrtr.io/OIFFMQ) in Paris or Nantes. **[Send me a tweet](https://twitter.com/FGRibreau) if you have any questions**!
## [The Story](http://blog.fgribreau.com/2015/12/match-when-pattern-matching-for-modern.html)
## [Changelog](/CHANGELOG.md)
