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Finally aclear,succinct andsafe syntax to do Pattern Matching in modern JavaScript.(backstory)

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The setup is pretty simple, simply require the library withmatch andwhen and you are ready to go!

const{match, when}=require('match-when');

or globally

require('match-when/register');// `match` and `when` are now globally available

Now let's see how we would write a factorial function:

constfact=match({[when(0)]:1,[when()]:(n)=>n*fact(n-1)});fact(10);// 3628800

Clear and simple right?

Alternatively,match(<input>, patternSpecification) can be used to instantly perform a match:

functionfact(n){returnmatch(n,{[when(0)]:1,[when()]:(n)=>n*fact(n-1)});}fact(10);// 3628800

Note thatwhen() is a catch-all pattern and, if used, should always be the last condition. If you forget itmatch() will throw aMissingCatchAllPattern exception if nothing was matched.

npm i match-when -S

match works well with high order functions likemap,filter (and so on) too:

[2,4,1,2].map(match({[when(1)]:"one",[when(2)]:"two",[when()]:"many"}));// [ 'two', 'many', 'one', 'two' ]

It also works witharrays:

functionlength(list){returnmatch({[when([])]:0,[when()]:([head, ...tail])=>1+length(tail)})(list);}length([1,1,1]);// 3

Sadly JavaScript does not offer us a way to overload operators so we're stuck withwhen.or:

functionparseArgument(arg){returnmatch({[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

constprotocols=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:'}))

match-when supportsregular expressions as well:

['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']

[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']

• { x1: pattern1, ..., xn: patternn } - matches any object with property namesx1 toxn matching patternspattern1 topatternn, respectively. Only the own properties of the pattern are used.
• [pattern0, ..., patternn] - matches any object with property names 0 to n matching patternspattern0 topatternn, 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 andhigh included.
• when.or(pattern0, ..., patternn) - matches if atleast onepattern matches.
• when.and(pattern0, ..., patternn) - matches ifeverypattern matches.

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.

*well, of course, they are not keywords but simple functions

I work atiAdvize as a Lead Developer and Architect. iAdvize is theleading real-time customer engagement platform in Europe and is used in 40 different countries. We are one of the french startup with thefastest growth and one ofthegreatest place to work inFrance.

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