[Code] Ternary-based pattern matching in JS - KaKi's blog
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# Ternary-based pattern matching in JS Folks are jealous of other programming languages having this kind of pattern matching abilities : kotlin fun compareNumbers( a: Int, b: Int ) = when { a > b -> "A is greater than B" b > a -> "B is greater than A" else -> "A and B are equal" } But, so do we : typescript const compareNumbers = ( a: number, b: number ) => a > b ? 'A is greater than B' : b > a ? 'B is greater than A' : 'A and B are equal'; And this example is at identical character count btw. For example, I use it here [https://git.kaki87.net/KaKi87/dynapt/src/commit/9c89046de317295d7cd2e20e96b1a877a7ccfc72/src/updatePackages.js#L42-L44] in dynapt(1) : javascript url = app.url ? app.url : app.github ? `https://api.github.com/repos/$%7Bapp.github.repo%7D/releases/latest` : undefined To define an app download url that is either already provided if arbitrary, or requires being generated if from a GitHub / string. I use it again in the same file a little lower [https://git.kaki87.net/KaKi87/dynapt/src/commit/9c89046de317295d7cd2e20e96b1a877a7ccfc72/src/updatePackages.js#L64-L66] : javascript isUnmodified = app.url ? response.status === 304 : app.github ? response.status === 304 || cache[app.id]?.id === response.data['id'] : undefined; To determine whethere the download file from the abovementioned URL is modified or not, which works a little differently for GitHub because of its API. In another project that uses Discord.JS and TypeScript, I use this syntax for a conditional return type : typescript createSelectRow = ({ type // ... }: { type: type, // ... }) : type extends 'channel' ? ActionRowBuilder : type extends 'role' ? ActionRowBuilder : type extends 'user' ? ActionRowBuilder : ActionRowBuilder => { // ... } Etc. — (1) dynapt is a dynamic APT repository. More information about that project here [https://gist.github.com/KaKi87/f620788e9901abbfef4978eb7ad358b4].
  • Ephera@lemmy.mlEnglish
    1·
    4 days ago

    Interesting and valid point, but Kotlin’s when is not actually pattern-matching. It really is just syntactic sugar for if-else, like you’ve demonstrated.
    Actual pattern-matching allows matching on the structure of a data type. So, you formulate a pattern to describe the structure and then you can choose different code paths based on whether the pattern matches or you can also access fields inside of this data type.

    Rust allows pattern-matching even in variable assignments, which is going to look funky, but I think it shows quite well that pattern-matching isn’t just a fancy switch statement: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=3a8d126fa0cb9736597793915b00dd5d

    So what happens is that:

    • there’s a check whether the data type has the described structure, and
    • where the pattern contains wildcards (like variable ‘slots’), the corresponding field values from the data type get put into those wildcards.

    When someone writes this in Rust:

    let x = 1;

    It’s still pattern matching under the hood, but the pattern x is just a singular wildcard, so the right-hand side just gets ‘assigned’ to that variable name.

    And with these simple examples, you might think that being able to access field values this way is silly, since you can also access them via my_data_type.foo, but it becomes more useful with deeply nested data types, and in particular in statically typed languages, it allows you to work with different data types without type casting. So, it would be more useful of a feature for TypeScript than it might be for JavaScript.

    • KaKi87@jlai.luOP
      1·
      4 days ago

      Actual pattern-matching allows matching on the structure of a data type

      That’s a condition, so it can fit in a ternary.