Functional programming style pattern matching in JavaScript

Question

I'm writing compiler from kind of functional language to JS. Compiler would run in browser. I need to implement pattern matching mechanics in JS, because original language have one. I've found Sparkler and Z. Sparkler can't be executed in browser as far as I know and Z doesn't have all possibilities I need.

So my language have semantics like this:

count x []         <- 0
count x [ x : xs ] <- 1 + count x xs
count x [ y : xs ] <- count x xs

This is what happens in this snippet:

First line is definition of a function, which takes two parameters: some variable x and empty list, and returns zero.

Second line is definition of a function, which also takes two parameters: some variable x and list, which starts with x, and returns 1 + count(x, xs)

Fot this example I want to generate code like this:

const count = (x, list) => {
    match(x, list) => (
        (x, []) => {...}
        (x, [ x : xs ]) => {...}
        (x, [ y : xs ]) => {...}
    )
} 

How properly unfold this kind of pattern matching into ifs and ors?

Answer 1

General case

There is a proposal for Pattern Matching in ECMAScript, but as of 2018 it's in a very early stage.

Currently, the Implementations section only lists:

• Babel Plugin
• Sweet.js macro (NOTE: this isn't based on the proposal, this proposal is partially based on it!)

List case

Use destructuring assignment, like:

const count = list => {
  const [x, ...xs] = list;
  if (x === undefined) {
    return 0;
  } else if (xs === undefined) {
    return 1;
  } else {
    return 1 + count(xs);
  }
}