Simple Javascript pattern matching

pattern-matching2.js

let a = [null, {status: 'OK'}];

switch_(
  (err, _) => console.log('error', err),
  (_, resp) => console.log('response', resp)  
)(a);

// logs 'response' { "status": "OK" }

let b = [{err: 401}, null];

switch_(
  (err, _) => console.log('error', err),
  (_, resp) => console.log('response', resp)
)(b);

// logs 'error' { "err": 401 }

let c = [1, 2, 3, 4];

let sumReduce = numbers => switch_(
  (x, $tail) => x + sumReduce($tail),
  (x) => x
)(numbers);

console.log(sumReduce(c)); // 10

let len = array => switch_( 
  (a, b, c) => 3,
  (a, b) => 2,
  (a) => 1,
  () => 'dont know'
)(array);

console.log(len([9,9,9,9,9,9])) // 'dont know'

let players = switch_(
  ({pool}) => `${pool} plays pool`,
  ({darts}) => `${darts} plays darts`,
  () => 'none'
);

console.log(players({darts: 'David'})); // David plays darts
console.log(players({pool: 'Paul'})); // Paul plays pool

let guess = switch_(
 (n = 1) => 'very low: ' + n,
 (n = 2) => 'low: ' + n,
 (n = 3) => 'almost: ' + n,  
 (n = 4) => 'just right: ' + n,
 (n) => 'give up: ' + n
);

console.log(guess(4)); // just right: 4

let guessHarder = switch_(
 (n = 3, p = 2) => 'WINNER!!!',  
 (a, b) => 'try again',
 () => 'guess 2 numbers'
);

console.log(guessHarder([4, 2])); // try again

//---

function switch_(...exps){
  return function(data = []){
    if(!Array.isArray(data)) data = [data];
    for(let i = 0; i < exps.length; i++){
      let exp = exps[i];
      var args = exp
        .toString()
        .match(/(?:function\s)?.*?\(([^)]*)\)/)[1]
        .split(',');
              
      let argValues = [];
      let usesTail = false;
      
      for(let j = 0; j < args.length; j++){
        let arg = args[j].trim();
        if(arg === '_'){
          argValues[j] = null;
          continue;
        }
        
        if(!data[j]) break;
        
        if(arg.indexOf('$') === 0) {
          argValues[j] = data.slice(j);
          usesTail = true;
          break;
        } else if(arg.match(/{\w+}/)){
           let name = (arg.match(/(\w+)/))[1];
           let result = eval_(data[j], name);
           if(result){
             argValues[j] = {[name]: result};             
           } else break;
        } else if(arg.indexOf('=') > -1){
           let [ref, val] = arg.split('=');
           ref = ref.trim();
           val = val.trim();
           let result = data[j] == val;
           if(result){
             argValues[j] = val;             
           } else break;
        } else {
          argValues[j] = data[j];
        }
      }
                  
      if((
          // common case where number of params and args match
          argValues.length === data.length && 
          argValues.length === args.length
        ) || (
          // empty catch all
          args.length === 1 &&
          args[0] === ""
        ) || 
        // $tail special case
        usesTail
      ){
        return exp.apply(null, argValues);
      }
    }
  }
}

function eval_(ctx, obj){
  try{
    return new Function(`return ${obj}`).call(ctx);
  } catch(e){
    return new Function(`return this.${obj}`).call(ctx);   
  }
}