javascript - Accumulator not initialized when using reduce.call -

why doesn't accumulator initialized? keep getting undefined.

var s = "sosstsros";  var radiatedletters = array.prototype.reduce.call(s,function(acc,curr){     if(!curr.match(/[so]/)){         acc++;                 } },0); console.log(radiatedletters); 

you need return accumulated value reducer function, not mutate it:

var s = "sosstsros";    var radiatedletters = array.prototype.reduce.call(s, function(acc, curr) {    if (!curr.match(/[so]/)) {      return acc + 1;    } else {      return acc;    }  }, 0);  console.log(radiatedletters);