.reduce is not a function javascript exercises

Hi, I’m having a problem with one of the java script exercises in fundamentals 4, it keeps saying error: finder.reduce is not a function. Does anyone know how to fix this? The code is below.
Thanks
const sumAll = function(a, b) {
var x = b + 1;
var y = a;
var finder = [];

 if(isNaN(b) || isNaN(a)){

console.log(‘ERROR’)
}
if (typeof a === ‘string’ || a instanceof String || typeof b === ‘string’ || b instanceof String){
console.log(‘ERROR’)
}
if(a < 0 || b < 0){
console.log(‘ERROR’)
}
if(a > b){
x = a;
y = b;
}

for (let i = y; i < x + 1; i++) {
finder += i;
}

finder.reduce((y, x) => y + x);
console.log(v);

}

js fiddle = https://jsfiddle.net/joshblake749/7Lst6dre/61/

this turns finder into a string, and it is no longer an array… so doesn’t have the reduce function anymore.