Brief :

use parseInt or Math.floor to have y/2 as integer, unleness you will not reach 0 which is the stopper of recursion .

Details

if you want to transalte [C Algo]:

int power(int x, unsigned int y)
{
    if( y == 0)
        return 1;
    else if (y%2 == 0)
        return power(x, y/2)*power(x, y/2);
    else
        return x*power(x, y/2)*power(x, y/2);
 
}

To [JS Algo] , you will have :

function power(x,y){
     if(y===0){return 1}
     else if (y%2 ===0){
         return power(x,parseInt(y/2))*power(x,parseInt(y/2))
     }else{
          return x*power(x,parseInt(y/2))*power(x,parseInt(y/2))
     }

}

DEMO :

    function power(x,y){
         if(y===0){return 1}
         else if (y%2 ===0){
             return power(x,parseInt(y/2))*power(x,parseInt(y/2))
         }else{
              return x*power(x,parseInt(y/2))*power(x,parseInt(y/2))
         }
    
    }


console.log(power(3,2))