复杂还是简单?
function multiply(a, b) {
    var mul=[1,1]
    var ab=[a,b].map((num,index)=>(''+num).replace(/\.(\d*)/,(_,$)=>{
        mul[index]=Math.pow(10,$.length)
        return $
    }))
    return ab.reduce((a,b)=>a*b)/mul.reduce((a,b)=>a*b)
}