js实现开平方 Math.sqrt

function openSquare(n){

if(n<0) return NaN;
if(n===1 || n===0) return n;
var data = n,number;
do{
	number = data;
	data = (data+n/data)/2;
}
  while (Math.abs(data - number) >= Number.EPSILON)
return data

}