Edit in JSFiddle

var x = 9.656;
var res1 = x.toPrecision(); // 9.656
var res2 = x.toPrecision(2); // 9.7
var res3 = x.toPrecision(4); // 9.656
var res4 = x.toPrecision(6); // 9.65600

//출력
document.write(res1 + "<br>");  
document.write(res2 + "<br>"); 
document.write(res3 + "<br>"); 
document.write(res4 + "<br>");