validation for number in java script

function data_change(field)
{
var check = true;
var value = field.value; //get characters
//check that all characters are digits, ., -, or ""
for(var i=0;i < new_key =" value.charAt(i);"> "9")) &&
!(new_key == ""))
{
check = false;
break;
}
}
//apply appropriate colour based on value
if(!check)
{
field.style.backgroundColor = "red";
}
else
{
field.style.backgroundColor = "white";
}
}