If they're meant to be separate values, try this:

var values = "554,20".split(",")
var v1 = parseFloat(values[0])
var v2 = parseFloat(values[1])

If they're meant to be a single value (like in French, where one-half is written 0,5)

var value = parseFloat("554,20".replace(",", "."));

Remove all non dot / digits:

var currency = "-$4,400.50";
var number = Number(currency.replace(/[^0-9.-]+/g,""));

Let string arithmetic do the string conversion, and then use parseFloat to get it back to a float:

var dA = 323423.23423423e4;
var sA = dA + '';
var dnA = parseFloat(sA);

Here's a fiddle to see it in action.

Note: All JavaScript numbers are doubles, so you don't need to draw a distinction between doubles and floats. Thus, the only place you'd need to worry about precision loss is in your first line. For example, if you did var dA = 987654321.0123456789 for your first line, it would be equivalent to var dA = 987654321.01234567, and dA would still equal dnA in the end.