JSBool JS_ValueToDouble()

Description	Extracts a function argument from a jsval, converts it to a double (if possible), and passes the converted value back to the caller.
Arguments	JSContext *cx, jsval v, double *dp
	cx is the opaque JSContext pointer that was passed to the JavaScript function. v is the jsval from which the string is to be extracted. dp is a pointer to an 8-byte double. This function stores the converted value in *dp.
Returns	A Boolean value indicating success (JS_TRUE) or failure (JS_FALSE).