Check for out of range float values

Remove unsupported attribute `[maxlength]` from `input[number]`.

app/helpers/application_helper.rb

@@ -101,22 +101,14 @@ module ApplicationHelper
     end
 
     def number_field(method, options = {})
-      attr_type = object.type_for_attribute(method)
-      case attr_type.type
+      case object.type_for_attribute(method).type
       when :float, :double
-        options[:value] = object.public_send(method)&.to_scientific
-        options[:step] ||= :any
-        options[:min] ||= Float::MIN_15
-        options[:max] ||= Float::MAX_15
-        # Longest possible number (written not using exponent):
-        #   sign (1), leading 0 (1), dot (1), exponent 0s (307), digits (15).
-        # This is only upper bound, which cannot guarantee the number won't fall
-        # out of range.
-        # TODO: add `[pattern]` to limit precision and (possibly) replace `[maxlength]`?
-        # NOTE: `[pattern]` is unavailable on `input[type=number]` and `[min]/[max]` is
-        # unavailable on `input[type=text]`.
-        options[:maxlength] ||= 3 + Float::MIN_10_EXP + Float::DIG
+        options[:min] ||= Float::MIN_15.to_scientific
+        options[:max] ||= Float::MAX_15.to_scientific
+        options[:oninput] = 'numberValidate(event)'
         options[:size] ||= 6
+        options[:step] ||= :any
+        options[:value] = object.public_send("#{method}_before_type_cast")
       end
       super
     end

app/javascript/application.js

@@ -53,6 +53,27 @@ function formValidate(event) {
 }
 window.formValidate = formValidate
 
+// Pattern for number precision verification. Should accept all number formats
+// accepted by Kernel#Float. Lookahead checks the number of significant digits
+// in mantissa, following expression makes sure there is only one dot.
+const numberRegExp = /^\-?(?=(?:\.?0)*(?:\.?\d){1,15}?(?:\.?0)*\.?(?:[eE]|$))(?<mantissa>\d*\.?\d+|\d+\.)(?:[eE]\-?\d{1,3})?$/
+
+// Validation of input[type=number] value precision.
+function numberValidate(event) {
+  var input = event.target
+
+  input.setCustomValidity("")
+  if (!input.validity.valid) return
+
+  var match = numberRegExp.exec(input.value)
+  if (!match ||
+      (parseFloat(input.value) == 0.0 && match.groups['mantissa'].match(/[1-9]/))) {
+    // TODO: move message to locales
+    input.setCustomValidity("Please select a value within range of double precision floating-point format.")
+  }
+}
+window.numberValidate = numberValidate
+
 
 // Turbo stream actions.
 const FORM_CONTROLS = ["BUTTON", "FIELDSET", "INPUT", "OPTGROUP", "OPTION", "SELECT",