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diff --git a/js/src/builtin/intl/DecimalNumber.h b/js/src/builtin/intl/DecimalNumber.h
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+++ b/js/src/builtin/intl/DecimalNumber.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_DecimalNumber_h
+#define builtin_intl_DecimalNumber_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "mozilla/Variant.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "jstypes.h"
+
+#include "js/TypeDecls.h"
+
+class JSLinearString;
+
+namespace JS {
+class JS_PUBLIC_API AutoCheckCannotGC;
+}
+
+namespace js::intl {
+
+/**
+ * Representation of a decimal number in normalized form.
+ *
+ * Examples of normalized forms:
+ * - "123" is normalized to "0.123e3".
+ * - "0.01e-4" is normalized to "0.1e-5".
+ * - "12.3" is normalized to "0.123e2".
+ *
+ * Note: Internally we leave the decimal point where it lies to avoid copying
+ * the string, but otherwise ignore it once we calculate the normalized
+ * exponent.
+ *
+ * TODO: Remove unused capabilities once there's a concrete PR for
+ * <https://github.com/tc39/proposal-intl-numberformat-v3/issues/98>.
+ */
+class MOZ_STACK_CLASS DecimalNumber final {
+  using Latin1String = mozilla::Span<const JS::Latin1Char>;
+  using TwoByteString = mozilla::Span<const char16_t>;
+
+  mozilla::Variant<Latin1String, TwoByteString> string_;
+
+  char charAt(size_t i) const {
+    if (string_.is<Latin1String>()) {
+      return static_cast<char>(string_.as<Latin1String>()[i]);
+    }
+    return static_cast<char>(string_.as<TwoByteString>()[i]);
+  }
+
+  // Decimal exponent. Valid range is (INT32_MIN, INT_MAX32].
+  int32_t exponent_ = 0;
+
+  // Start and end position of the significand.
+  size_t significandStart_ = 0;
+  size_t significandEnd_ = 0;
+
+  // Flag if the number is zero.
+  bool zero_ = false;
+
+  // Flag for negative numbers.
+  bool negative_ = false;
+
+  // Error flag when the exponent is too large.
+  bool exponentTooLarge_ = false;
+
+  template <typename CharT>
+  explicit DecimalNumber(mozilla::Span<const CharT> string) : string_(string) {}
+
+ public:
+  /** Return true if this decimal is zero. */
+  bool isZero() const { return zero_; }
+
+  /** Return true if this decimal is negative. */
+  bool isNegative() const { return negative_; }
+
+  /** Return true if the exponent is too large. */
+  bool exponentTooLarge() const { return exponentTooLarge_; }
+
+  /** Return the exponent of this decimal. */
+  int32_t exponent() const { return exponent_; }
+
+  // Exposed for testing.
+  size_t significandStart() const { return significandStart_; }
+  size_t significandEnd() const { return significandEnd_; }
+
+  /**
+   * Compare this decimal to another decimal. Returns a negative value if this
+   * decimal is smaller; zero if this decimal is equal; or a positive value if
+   * this decimal is larger than the input.
+   */
+  int32_t compareTo(const DecimalNumber& other) const;
+
+  /**
+   * Create a decimal number from the input. Returns |mozilla::Nothing| if the
+   * input can't be parsed.
+   */
+  template <typename CharT>
+  static mozilla::Maybe<DecimalNumber> from(mozilla::Span<const CharT> chars);
+
+  /**
+   * Create a decimal number from the input. Returns |mozilla::Nothing| if the
+   * input can't be parsed.
+   */
+  static mozilla::Maybe<DecimalNumber> from(JSLinearString* str,
+                                            JS::AutoCheckCannotGC& nogc);
+};
+}  // namespace js::intl
+
+#endif /* builtin_intl_DecimalNumber_h */