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diff --git a/js/src/util/DoubleToString.h b/js/src/util/DoubleToString.h
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+++ b/js/src/util/DoubleToString.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 util_DoubleToString_h
+#define util_DoubleToString_h
+
+/*
+ * Public interface to portable double-precision floating point to string
+ * and back conversion package.
+ */
+
+struct DtoaState;
+
+namespace js {
+
+extern DtoaState* NewDtoaState();
+
+extern void DestroyDtoaState(DtoaState* state);
+
+}  // namespace js
+
+/* Maximum number of characters (including trailing null) that a DTOSTR_STANDARD
+ * or DTOSTR_STANDARD_EXPONENTIAL conversion can produce.  This maximum is
+ * reached for a number like -0.0000012345678901234567. */
+#define DTOSTR_STANDARD_BUFFER_SIZE 26
+
+/*
+ * DO NOT USE THIS FUNCTION IF YOU CAN AVOID IT.  js::NumberToCString() is a
+ * better function to use.
+ *
+ * Convert d to a string in the given base.  The integral part of d will be
+ * printed exactly in that base, regardless of how large it is, because there
+ * is no exponential notation for non-base-ten numbers.  The fractional part
+ * will be rounded to as few digits as possible while still preserving the
+ * round-trip property (analogous to that of printing decimal numbers).  In
+ * other words, if one were to read the resulting string in via a hypothetical
+ * base-number-reading routine that rounds to the nearest IEEE double (and to
+ * an even significand if there are two equally near doubles), then the result
+ * would equal d (except for -0.0, which converts to "0", and NaN, which is
+ * not equal to itself).
+ *
+ * Return nullptr if out of memory.  If the result is not nullptr, it must be
+ * released via js_free().
+ */
+char* js_dtobasestr(DtoaState* state, int base, double d);
+
+#endif /* util_DoubleToString_h */