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diff --git a/js/public/BigInt.h b/js/public/BigInt.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+/* BigInt. */
+
+#ifndef js_BigInt_h
+#define js_BigInt_h
+
+#include "mozilla/Span.h"  // mozilla::Span
+
+#include <limits>       // std::numeric_limits
+#include <stdint.h>     // int64_t, uint64_t
+#include <type_traits>  // std::enable_if_t, std::{true,false}_type, std::is_{integral,signed,unsigned}_v
+
+#include "jstypes.h"  // JS_PUBLIC_API
+#include "js/TypeDecls.h"
+
+namespace mozilla {
+template <typename T>
+class Range;
+}
+
+namespace JS {
+
+class JS_PUBLIC_API BigInt;
+
+namespace detail {
+
+using Int64Limits = std::numeric_limits<int64_t>;
+using Uint64Limits = std::numeric_limits<uint64_t>;
+
+extern JS_PUBLIC_API BigInt* BigIntFromInt64(JSContext* cx, int64_t num);
+extern JS_PUBLIC_API BigInt* BigIntFromUint64(JSContext* cx, uint64_t num);
+extern JS_PUBLIC_API BigInt* BigIntFromBool(JSContext* cx, bool b);
+
+template <typename T, typename = void>
+struct NumberToBigIntConverter;
+
+template <typename SignedIntT>
+struct NumberToBigIntConverter<
+    SignedIntT,
+    std::enable_if_t<
+        std::is_integral_v<SignedIntT> && std::is_signed_v<SignedIntT> &&
+        Int64Limits::min() <= std::numeric_limits<SignedIntT>::min() &&
+        std::numeric_limits<SignedIntT>::max() <= Int64Limits::max()>> {
+  static BigInt* convert(JSContext* cx, SignedIntT num) {
+    return BigIntFromInt64(cx, num);
+  }
+};
+
+template <typename UnsignedIntT>
+struct NumberToBigIntConverter<
+    UnsignedIntT,
+    std::enable_if_t<
+        std::is_integral_v<UnsignedIntT> && std::is_unsigned_v<UnsignedIntT> &&
+        std::numeric_limits<UnsignedIntT>::max() <= Uint64Limits::max()>> {
+  static BigInt* convert(JSContext* cx, UnsignedIntT num) {
+    return BigIntFromUint64(cx, num);
+  }
+};
+
+template <>
+struct NumberToBigIntConverter<bool> {
+  static BigInt* convert(JSContext* cx, bool b) {
+    return BigIntFromBool(cx, b);
+  }
+};
+
+extern JS_PUBLIC_API bool BigIntIsInt64(BigInt* bi, int64_t* result);
+extern JS_PUBLIC_API bool BigIntIsUint64(BigInt* bi, uint64_t* result);
+
+template <typename T, typename = void>
+struct BigIntToNumberChecker;
+
+template <typename SignedIntT>
+struct BigIntToNumberChecker<
+    SignedIntT,
+    std::enable_if_t<
+        std::is_integral_v<SignedIntT> && std::is_signed_v<SignedIntT> &&
+        Int64Limits::min() <= std::numeric_limits<SignedIntT>::min() &&
+        std::numeric_limits<SignedIntT>::max() <= Int64Limits::max()>> {
+  using TypeLimits = std::numeric_limits<SignedIntT>;
+
+  static bool fits(BigInt* bi, SignedIntT* result) {
+    int64_t innerResult;
+    if (!BigIntIsInt64(bi, &innerResult)) {
+      return false;
+    }
+    if (TypeLimits::min() <= innerResult && innerResult <= TypeLimits::max()) {
+      *result = SignedIntT(innerResult);
+      return true;
+    }
+    return false;
+  }
+};
+
+template <typename UnsignedIntT>
+struct BigIntToNumberChecker<
+    UnsignedIntT,
+    std::enable_if_t<
+        std::is_integral_v<UnsignedIntT> && std::is_unsigned_v<UnsignedIntT> &&
+        std::numeric_limits<UnsignedIntT>::max() <= Uint64Limits::max()>> {
+  static bool fits(BigInt* bi, UnsignedIntT* result) {
+    uint64_t innerResult;
+    if (!BigIntIsUint64(bi, &innerResult)) {
+      return false;
+    }
+    if (innerResult <= std::numeric_limits<UnsignedIntT>::max()) {
+      *result = UnsignedIntT(innerResult);
+      return true;
+    }
+    return false;
+  }
+};
+
+}  // namespace detail
+
+/**
+ * Create a BigInt from an integer value. All integral types not larger than 64
+ * bits in size are supported.
+ */
+template <typename NumericT>
+static inline BigInt* NumberToBigInt(JSContext* cx, NumericT val) {
+  return detail::NumberToBigIntConverter<NumericT>::convert(cx, val);
+}
+
+/**
+ * Create a BigInt from a floating-point value. If the number isn't integral
+ * (that is, if it's NaN, an infinity, or contains a fractional component),
+ * this function returns null and throws an exception.
+ *
+ * Passing -0.0 will produce the bigint 0n.
+ */
+extern JS_PUBLIC_API BigInt* NumberToBigInt(JSContext* cx, double num);
+
+/**
+ * Create a BigInt by parsing a string using the ECMAScript StringToBigInt
+ * algorithm (https://tc39.es/ecma262/#sec-stringtobigint). Latin1 and two-byte
+ * character ranges are supported. It may be convenient to use
+ * JS::ConstLatin1Chars or JS::ConstTwoByteChars.
+ *
+ * (StringToBigInt performs parsing similar to that performed by the |Number|
+ * global function when passed a string, but it doesn't allow infinities,
+ * decimal points, or exponential notation, and neither algorithm allows numeric
+ * separators or an 'n' suffix character. This fast-and-loose description is
+ * offered purely as a convenience to the reader: see the specification
+ * algorithm for exact behavior.)
+ *
+ * If parsing fails, this function returns null and throws an exception.
+ */
+extern JS_PUBLIC_API BigInt* StringToBigInt(
+    JSContext* cx, mozilla::Range<const Latin1Char> chars);
+
+extern JS_PUBLIC_API BigInt* StringToBigInt(
+    JSContext* cx, mozilla::Range<const char16_t> chars);
+
+/**
+ * Create a BigInt by parsing a string consisting of an optional sign character
+ * followed by one or more alphanumeric ASCII digits in the provided radix.
+ *
+ * If the radix is not in the range [2, 36], or the string fails to parse, this
+ * function returns null and throws an exception.
+ */
+extern JS_PUBLIC_API BigInt* SimpleStringToBigInt(
+    JSContext* cx, mozilla::Span<const char> chars, uint8_t radix);
+
+/**
+ * Convert a JS::Value to a BigInt using the ECMAScript ToBigInt algorithm
+ * (https://tc39.es/ecma262/#sec-tobigint).
+ *
+ * (Note in particular that this will throw if passed a value whose type is
+ * 'number'. To convert a number to a BigInt, use one of the overloads of
+ * JS::NumberToBigInt().)
+ */
+extern JS_PUBLIC_API BigInt* ToBigInt(JSContext* cx, Handle<Value> val);
+
+/**
+ * Convert the given BigInt, modulo 2**64, to a signed 64-bit integer.
+ */
+extern JS_PUBLIC_API int64_t ToBigInt64(BigInt* bi);
+
+/**
+ * Convert the given BigInt, modulo 2**64, to an unsigned 64-bit integer.
+ */
+extern JS_PUBLIC_API uint64_t ToBigUint64(BigInt* bi);
+
+/**
+ * Convert the given BigInt to a Number value as if calling the Number
+ * constructor on it
+ * (https://tc39.es/ecma262/#sec-number-constructor-number-value). The value
+ * may be rounded if it doesn't fit without loss of precision.
+ */
+extern JS_PUBLIC_API double BigIntToNumber(BigInt* bi);
+
+/**
+ * Return true if the given BigInt is negative.
+ */
+extern JS_PUBLIC_API bool BigIntIsNegative(BigInt* bi);
+
+/**
+ * Return true if the given BigInt fits inside the given NumericT type without
+ * loss of precision, and store the value in the out parameter. Otherwise return
+ * false and leave the value of the out parameter unspecified.
+ */
+template <typename NumericT>
+static inline bool BigIntFits(BigInt* bi, NumericT* out) {
+  return detail::BigIntToNumberChecker<NumericT>::fits(bi, out);
+}
+
+/**
+ * Same as BigIntFits(), but checks if the value fits inside a JS Number value.
+ */
+extern JS_PUBLIC_API bool BigIntFitsNumber(BigInt* bi, double* out);
+
+/**
+ * Convert the given BigInt to a String value as if toString() were called on
+ * it.
+ *
+ * If the radix is not in the range [2, 36], then this function returns null and
+ * throws an exception.
+ */
+extern JS_PUBLIC_API JSString* BigIntToString(JSContext* cx, Handle<BigInt*> bi,
+                                              uint8_t radix);
+
+}  // namespace JS
+
+#endif /* js_BigInt_h */