diff --git a/src/clp_ffi_js/constants.hpp b/src/clp_ffi_js/constants.hpp
index 9bf336a9..045227da 100644
--- a/src/clp_ffi_js/constants.hpp
+++ b/src/clp_ffi_js/constants.hpp
@@ -2,14 +2,31 @@
 #define CLP_FFI_JS_CONSTANTS_HPP
 
 #include <array>
-#include <cstddef>
+#include <cstdint>
 #include <string_view>
 
 namespace clp_ffi_js {
-constexpr size_t cLogLevelNone{0};
-constexpr size_t cValidLogLevelsBeginIdx{cLogLevelNone + 1};
+/**
+ * Enum of known log levels.
+ */
+enum class LogLevel : std::uint8_t {
+    NONE = 0,  // This isn't a valid log level.
+    TRACE,
+    DEBUG,
+    INFO,
+    WARN,
+    ERROR,
+    FATAL,
+};
+constexpr LogLevel cValidLogLevelsBeginIdx{LogLevel::TRACE};
+
+/**
+ * Strings corresponding to `LogLevel`.
+ *
+ * NOTE: These must be kept in sync manually.
+ */
 constexpr std::array<std::string_view, 7> cLogLevelNames{
-        "NONE",  // This should not be used.
+        "NONE",  // This isn't a valid log level.
         "TRACE",
         "DEBUG",
         "INFO",
diff --git a/src/clp_ffi_js/ir/LogEventWithLevel.hpp b/src/clp_ffi_js/ir/LogEventWithLevel.hpp
new file mode 100644
index 00000000..22a00af3
--- /dev/null
+++ b/src/clp_ffi_js/ir/LogEventWithLevel.hpp
@@ -0,0 +1,41 @@
+#ifndef CLP_FFI_JS_IR_LOGEVENTWITHLEVEL_HPP
+#define CLP_FFI_JS_IR_LOGEVENTWITHLEVEL_HPP
+
+#include <utility>
+
+#include <clp/ir/EncodedTextAst.hpp>
+#include <clp/ir/LogEvent.hpp>
+#include <clp/ir/types.hpp>
+#include <clp/time_types.hpp>
+
+#include <clp_ffi_js/constants.hpp>
+
+namespace clp_ffi_js::ir {
+/**
+ * A class derived from `clp::ir::LogEvent` with an additional member for the log level.
+ *
+ * NOTE: Once we move to the next IR format, this class should no longer be necessary since each
+ * IR log event will contain a set of key-value pairs, one of which should be the log level.
+ * @tparam encoded_variable_t The type of encoded variables in the event
+ */
+template <typename encoded_variable_t>
+class LogEventWithLevel : public clp::ir::LogEvent<encoded_variable_t> {
+public:
+    // Constructors
+    LogEventWithLevel(
+            clp::ir::epoch_time_ms_t timestamp,
+            clp::UtcOffset utc_offset,
+            clp::ir::EncodedTextAst<encoded_variable_t> message,
+            LogLevel log_level
+    )
+            : clp::ir::LogEvent<encoded_variable_t>{timestamp, utc_offset, std::move(message)},
+              m_log_level{log_level} {}
+
+    [[nodiscard]] auto get_log_level() const -> LogLevel { return m_log_level; }
+
+private:
+    LogLevel m_log_level;
+};
+}  // namespace clp_ffi_js::ir
+
+#endif  // CLP_FFI_JS_IR_LOGEVENTWITHLEVEL_HPP
diff --git a/src/clp_ffi_js/ir/StreamReader.cpp b/src/clp_ffi_js/ir/StreamReader.cpp
index b5682a69..b9c86b6b 100644
--- a/src/clp_ffi_js/ir/StreamReader.cpp
+++ b/src/clp_ffi_js/ir/StreamReader.cpp
@@ -5,27 +5,29 @@
 #include <cstdint>
 #include <iterator>
 #include <memory>
-#include <span>
+#include <optional>
 #include <string>
 #include <string_view>
 #include <system_error>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
 #include <clp/Array.hpp>
 #include <clp/ErrorCode.hpp>
 #include <clp/ffi/ir_stream/decoding_methods.hpp>
-#include <clp/ir/LogEvent.hpp>
 #include <clp/ir/LogEventDeserializer.hpp>
 #include <clp/ir/types.hpp>
 #include <clp/streaming_compression/zstd/Decompressor.hpp>
 #include <clp/TraceableException.hpp>
+#include <clp/type_utils.hpp>
 #include <emscripten/em_asm.h>
 #include <emscripten/val.h>
 #include <spdlog/spdlog.h>
 
 #include <clp_ffi_js/ClpFfiJsException.hpp>
 #include <clp_ffi_js/constants.hpp>
+#include <clp_ffi_js/ir/LogEventWithLevel.hpp>
 #include <clp_ffi_js/ir/StreamReaderDataContext.hpp>
 
 using namespace std::literals::string_literals;
@@ -99,25 +101,47 @@ auto StreamReader::get_num_events_buffered() const -> size_t {
     return m_encoded_log_events.size();
 }
 
-auto StreamReader::deserialize_range(size_t begin_idx, size_t end_idx) -> size_t {
-    constexpr size_t cFullRangeEndIdx{0};
-    if (0 != begin_idx || cFullRangeEndIdx != end_idx) {
-        throw ClpFfiJsException{
-                clp::ErrorCode::ErrorCode_Unsupported,
-                __FILENAME__,
-                __LINE__,
-                "Partial range deserialization is not yet supported."
-        };
+auto StreamReader::get_filtered_log_event_map() const -> FilteredLogEventMapTsType {
+    if (false == m_filtered_log_event_map.has_value()) {
+        return FilteredLogEventMapTsType{emscripten::val::null()};
     }
-    if (nullptr != m_stream_reader_data_context) {
-        constexpr size_t cDefaultNumReservedLogEvents{500'000};
-        m_encoded_log_events.reserve(cDefaultNumReservedLogEvents);
-        while (true) {
-            auto result{m_stream_reader_data_context->get_deserializer().deserialize_log_event()};
-            if (false == result.has_error()) {
-                m_encoded_log_events.emplace_back(std::move(result.value()));
-                continue;
-            }
+
+    return FilteredLogEventMapTsType{emscripten::val::array(m_filtered_log_event_map.value())};
+}
+
+void StreamReader::filter_log_events(emscripten::val const& log_level_filter) {
+    if (log_level_filter.isNull()) {
+        m_filtered_log_event_map.reset();
+        return;
+    }
+
+    m_filtered_log_event_map.emplace();
+    auto filter_levels{emscripten::vecFromJSArray<std::underlying_type_t<LogLevel>>(log_level_filter
+    )};
+    for (size_t log_event_idx = 0; log_event_idx < m_encoded_log_events.size(); ++log_event_idx) {
+        auto const& log_event = m_encoded_log_events[log_event_idx];
+        if (std::ranges::find(
+                    filter_levels,
+                    clp::enum_to_underlying_type(log_event.get_log_level())
+            )
+            != filter_levels.end())
+        {
+            m_filtered_log_event_map->emplace_back(log_event_idx);
+        }
+    }
+}
+
+auto StreamReader::deserialize_stream() -> size_t {
+    if (nullptr == m_stream_reader_data_context) {
+        return m_encoded_log_events.size();
+    }
+
+    constexpr size_t cDefaultNumReservedLogEvents{500'000};
+    m_encoded_log_events.reserve(cDefaultNumReservedLogEvents);
+
+    while (true) {
+        auto result{m_stream_reader_data_context->get_deserializer().deserialize_log_event()};
+        if (result.has_error()) {
             auto const error{result.error()};
             if (std::errc::no_message_available == error) {
                 break;
@@ -133,51 +157,76 @@ auto StreamReader::deserialize_range(size_t begin_idx, size_t end_idx) -> size_t
                     "Failed to deserialize: "s + error.category().name() + ":" + error.message()
             };
         }
-        m_stream_reader_data_context.reset(nullptr);
-    }
+        auto const& log_event = result.value();
+        auto const& message = log_event.get_message();
 
+        auto const& logtype = message.get_logtype();
+        constexpr size_t cLogLevelPositionInMessages{1};
+        LogLevel log_level{LogLevel::NONE};
+        if (logtype.length() > cLogLevelPositionInMessages) {
+            // NOLINTNEXTLINE(readability-qualified-auto)
+            auto const log_level_name_it{std::find_if(
+                    cLogLevelNames.begin() + static_cast<size_t>(cValidLogLevelsBeginIdx),
+                    cLogLevelNames.end(),
+                    [&](std::string_view level) {
+                        return logtype.substr(cLogLevelPositionInMessages).starts_with(level);
+                    }
+            )};
+            if (log_level_name_it != cLogLevelNames.end()) {
+                log_level = static_cast<LogLevel>(
+                        std::distance(cLogLevelNames.begin(), log_level_name_it)
+                );
+            }
+        }
+
+        auto log_viewer_event{LogEventWithLevel<four_byte_encoded_variable_t>(
+                log_event.get_timestamp(),
+                log_event.get_utc_offset(),
+                message,
+                log_level
+        )};
+        m_encoded_log_events.emplace_back(std::move(log_viewer_event));
+    }
+    m_stream_reader_data_context.reset(nullptr);
     return m_encoded_log_events.size();
 }
 
-auto StreamReader::decode_range(size_t begin_idx, size_t end_idx) const -> DecodedResultsTsType {
-    if (m_encoded_log_events.size() < end_idx || begin_idx >= end_idx) {
-        return DecodedResultsTsType(emscripten::val::null());
+auto StreamReader::decode_range(size_t begin_idx, size_t end_idx, bool use_filter) const
+        -> DecodedResultsTsType {
+    if (use_filter && false == m_filtered_log_event_map.has_value()) {
+        return DecodedResultsTsType{emscripten::val::null()};
+    }
+
+    size_t length{0};
+    if (use_filter) {
+        length = m_filtered_log_event_map->size();
+    } else {
+        length = m_encoded_log_events.size();
+    }
+    if (length < end_idx || begin_idx > end_idx) {
+        return DecodedResultsTsType{emscripten::val::null()};
     }
 
-    std::span const log_events_span{
-            m_encoded_log_events.begin()
-                    + static_cast<decltype(m_encoded_log_events)::difference_type>(begin_idx),
-            m_encoded_log_events.begin()
-                    + static_cast<decltype(m_encoded_log_events)::difference_type>(end_idx)
-    };
     std::string message;
     constexpr size_t cDefaultReservedMessageLength{512};
     message.reserve(cDefaultReservedMessageLength);
-    size_t log_num{begin_idx + 1};
     auto const results{emscripten::val::array()};
-    for (auto const& log_event : log_events_span) {
-        message.clear();
+
+    for (size_t i = begin_idx; i < end_idx; ++i) {
+        size_t log_event_idx{0};
+        if (use_filter) {
+            log_event_idx = m_filtered_log_event_map->at(i);
+        } else {
+            log_event_idx = i;
+        }
+        auto const& log_event{m_encoded_log_events[log_event_idx]};
 
         auto const parsed{log_event.get_message().decode_and_unparse()};
         if (false == parsed.has_value()) {
             SPDLOG_ERROR("Failed to decode message.");
             break;
         }
-        message.append(parsed.value());
-
-        constexpr size_t cLogLevelPositionInMessages{1};
-        size_t log_level{cLogLevelNone};
-        // NOLINTNEXTLINE(readability-qualified-auto)
-        auto const log_level_name_it{std::find_if(
-                cLogLevelNames.begin() + cValidLogLevelsBeginIdx,
-                cLogLevelNames.end(),
-                [&](std::string_view level) {
-                    return message.substr(cLogLevelPositionInMessages).starts_with(level);
-                }
-        )};
-        if (log_level_name_it != cLogLevelNames.end()) {
-            log_level = std::distance(cLogLevelNames.begin(), log_level_name_it);
-        }
+        message = parsed.value();
 
         m_ts_pattern.insert_formatted_timestamp(log_event.get_timestamp(), message);
 
@@ -186,10 +235,9 @@ auto StreamReader::decode_range(size_t begin_idx, size_t end_idx) const -> Decod
                 results.as_handle(),
                 message.c_str(),
                 log_event.get_timestamp(),
-                log_level,
-                log_num
+                log_event.get_log_level(),
+                log_event_idx + 1
         );
-        ++log_num;
     }
 
     return DecodedResultsTsType(results);
@@ -211,6 +259,8 @@ EMSCRIPTEN_BINDINGS(ClpIrStreamReader) {
     emscripten::register_type<clp_ffi_js::ir::DecodedResultsTsType>(
             "Array<[string, number, number, number]>"
     );
+    emscripten::register_type<clp_ffi_js::ir::FilteredLogEventMapTsType>("number[] | null");
+
     emscripten::class_<clp_ffi_js::ir::StreamReader>("ClpIrStreamReader")
             .constructor(
                     &clp_ffi_js::ir::StreamReader::create,
@@ -220,7 +270,12 @@ EMSCRIPTEN_BINDINGS(ClpIrStreamReader) {
                     "getNumEventsBuffered",
                     &clp_ffi_js::ir::StreamReader::get_num_events_buffered
             )
-            .function("deserializeRange", &clp_ffi_js::ir::StreamReader::deserialize_range)
+            .function(
+                    "getFilteredLogEventMap",
+                    &clp_ffi_js::ir::StreamReader::get_filtered_log_event_map
+            )
+            .function("filterLogEvents", &clp_ffi_js::ir::StreamReader::filter_log_events)
+            .function("deserializeStream", &clp_ffi_js::ir::StreamReader::deserialize_stream)
             .function("decodeRange", &clp_ffi_js::ir::StreamReader::decode_range);
 }
 }  // namespace
diff --git a/src/clp_ffi_js/ir/StreamReader.hpp b/src/clp_ffi_js/ir/StreamReader.hpp
index 15feda71..dec6c360 100644
--- a/src/clp_ffi_js/ir/StreamReader.hpp
+++ b/src/clp_ffi_js/ir/StreamReader.hpp
@@ -3,19 +3,21 @@
 
 #include <cstddef>
 #include <memory>
+#include <optional>
 #include <vector>
 
-#include <clp/ir/LogEvent.hpp>
 #include <clp/ir/types.hpp>
 #include <clp/TimestampPattern.hpp>
 #include <emscripten/bind.h>
 #include <emscripten/val.h>
 
+#include <clp_ffi_js/ir/LogEventWithLevel.hpp>
 #include <clp_ffi_js/ir/StreamReaderDataContext.hpp>
 
 namespace clp_ffi_js::ir {
 EMSCRIPTEN_DECLARE_VAL_TYPE(DataArrayTsType);
 EMSCRIPTEN_DECLARE_VAL_TYPE(DecodedResultsTsType);
+EMSCRIPTEN_DECLARE_VAL_TYPE(FilteredLogEventMapTsType);
 
 /**
  * Class to deserialize and decode Zstandard-compressed CLP IR streams as well as format decoded
@@ -23,6 +25,12 @@ EMSCRIPTEN_DECLARE_VAL_TYPE(DecodedResultsTsType);
  */
 class StreamReader {
 public:
+    /**
+     * Mapping between an index in the filtered log events collection to an index in the unfiltered
+     * log events collection.
+     */
+    using FilteredLogEventsMap = std::optional<std::vector<size_t>>;
+
     /**
      * Creates a StreamReader to read from the given array.
      *
@@ -50,32 +58,42 @@ class StreamReader {
     [[nodiscard]] auto get_num_events_buffered() const -> size_t;
 
     /**
-     * Deserializes and buffers log events in the range `[beginIdx, endIdx)`. After the stream has
-     * been exhausted, it will be deallocated.
+     * @return The filtered log events map.
+     */
+    [[nodiscard]] auto get_filtered_log_event_map() const -> FilteredLogEventMapTsType;
+
+    /**
+     * Generates a filtered collection from all log events.
      *
-     * NOTE: Currently, this class only supports deserializing the full range of log events in the
-     * stream.
+     * @param log_level_filter Array of selected log levels
+     */
+    void filter_log_events(emscripten::val const& log_level_filter);
+
+    /**
+     * Deserializes all log events in the stream. After the stream has been exhausted, it will be
+     * deallocated.
      *
-     * @param begin_idx
-     * @param end_idx
      * @return The number of successfully deserialized ("valid") log events.
      */
-    [[nodiscard]] auto deserialize_range(size_t begin_idx, size_t end_idx) -> size_t;
+    [[nodiscard]] auto deserialize_stream() -> size_t;
 
     /**
-     * Decodes the deserialized log events in the range `[beginIdx, endIdx)`.
+     * Decodes log events in the range `[beginIdx, endIdx)` of the filtered or unfiltered
+     * (depending on the value of `useFilter`) log events collection.
      *
      * @param begin_idx
      * @param end_idx
+     * @param use_filter Whether to decode from the filtered or unfiltered log events collection.
      * @return An array where each element is a decoded log event represented by an array of:
      * - The log event's message
      * - The log event's timestamp as milliseconds since the Unix epoch
      * - The log event's log level as an integer that indexes into `cLogLevelNames`
      * - The log event's number (1-indexed) in the stream
-     * @return null if any log event in the range doesn't exist (e.g., the range exceeds the number
-     * of log events in the file).
+     * @return null if any log event in the range doesn't exist (e.g. the range exceeds the number
+     * of log events in the collection).
      */
-    [[nodiscard]] auto decode_range(size_t begin_idx, size_t end_idx) const -> DecodedResultsTsType;
+    [[nodiscard]] auto
+    decode_range(size_t begin_idx, size_t end_idx, bool use_filter) const -> DecodedResultsTsType;
 
 private:
     // Constructor
@@ -83,9 +101,10 @@ class StreamReader {
                                   stream_reader_data_context);
 
     // Variables
-    std::vector<clp::ir::LogEvent<clp::ir::four_byte_encoded_variable_t>> m_encoded_log_events;
+    std::vector<LogEventWithLevel<clp::ir::four_byte_encoded_variable_t>> m_encoded_log_events;
     std::unique_ptr<StreamReaderDataContext<clp::ir::four_byte_encoded_variable_t>>
             m_stream_reader_data_context;
+    FilteredLogEventsMap m_filtered_log_event_map;
     clp::TimestampPattern m_ts_pattern;
 };
 }  // namespace clp_ffi_js::ir