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Dart2js Info

This package contains libraries and tools you can use to process info files produced when running dart2js with --dump-info.

The info files contain data about each element included in the output of your program. The data includes information such as:

  • the size that each function adds to the .dart.js output,
  • dependencies between functions,
  • how the code is clustered when using deferred libraries, and
  • the declared and inferred type of each function argument.

All of this information can help you understand why some piece of code is included in your compiled application, and how far was dart2js able to understand your code. This data can help you make changes to improve the quality and size of your framework or app.

This package focuses on gathering libraries and tools that summarize all of that information. Bear in mind that even with all these tools, it is not trivial to isolate code-size issues. We just hope that these tools make things a bit easier.


Currently, most tools available here can be used to analyze code-size and attribution of code-size to different parts of your app. With time, we hope to add more data to the info files, and include better tools to help understand the results of type inference.

This package is still in flux and we might make breaking changes at any time. Our current goal is not to provide a stable API, we mainly want to expose the functionality and iterate on it. We recommend that you pin a specific version of this package and update when needed.


All tools are provided as commands of a single command-line interface. To install:

pub global activate dart2js_info

To run a tool, then run:

dart2js_info <command> [arguments]

There is a short help available on the tool, and more details are provided below.


Dart2js info files are produced in either a binary or JSON format.

Info API

This package also exposes libraries to parse and represent the information from the info files. If there is data that is stored in the info files but not exposed by one of our tools, you may be able to use the info APIs to quickly put together your own tool.

AllInfo exposes a Dart representation of all of the collected information. There are deserialization libraries in this package to decode any info file produced by the dart2js --dump-info option. See lib/binary_serialization.dart and lib/json_info_codec.dart to find the binary and JSON decoders respectively. For convenience, package:dart2js_info/src/io.dart also exposes a helper method that can choose, depending on the extension of the info file, whether to deserialize it using the binary or JSON decoder. For example:

import 'dart:convert';
import 'dart:io';

import 'package:dart2js_info/info.dart';
import 'package:dart2js_info/src/io.dart';

main(args) async {
  var infoPath = args[0];
  var info = await infoFromFile(infoPath);

Available tools

The following tools are a available today:

  • code_deps: simple tool that can answer queries about the dependency between functions and fields in your program. Currently it only supports the some_path query, which shows a dependency path from one function to another.

  • common: a tool that reports the common elements of two info files. Commonality is determined by the element's name, file path, and URI but not code size.

  • diff: a tool that diffs two info files and reports which program elements have been added, removed, or changed size. This also tells which elements are no longer deferred or have become deferred.

  • library_size: a tool that shows how much code was attributed to each library. This tool is configurable so it can group data in many ways (e.g. to tally together all libraries that belong to a package, or all libraries that match certain name pattern).

  • deferred_check: a tool that verifies that code was split into deferred parts as expected. This tool takes a specification of the expected layout of code into deferred parts, and checks that the output from dart2js meets the specification.

  • deferred_size: a tool that gives a breakdown of the sizes of the deferred parts of the program. This can show how much of your total code size can be loaded deferred.

  • deferred_layout: a tool that reports which code is included on each output unit.

  • function_size: a tool that shows how much code was attributed to each function. This tool also uses dependency information to compute dominance and reachability data. This information can sometimes help determine how much savings could come if the function was not included in the program.

  • coverage_server and coverage_analysis: dart2js has an experimental feature to gather coverage data of your application. The coverage_log_server can record this data, and live_code_size_analysis can correlate that with the info file, so you determine why code that is not used is being included in your app.

  • convert: a tool that converts info files from one format to another. Accepted inputs are JSON or the internal binary form, outputs can be JSON, backward-compatible JSON, binary, or protobuf schema (as defined in info.proto).

  • runtime_coverage: dart2js has an experimental feature to gather runtime coverage data of your application. This tool correlates that with the info file and can output a package-level breakdown of which files were not used during the runtime of your app.

  • show: a tool that dumps info files in a readable text format.

Next we describe in detail how to use each of these tools.

Code deps tool

This command-line tool can be used to query for code dependencies. Currently this tool only supports the some_path query, which gives you the shortest path for how one function depends on another.

Run this tool as follows:

# activate is only needed once to install the dart2js_info tool
$ pub global activate dart2js_info
$ dart2js_info code_deps some_path main foo

The arguments to the query are regular expressions that can be used to select a single element in your program. If your regular expression is too general and has more than one match, this tool will pick the first match and ignore the rest. Regular expressions are matched against a fully qualified element name, which includes the library and class name (if any) that contains it. A typical qualified name is of this form:


If the name of a function your are looking for is unique enough, it might be sufficient to just write that name as your regular expression.

Common tool

This command-line tool shows common elements between two info files. It can be run as follows:

$ pub global activate dart2js_info # only needed once
$ dart2js_info common

The tool gives a breakdown of the common elements between the two info files, reporting code size discrepancies if they exist. Here's an example output snippet:

COMMON ELEMENTS (455 common elements, 70334 bytes -> 70460 bytes)
dart:_foreign_helper::: 141 bytes
dart:_foreign_helper::JS_CONST: 141 bytes
dart:_foreign_helper::JS_CONST.code: 0 bytes
dart:_interceptors::: 4052 -> 7968 bytes
dart:_interceptors::ArrayIterator: 805 bytes
dart:_interceptors::ArrayIterator.ArrayIterator: 0 bytes
dart:_interceptors::ArrayIterator._current: 91 bytes
dart:_interceptors::ArrayIterator._index: 0 bytes
dart:_interceptors::ArrayIterator._iterable: 0 bytes
dart:_interceptors::ArrayIterator._length: 0 bytes
dart:_interceptors::ArrayIterator.current: 0 bytes
dart:_interceptors::ArrayIterator.moveNext: 406 bytes
dart:_interceptors::Interceptor: 198 bytes
dart:_interceptors::Interceptor.toString: 104 -> 182 bytes

Common elements are sorted by name by default but can be sorted by size with the --order-by-size flag. Additionally, the tool can be restricted to just packages with the --packages-only flag.

Diff tool

This command-line tool shows a diff between two info files. It can be run as follows:

$ pub global activate dart2js_info # only needed once
$ dart2js_info diff [--summary]

The tool gives a breakdown of the difference between the two info files. Here's an example output:

total_size_difference -2688
total_added 0
total_removed 2321
total_size_changed -203
total_became_deferred 0
total_no_longer_deferred 0

ADDED (0 bytes)

REMOVED (2321 bytes)
dart:_js_helper::getRuntimeTypeString: 488 bytes
dart:_js_helper::substitute: 479 bytes
dart:_js_helper::TypeImpl.toString: 421 bytes
dart:_js_helper::computeSignature: 204 bytes
dart:_js_helper::getRuntimeTypeArguments: 181 bytes
dart:_js_helper::extractFunctionTypeObjectFrom: 171 bytes
dart:_js_helper::getTypeArgumentByIndex: 147 bytes
dart:_js_helper::runtimeTypeToString: 136 bytes
dart:_js_helper::setRuntimeTypeInfo: 94 bytes
dart:core::Object.runtimeType: 0 bytes
dart:_js_helper::getRawRuntimeType: 0 bytes
dart:_js_helper::invoke: 0 bytes
dart:_js_helper::invokeOn: 0 bytes
dart:_js_helper::getField: 0 bytes
dart:_js_helper::getClassName: 0 bytes
dart:_js_helper::getRuntimeType: 0 bytes
dart:_js_helper::TypeImpl.TypeImpl: 0 bytes

CHANGED SIZE (-203 bytes)
dart:_interceptors::JSUnmodifiableArray: -3 bytes
dart:core::List: -3 bytes
dart:_interceptors::ArrayIterator: -4 bytes
dart:_js_helper::TypeImpl._typeName: -10 bytes
dart:_js_helper::TypeImpl._unmangledName: -15 bytes
dart:_js_names::: -30 bytes
dart:_js_names::extractKeys: -30 bytes
dart:core::StringBuffer: -40 bytes
dart:core::StringBuffer._writeAll: -40 bytes
dart:core::: -43 bytes
dart:_interceptors::JSArray.+: -63 bytes
dart:_interceptors::JSArray: -66 bytes
dart:_interceptors::: -73 bytes
dart:_js_helper::TypeImpl: -481 bytes
dart:_js_helper::: -2445 bytes



You can also pass --summary to only show the summary section.

Library size split tool

This command-line tool shows the size distribution of generated code among libraries. It can be run as follows:

$ pub global activate dart2js_info # only needed once
$ dart2js_info library_size

Libraries can be grouped using regular expressions. You can specify what regular expressions to use by providing a grouping.yaml file with the --grouping flag:

$ dart2js_info library_size --grouping grouping.yaml

The format of the grouping.yaml file is as follows:

- { regexp: "package:(foo)/*.dart", name: "group name 1", cluster: 2}
- { regexp: "dart:.*",              name: "group name 2", cluster: 3}

The file should include a single key groups containing a list of group specifications. Each group is specified by a map of 3 entries:

  • regexp (required): a regexp used to match entries that belong to the group.

  • name (optional): the name given to this group in the output table. If omitted, the name is derived from the regexp as the match's group(1) or group(0) if no group was defined. When names are omitted the group specification implicitly defines several groups, one per observed name.

  • cluster (optional): a clustering index for how data is shown in a table. Groups with higher cluster indices are shown later in the table after a dividing line. If missing, the cluster index defaults to 0.

Here is an example configuration, with comments about what each entry does:

# This group shows the total size for all libraries that were loaded from
# file:// urls, it is shown in cluster #2, which happens to be the last
# cluster in this example before the totals are shown:
- name: "Loose files"
  regexp: "file://.*"
  cluster: 2

# This group shows the total size of all code loaded from packages:
- { name: "All packages", regexp: "package:.*", cluster: 2}

# This group shows the total size of all code loaded from core libraries:
- { name: "Core libs", regexp: "dart:.*", cluster: 2}

# This group shows the total size of all libraries in a single package. Here
# we omitted the `name` entry, instead we extract it from the regexp
# directly.  In this case, the name will be the package-name portion of the
# package-url (determined by group(1) of the regexp).
- { regexp: "package:([^/]*)", cluster: 1}

# The next two groups match the entire library url as the name of the group.
- regexp: "package:.*"
- regexp: "dart:.*"

# If your code lives under /my/project/dir, this will match any file loaded
from a file:// url, and we use as a name the relative path to it.
- regexp: "file:///my/project/dir/(.*)"

Regardless of the grouping configuration, the tool will display the total code size attributed of all libraries, constants, and the program size.

Note: eventually you should expect all numbers to add up to the program size. Currently dart2js's --dump-info is not complete, so numbers for bootstrapping code and lazy static initializers are missing.

Deferred library verification

This tool checks that the output from dart2js meets a given specification, given in a YAML file. It can be run as follows:

$ pub global activate dart2js_info # only needed once
$ dart2js_info deferred_check manifest.yaml

The format of the YAML file is:

    - some_package
    - other_package
    - some_other_package

    - foo
    - bar

    - baz
    - quux
    - zardoz

The YAML file consists of a list of declarations, one for each deferred part expected in the output. At least one of these parts must be named “main”; this is the main part that contains the program entrypoint. Each top-level part contains a list of package names that are expected to be contained in that part, a list of package names that are expected to be in another part, or both. For instance, in the example YAML above the part named “baz” is expected to contain the packages “baz” and “quux” and exclude the package “zardoz”.

The names for parts given in the specification YAML file (besides “main”) are the same as the name given to the deferred import in the dart file. For instance, if you have import 'package:foo/bar.dart' deferred as baz; in your dart file, then the corresponding name in the specification file is ‘baz’.

Deferred library size tool

This tool gives a breakdown of all of the deferred code in the program by size. It can show how much of the total code size is deferred. It can be run as follows:

pub global activate dart2js_info # only needed once
dart2js_info deferred_size

The tool will output a table listing all of the deferred imports in the program as well as the “main” chunk, which is not deferred. The output looks like:

Size by library
main                                    12345678
foo                                      7654321
bar                                      1234567
Main chunk size                         12345678
Deferred code size                       8888888
Percent of code deferred                  41.86%

Deferred library layout tool

This tool reports which code is included in each output unit. It can be run as follows:

$ pub global activate dart2js_info # only needed once
$ dart2js_info deferred_layout

The tool will output a table listing all of the deferred output units or chunks, for each unit it will list the set of libraries that contribute code to this unit. If a library contributes to more than one output unit, the tool lists which elements are in one or another output unit. For example, the output might look like this:

Output unit main:
  loaded by default
     - hello_world.dart
     - dart:core

Output unit 2:
  loaded by importing: [b]
     - c.dart:
       - function d
     - b.dart

Output unit 1:
  loaded by importing: [a]
     - c.dart:
       - function c
     - a.dart

In this example, all the code of b.dart after tree-shaking was included in the output unit 2, but c.dart was split between output unit 1 and output unit 2.

Function size analysis tool

This command-line tool presents how much each function contributes to the total code of your application. We use dependency information to compute dominance and reachability data as well.

When you run:

$ pub global activate dart2js_info # only needed once
$ dart2js_info function_size

the tool produces a table output with lots of entries. Here is an example entry with the corresponding table header:

 --- Results per element (field or function) ---
    element size     dominated size     reachable size Element identifier
     275   0.01%     283426  13.97%    1506543  74.28%

Such entry means that the function myMethodName uses 275 bytes, which is 0.01% of the application. That function however calls other functions, which transitively can include up to 74.28% of the application size. Of all those reachable functions, some of them are reachable from other parts of the program, but a subset are dominated by myMethodName, that is, other parts of the program starting from main would first go through myMethodName before reaching those functions. In this example, that subset is 13.97% of the application size. This means that if you somehow can remove your dependency on myMethodName, you will save at least that 13.97%, and possibly some more from the reachable size, but how much of that we are not certain.

Coverage Server Analysis

Coverage information requires a bit more setup and work to get them running. The steps are as follows:

  • Compile an app with dart2js using --dump-info and --experiment-call-instrumentation
$ dart2js --dump-info --experiment-call-instrumentation main.dart

The flag only works dart2js version 2.2.0 or newer.

  • Launch the coverage server tool to serve up the JS code of your app:
$ dart2js_info coverage_server main.dart.js
  • (optional) If you have a complex application setup, you may need to serve an html file or integrate your application server to proxy to the log server any GET request for the .dart.js file and /coverage POST requests that send coverage data.

  • Load your app and use it to exercise the entire code.

  • Shut down the coverage server (Ctrl-C). This will emit a file named mail.dart.js.coverage.json

  • Finally, run the live code analysis tool given it both the info and coverage json files:

$ dart2js_info coverage_analysis main.dart.coverage.json

Runtime Code Analysis

Runtime code analysis requires both an info file and a runtime data file.

The info file is emitted by compiling a dart2js app with --dump-info:

$ dart2js --dump-info main.dart

Enable the collection of runtime data by compiling a dart2js app with an experimental flag:

$ dart2js --experimental-track-allocations main.dart

After using your app (manually or via integration tests), dump the top-level window object below to a text file:


Finally run this tool:

$ dart2js_info runtime_coverage

And with the following to view package-level information:

$ dart2js_info runtime_coverage --show-packages

Here's an example output snippet:

Runtime Coverage Summary
                                   bytes      %
 Program size                   96860754 100.00%
 Libraries (excluding statics)  94394961  97.45%
 Code (classes + closures)      91141701  94.10%
 Used                            3519239   3.63%

                                   count      %
 Classes + closures                15902 100.00%
 Used                               5661  35.60%

Runtime Coverage Breakdown (packages) (87622462 bytes)
 package:angular_components.material_datepicker (29881 bytes unused)
   proportion of package used:                      43394/73275 (59.22%)
   proportion of unused code to all code:           29881/91141701 (0.03%)
   proportion of unused code to all unused code:    29881/87622462 (0.03%)
   proportion of main unit code to package code:    8142/73275 (11.11%)
   proportion of main unit code that is unused:     3088/8142 (37.93%)
   package breakdown:
     [-D] package:angular_components.material_datepicker/material_datepicker.dart:_ViewMaterialDatepickerComponent5: 656 bytes (0.90% of package)
     [+D] package:angular_components.material_datepicker/calendar.dart:CalendarSelection: 645 bytes (0.88% of package)
     [+M] package:angular_components.material_datepicker/range.dart:MonthRange: 629 bytes (0.86% of package)
     [-M] package:angular_components.material_datepicker/range.dart:QuarterRange: 629 bytes (0.86% of package)

A +/- indicates whether or not the element was used at runtime. A M/D indicates whether or not the element was in the main or deferred output unit.

Code location, features and bugs

This package is developed in github. Please file feature requests and bugs at the issue tracker.