Dart2js Info

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

The .info.json 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 earier.


Currently, most tools available here can be used to analyze code-size and attibution of code-size to different parts of your app. With time, we hope to add more data to the .info.json 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.

Info API

AllInfo exposes a Dart representation of the .info.json files. You can parse the information using AllInfo.parseFromJson. For example:

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

import 'package:dart2js_info/info.dart';

main(args) {
  var infoPath = args[0];
  var json = JSON.decode(new File(infoPath).readAsStringSync());
  var info = AllInfo.parseFromJson(json);

Available tools

The following tools are a available today:

  • code_deps.dart: 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.

  • library_size_split: 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).

  • function_size_analysis: a tool that shows how much code was attributed to each function. This tool also uses depedency 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_log_server and live_code_size_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.json, so you determine why code that is not used is being included in your app.

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:

pub global activate dart2js_info # only needed once
pub global run dart2js_info:code_deps out.js.info.json 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.

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
pub global run dart2js_info:library_size_split out.js.info.json

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

pub global run dart2js_info:library_size_split out.js.info.json 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 of all libraries together, it is shown in
# cluster #3, which happens to be the last cluster in this example:
- name: "Total (excludes preambles, statics & consts)"
  regexp: ".*"
  cluster: 3

# This group shows the total size for all libraries that were loaded from
# file:// urls:
- { 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/(.*)"

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
pub global run dart2js_info:function_size_analysis out.js.info.json

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% some.library.name::ClassName.myMethodName

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 tools

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 defining the Dart environment traceCalls=post:

DART_VM_OPTIONS="-DtraceCalls=post" dart2js --dump-info main.dart

Because coverage/tracing data is currently experimental, the feature is not exposed as a flag in dart2js, but you can enable it using the Dart environment flag. The flag only works dart2js version 1.13.0-dev.0.0 or newer.

  • Launch the coverage server tool to serve up the JS code of your app:
pub global run dart2js_info:coverage_log_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 excersize 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 converage json files:

pub global run dart2js_info:live_code_size_analysis main.dart.info.json main.dart.coverage.json

Code location, features and bugs

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



Data produced by dart2js when run with the --dump-info flag.