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0.37.0 • Public • Published

Tsickle - TypeScript to Closure Translator Build Status Windows build

Tsickle converts TypeScript code into a form acceptable to the Closure Compiler. This allows using TypeScript to transpile your sources, and then using Closure Compiler to bundle and optimize them, while taking advantage of type information in Closure Compiler.

What conversion means

A (non-exhaustive) list of the sorts of transformations Tsickle applies:

  • inserts Closure-compatible JSDoc annotations on functions/classes/etc
  • converts ES6 modules into goog.module modules
  • generates externs.js from TypeScript d.ts (and declare, see below)
  • declares types for class member variables
  • translates export * from ... into a form Closure accepts
  • converts TypeScript enums into a form Closure accepts
  • reprocesses all jsdoc to strip Closure-invalid tags

In general the goal is that you write valid TypeScript and Tsickle handles making it valid Closure Compiler code.

Warning: work in progress

We already use tsickle within Google to minify our apps (including those using Angular), but we have less experience using tsickle with the various JavaScript builds that are seen outside of Google.

We would like to make tsickle usable for everyone but right now if you'd like to try it you should expect to spend some time debugging and reporting bugs.


Project Setup

Tsickle works by wrapping tsc. To use it, you must set up your project such that it builds correctly when you run tsc from the command line, by configuring the settings in tsconfig.json.

If you have complicated tsc command lines and flags in a build file (like a gulpfile etc.) Tsickle won't know about it. Another reason it's nice to put everything in tsconfig.json is so your editor inherits all these settings as well.


Run tsickle --help for the full syntax, but basically you provide any tsickle specific options and use it as a TypeScript compiler.

Output format

Tsickle is designed to do whatever is necessary to make the code acceptable by Closure compiler. We view its output as a necessary intermediate form for communicating to the Closure compiler, and not something for humans. This means the tsickle output may be kind of ugly to read. Its only real use is to pass it on to the compiler.

For one example, the syntax of types tsickle produces are specific to Closure. The type {!Foo} means "Foo, excluding null" and a type alias becomes a var statement that is tagged with @typedef.

Tsickle emits modules using Closure's goog.module module system. This system is similar to but different from ES modules, and was supported by Closure before the ES module system was finalized.

Differences from TypeScript

Closure and TypeScript are not identical. Tsickle hides most of the differences, but users must still be aware of some differences.


Any declaration in a .d.ts file, as well as any declaration tagged with declare ..., is intepreted by Tsickle as a name that should be preserved through Closure compilation (i.e. not renamed into something shorter). Use it any time the specific string names of your fields are significant. That would most often happen when the object either coming from outside your program, or being passed out of the program.


declare interface JSONResult {
    username: string;
let r = JSON.parse(input) as JSONResult;

By adding declare to the interface (or if it were in a .d.ts file), Tsickle will inform Closure that it must use exactly the field name .username (and not e.g. .a) in the output JS. This matters for this example because the input JSON probably uses the string 'username' and not whatever name Closure would invent for it. (Note: declare on an interface has no additional meaning in pure TypeScript.)

Exporting decorators

An exporting decorator is a decorator that has @ExportDecoratedItems in its JSDoc.

The names of elements that have an exporting decorator are preserved through the Closure compilation process by applying an @export tag to them.


/** @ExportDecoratedItems */
function myDecorator() {
  // ...

class DoNotRenameThisClass { ... }


Bazel install

We use bazel to build, and are pinned to an old version of it in package.json. So unless you have that specific version of bazel installed, you should run run yarn bazel instead of bazel in any of the below commands.

One-time setup

Run bazel run @nodejs//:yarn && ./ to install the dependencies.

This avoids occupying the bazel server, so that yarn can call bazel again. Ideally we should just use @nodejs//:yarn, see

Test commands

  • ibazel test test:unit_test executes the unit tests in watch mode (use bazel test test:unit_test for a single run),
  • bazel test test:e2e_test executes the e2e tests,
  • bazel test test:golden_test executes the golden tests,
  • node check-format.js checks the source code formatting using clang-format,
  • yarn test runs unit tests, e2e tests and checks the source code formatting.


You can debug tests by using bazel run and passing --node_options=--inspect. For example, to debug a specific golden test:

TEST_FILTER=my_golden_test ibazel run //test:golden_test -- --node_options=--inspect

Then open [about:inspect] in Chrome and choose "about:inspect". Chrome will launch a debugging session on any node process that starts with a debugger listening on one of the listed ports. The tsickle tests and Chrome both default to localhost:9229, so things should work out of the box.

VS Code can also connect using the inspect protocol. It doesn't support automatically reconnecting or any way to re-run the test suite though, so it is a less convenient. You can start the node process passing an extra --node_options=--debug-brk (in addition to the parameters above) to have Node wait before program execution, so you have time to attach VS Code.

Updating Goldens

Run UPDATE_GOLDENS=y bazel run test:golden_test to have the test suite update the goldens in test_files/....

Environment variables

Pass the flag --action_env=TEST_FILTER=<REGEX> to bazel test to limit the end-to-end test (found in test_files/...) run tests with a name matching the regex.


On a new branch, run

# tsickle releases are all minor releases for now, see npm help version.
$ npm version minor

This will update the version in package.json, commit the changes, and create a git tag.

Push the branch and get it reviewed, but do not merge. If you click the "rebase and merge" button in the Github UI it changes the commit, so the git tag that was created would point at the wrong commit.

Instead, push the branch to master directly via:

$ git push origin mybranch:master

Note that Github will block non-fast-forward pushes to master, so if there have been other intervening commits you'll need to recreate the release.

Also push the tag.

$ git push origin v0.32.0  # but use correct version

Once the versioned tag is pushed to Github the release (as found on will be implicitly created.

Run bazel run :npm_package.publish from the master branch (you must be logged into the angular shared npm account).


npm i tsickle

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