tsickle

What is tsickle?

Tsickle is a TypeScript to Closure Compiler transformer that helps in generating Closure-compatible JavaScript from TypeScript code. It is primarily used to bridge the gap between TypeScript and Google's Closure Compiler, enabling advanced optimizations and type-checking.

What are tsickle's main functionalities?

Type Annotations to JSDoc

Tsickle converts TypeScript type annotations into JSDoc comments, which can be understood by the Closure Compiler for type-checking and optimizations.

/* TypeScript Code */
function add(a: number, b: number): number {
  return a + b;
}

/* Transformed JavaScript Code */
/**
 * @param {number} a
 * @param {number} b
 * @return {number}
 */
function add(a, b) {
  return a + b;
}

Closure Compiler Compatibility

Tsickle transforms TypeScript classes and other constructs into a form that is compatible with the Closure Compiler, enabling advanced optimizations.

/* TypeScript Code */
class Example {
  private value: string;
  constructor(value: string) {
    this.value = value;
  }
}

/* Transformed JavaScript Code */
/**
 * @constructor
 * @param {string} value
 */
var Example = function(value) {
  /** @private {string} */
  this.value = value;
};

Type Checking with Closure Compiler

By converting TypeScript types to JSDoc, Tsickle allows the Closure Compiler to perform type-checking on the generated JavaScript code.

/* TypeScript Code */
function greet(name: string): string {
  return 'Hello, ' + name;
}

/* Transformed JavaScript Code */
/**
 * @param {string} name
 * @return {string}
 */
function greet(name) {
  return 'Hello, ' + name;
}

Other packages similar to tsickle

babel

Babel is a JavaScript compiler that allows you to use next-generation JavaScript, today. It can transform TypeScript code into JavaScript, but it does not provide the same level of integration with the Closure Compiler as Tsickle.

typescript

The TypeScript compiler (tsc) itself can compile TypeScript to JavaScript. While it does not specifically target Closure Compiler compatibility, it is a robust and widely-used tool for TypeScript development.

google-closure-compiler

Google Closure Compiler is a tool for making JavaScript download and run faster. While it can be used in conjunction with Tsickle, it does not itself transform TypeScript code.

README

Tsickle - TypeScript to Closure Translator

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.

Usage

Tsickle is a library, designed to be used by a larger program that interacts with TypeScript and the Closure compiler.

Some known clients are:

1. Within Google we use tsickle inside the Bazel build system. That code is published as open source as part of Bazel's nodejs/TypeScript build rules.
2. tscc wraps tsickle and closure compiler, and interops with rollup.
3. We publish a simple demo program in the demo/ subdirectory.

Design details

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.

declare

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.

Example:

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

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.

Example:

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

@myDecorator()
class DoNotRenameThisClass { ... }

Development

Dependencies

• nodejs. Install from your operating system's package manger, by following instructions on https://nodejs.org/en/, or by using NVM
• yarn. Install from your operating system's package manager or by following instructions on yarnpkg.com.

One-time setup

Run yarn to install dependencies.

Build & Test commands

• yarn build builds the code base.
• Run tsc --watch for an interactive, incremental, and continuous build.
• yarn lint checks for lint.
• yarn test runs unit tests, e2e tests and checks for lint (but make sure to yarn build first or run tsc!). Set the TEST_FILTER environment variable to filter what golden tests to run.

TypeScript AST help

https://astexplorer.net/ and https://ts-ast-viewer.com/ are convenient tools to visualize and inspect a TypeScript AST.

Debugging

You can debug tests by passing --node_options=--inspect or --node_options=--inspect-brk (to suspend execution directly after startup).

For example, to debug a specific golden test:

TEST_FILTER=my_golden_test node --inspect-brk=4332 ./node_modules/.bin/jasmine out/test/*.js

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.

The break in specific code locations you can add debugger; statements in the source code.

Updating Goldens

Run UPDATE_GOLDENS=y yarn test to have the test suite update the goldens in test_files/....

Environment variables

Set the environment variable TEST_FILTER=<REGEX> to limit the golden tests (found in test_files/...) to only run tests with a name matching the regex.

Releasing

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 https://github.com/angular/tsickle/releases) will be implicitly created.

From the master branch run:

npm config set registry https://wombat-dressing-room.appspot.com
npm login
npm publish