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Hello World with TypeScript

To create a new TypeScript project, use npx projen new typescript:

npx projen new typescript

This will create a .projenrc.ts file which defines your project.

npx projen new will synthesize a standard project directory structure with some sample code.

├── src
│   ├── index.ts
├── test
│ ├── hello.test.ts
├── node_modules
├── package.json
└── tsconfig.json

The default options will set up your tsconfig root directory as src and your out directory as lib. By default the test directory is not compiled to JavaScript and instead compiled when npx projen test is executed.

The npx projen new command will also generate a .projenrc.js file which includes the definition of your project with any options you specified in the command line:

const { typescript } = require("projen");
const project = new typescript.TypeScriptProject({
defaultReleaseBranch: "main",
name: "my-project",

To modify your project definitions, edit .projenrc.ts and run projen again to re-synthesize your project. The following sections describe the various features of your TypeScript project.


You can specify dependencies in your project via the deps, devDeps, and peerDeps options similar to what you might expect in a package.json file.

The recommendation is to only specify the module name here (e.g. express). This will behave similar to yarn add or npm install in the sense that it will add the module as a dependency to your package.json file with the latest version (^). You can specify semver requirements in the same syntax passed to npm i or yarn add (e.g. express@^2) and this will be what your package.json will eventually include.

Migrating your TypeScript Project to Projen

Projen provides some opinionated defaults that may be different than the defaults you rely on in your existing TypeScript Project. This section attempts to document some of the common themes to look out for when migrating an existing TypeScript project to Projen.

Migrating .projenrc.js to .projenrc.ts

If you'd like to upgrade an existing project that was creating using a JS-based projen RC file (.projenrc.js) to a TS-based projen RC file (.projenrc.ts):

  1. Add projenrcTs: true to your project.
  2. Run npx projen.
  3. Rename .projenrc.js to .projenrc.ts.
  4. Update requires to imports.
  5. Run npx projen.

Default Directory Structure

The most important is the default project structure. Projen expects all source code to be in the src directory and will write compiled .js and .d.ts files to the lib directory. Test files are expected in the test directory and not compiled to javascript. Instead, Projen configures Jest to compile .ts files when testing.

As this is only one of many structures for valid TypeScript projects, your migrated project may not work out-of-the-box with Projen. We recommend that you take the time to fully migrate to the expected project structure so that you can take full advantage of Projen management on your repository.

However, you can also configure your .projenrc.js file to reflect your current project structure. A TypeScriptProject exposes all options in a tsConfig file so you can manually set the structure you want. Here is an example for specifying a structure where the TypeScript compiler finds all TypeScript files recursively in your repository:

const { typescript } = require('projen');
const project = new typescript.TypeScriptProject({
tsConfig: {
compilerOptions: {
rootDir: '.',
outDir: '.',
include: ['**/*.ts'], // all typescript files recursively

Dependency Versions

As described in the section above, Projen recommends that you list your dependencies only by module name and have Projen install the latest version of the package. A consequence of this recommendation is that when migrating, you may unknowingly update your dependencies to incompatible versions. You can always provide specific semvar requirements (e.g. express@2.1.0) if necessary.