Developing LoopBack

This document describes how to develop modules living in loopback-next monorepo. See Monorepo overview for a list of all packages.

Setting up development environment

Before you can start developing LoopBack, you need to install and configure few dependencies.

You may want to configure your IDE or editor to get better support for TypeScript too.

  • VisualStudio Code
  • Missing your favorite IDE/editor here? We would love to have documentation for more IDEs/editors! Please send a pull request to add recommended setup for your tool.

Before getting started, it is recommended to configure git so that it knows who you are:

git config --global "J. Random User"
git config --global ""

Please make sure this local email is also added to your GitHub email list so that your commits will be properly associated with your account and you will be promoted to Contributor once your first commit is landed.

Building the project

Whenever you pull updates from GitHub or switch between feature branches, make sure to updated installed dependencies in all monorepo packages. The following command will install npm dependencies for all packages and create symbolic links for intra-dependencies:

npm install

The next step is to compile all packages from TypeScript to JavaScript:

npm run build

Please note that we are automatically running the build from pretest script, therefore you should not need to run this command as part of your red-green-refactor cycle.

Running tests

This is the only command you should need while developing LoopBack:

npm test

It does all you need:

  • Compile TypeScript
  • Run all tests
  • Check code formatting using Prettier
  • Lint the code using TSLint

Coding rules

  • All features and bug fixes must be covered by one or more automated tests.

  • All public methods must be documented with typedoc comments (see API Documentation below).

  • Follow our style guide as documented on Code style guide.

Linting and formatting

We use two tools to keep our codebase healthy:

  • TSLint to statically analyse our source code and detect common problems.
  • Prettier to keep our code always formatted the same way, avoid style discussions in code reviews, and save everybody’s time an energy.

You can run both linters via the following npm script, just keep in mind that npm test is already running them for you.

npm run lint

Many problems (especially formatting) can be automatically fixed by running the npm script lint:fix.

npm run lint:fix

File naming convention

For consistency, we follow Angular’s file naming convention. It helps to derive the usage of files by inspecting the names. Besides the LoopBack 4 codebase, we also follow this naming convention in our generated artifacts from the CLI tooling: {name}.{artifact-type}.ts

Examples are:


In addition, files under test folder are categorized according to the type of tests (unit, acceptance and integration), with the convention {name}.{test-type}.ts.

Examples are:


API Documentation

We use strong-docs to generate API documentation for all our packages. This documentation is generated when publishing new releases to and it’s picked up by

You can preview API docs locally by opening the file docs/apidocs.html in your browser.

Commit message guidelines

Note: we have recently changed our commit message conventions. Most of other LoopBack repositories (e.g. strongloop/ use the older convention as described on

A good commit message should describe what changed and why.

Our commit messages are formatted according to Conventional Commits, we use commitlint to verify and enforce this convention. These rules lead to more readable messages that are easy to follow when looking through the project history. But also, we use the git commit messages to generate change logs when publishing new versions.

Commit Message Format

Each commit message consists of a header, a body and a footer. The header has a special format that includes a type, an optional scope and a subject:

<type>(<scope>): <subject>


The type must be one of the following:

  • feat: A new feature
  • fix: A bug fix
  • docs: Documentation only changes
  • style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
  • refactor: A code change that neither fixes a bug nor adds a feature
  • perf: A code change that improves performance
  • test: Adding missing or correcting existing tests
  • build: Changes that affect the build system or external dependencies
  • ci: Changes to our CI configuration files and scripts
  • chore: Changes to the auxiliary tools and libraries such as documentation generation
  • revert: Reverts a previous commit


The scope must be a list of one or more packages contained in this monorepo. Each scope name must match a directory name in packages/, e.g. core or context.

Note: If multiple packages are affected by a pull request, don’t list the scopes as the commit linter currently only supports only one scope being listed at most. The CHANGELOG for each affected package will still show the commit. Commit linter will be updated to allow listing of multiple affected scopes, see issue #581


The subject contains succinct description of the change:

  • use the imperative, present tense: “change” not “changed” nor “changes”
  • don’t capitalize first letter
  • no dot (.) at the end


The body provides more details, it should include the motivation for the change and contrast this with previous behavior.

Just as in the subject, use the imperative, present tense: “change” not “changed” nor “changes”a

Paragraphs or bullet points are ok (must not exceed 100 characters per line). Typically a hyphen or asterisk is used for the bullet, followed by a single space, with blank lines in between.

The footer should contain any information about Breaking Changes introduced by this commit.

This section must start with the upper case text BREAKING CHANGE followed by a colon (:) and a space (``). A description must be provided, describing what has changed and how to migrate from older versions.

Tools to help generate a commit message

This repository has commitizen support enabled. Commitizen can help you generate your commit messages automatically. You must install it globally as follows:

npm i -g commitizen

And to use it, simply call git cz instead of git commit. The tool will help you generate a commit message that follows the above guidelines.

Releasing new versions

When we are ready to tag and publish a release, run the following commands:

cd loopback-next
git checkout master
git pull
npm run release

The release script will automatically perform the tasks for all packages:

  • Clean up node_modules
  • Install/link dependencies
  • Transpile TypeScript files into JavaScript
  • Run mocha tests
  • Check lint (tslint and prettier) issues

If all steps are successful, it prompts you to publish packages into npm repository.

Adding a new package

Create a new package

To add a new package, create a folder in packages as the root directory of your module. For example,

cd loopback-next/packages
mkdir <a-new-package>

The package follows the node/npm module layout. You can use npm init or lb4 extension command to scaffold the module, copy/paste from an existing package, or manually add files including package.json.

Make sure you add LICENSE file properly and all source code files have the correct copyright header.

Keep shared configuration in root

We have some configuration files at the top level (loopback-next/):

  • .gitignore
  • .prettierignore
  • .nycrc.yml

For consistency across all packages, do not add them at package level unless specific customization is needed.

Make a scoped package public

By default, npm publishes scoped packages with private access. There are two options to make a new scoped package with public access.

Either add the following section to package.json:

  "publishConfig": {
    "access": "public"

Or explicitly publish the package with --access=public:

cd packages/<a-new-package>
npm publish --access=public

Register the new package

Please register the new package in the following files:

How to test infrastructure changes

When making changes to project infrastructure, e.g. modifying tsc or tslint configuration, it’s important to verify that all usage scenarios keep working.

Verify TypeScript setup

  1. Open any existing TypeScript file, e.g. packages/src/index.ts

  2. Add a small bit of code to break TypeScript’s type checks, for example:

    const foo: number = 'bar';
  3. Run npm test

  4. Verify that the build failed and the compiler error message shows a path relative to monorepo root, e.g. packages/src/index.ts.

    (This is does not work now, tsc is reporting paths relative to individual package directories. See

  5. Test integration with supported IDEs:

Verify TSLint setup

  1. Open any existing TypeScript file, e.g. packages/src/index.ts

  2. Introduce two kinds linting problems - one that does and another that does not require type information to be detected. For example, you can add the following line at the end of the opened index.ts:

    const foo: any = 'bar';
  3. Run npm test

  4. Verify that the build failed and both linting problems are reported:

    ERROR: /Users/(...)/packages/core/src/index.ts[16, 7]: 'foo' is declared but its value is never read.
    ERROR: /Users/(...)/packages/core/src/index.ts[16, 12]: Type declaration of 'any' loses type-safety. Consider replacing it with a more precise type.
  5. Test integration with supported IDEs: