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How to Contribute

Reporting security issues

Please don't open issues for security issues. Instead, file a report at

Reporting Issues

Please see our FAQ for common issues that people run into.

Should you run into other issues with the project, please don't hesitate to let us know by filing an issue!

In general we are going to ask for an example of the problem failing, which can be as simple as a jsfiddle/jsbin/etc. We've put together a jsfiddle template to ease this. (We will keep this link up to date as new releases occur, so feel free to check back here).

Pull requests containing only failing tests demonstrating the issue are welcomed and this also helps ensure that your issue won't regress in the future once it's fixed.

Documentation issues on the site should be reported on handlebars-lang/docs.


  • The branch 4.x contains the currently released version. Bugfixes should be made in this branch.
  • The branch master contains the next version. A release date is not yet specified. Maintainers should merge the branch 4.x into the master branch regularly.
  • The branch 3.x contains the legacy version 3.x. Bugfixes are applied separately (if needed). The branch will not be merged with any of the other branches.

Pull Requests

We also accept pull requests!

Generally we like to see pull requests that

  • Maintain the existing code style
  • Are focused on a single change (i.e. avoid large refactoring or style adjustments in untouched code if not the primary goal of the pull request)
  • Have good commit messages
  • Have tests
  • Don't significantly decrease the current code coverage (see coverage/lcov-report/index.php)


To build Handlebars.js you'll need a few things installed.

Before building, you need to make sure that the Git submodule spec/mustache is included (i.e. the directory spec/mustache should not be empty). To include it, if using Git version 1.6.5 or newer, use git clone --recursive rather than git clone. Or, if you already cloned without --recursive, use git submodule update --init.

Project dependencies may be installed via npm install.

To build Handlebars.js from scratch, you'll want to run grunt in the root of the project. That will build Handlebars and output the results to the dist/ folder. To re-run tests, run grunt test or npm test. You can also run our set of benchmarks with grunt bench.

The grunt dev implements watching for tests and allows for in browser testing at http://localhost:9999/spec/.

If you notice any problems, please report them to the GitHub issue tracker at

Running Tests

To run tests locally, first install all dependencies.

npm install

Clone the mustache specs into the spec/mustache folder.

cd spec
rm -r mustache
git clone mustache

From the root directory, run the tests.

npm test

Linting and Formatting

Handlebars uses eslint to enforce best-practices and prettier to auto-format files. We do linting and formatting in two phases:

  • Committed files are linted and formatted in a pre-commit hook. In this stage eslint-errors are forbidden, while warnings are allowed.
  • The GitHub CI job also lints all files and checks if they are formatted correctly. In this stage, warnings are forbidden.

You can use the following scripts to make sure that the CI job does not fail:

  • npm run lint will run eslint and fail on warnings
  • npm run format will run prettier on all files
  • npm run check-before-pull-request will perform all most checks that our CI job does in its build-job, excluding the "integration-test".
  • npm run integration-test will run integration tests (using old NodeJS versions and integrations with webpack, babel and so on) These tests only work on a Linux-machine with nvm installed (for running tests in multiple versions of NodeJS).

Releasing the latest version

Before attempting the release Handlebars, please make sure that you have the following authorizations:

  • Push-access to handlebars-lang/handlebars.js
  • Publishing rights on for the handlebars package
  • Publishing rights on gemfury for the handlebars-source package
  • Push-access to the repo for legacy package managers: components/handlebars
  • Push-access to the production-repo of the handlebars site: handlebars-lang/

When releasing a previous version of Handlebars, please look into the in the corresponding branch.

A full release may be completed with the following:

npm ci
npx grunt
npm publish

cd dist/components/
gem build handlebars-source.gemspec
gem push handlebars-source-*.gem

After the release, you should check that all places have really been updated. Especially verify that the latest-tags in those places still point to the latest version

When everything is OK, the handlebars site needs to be updated.

Go to the master branch of the repo handlebars-lang/docs and make a minimal change to the README. This will invoke a github-action that redeploys the site, fetching the latest version-number from the npm-registry. (note that the default-branch of this repo is not the master and regular changes are done in the handlebars-lang/docs-repo).