Espree v3.0.0 Alpha 2 released

We are pleased to share that we've just pushed the second alpha release of Espree v3.0.0. This is a major release with breaking changes, so please be sure to read this post thoroughly.


Since Espree v3.0.0 is a prerelease version, you’ll need to install it using the @next tag:

$ npm install espree@next

The @latest tag is still on the latest v2.x version of Espree.

Note: We are providing this release to gather feedback. Help out by filing issues if you find a problem.


Here are some highlights of changes in this release.

Removal of most ecmaFeatures

We introduced ecmaFeatures as a way to start incrementally implementing ECMAScript 6 at a time when it was constantly changing. We did this in order to allow people to start using those features that were ready without forcing usage of all features.

Now that ES6 is finalized and all the features are implemented, most people are using all ES6 features and it’s a pain to have to manually configure them all. Additionally, there’s no good reason to turn on just some features and not all of them (as that’s not how the real world works).

In this second alpha release, we’ve made the following changes:

  • Introduced ecmaVersion as the way to indicate which version of ECMAScript you want to target. Possible values are 3, 5, and 6.
  • Removed all ES6 flags from ecmaFeatures (globalReturn, jsx, and experimental features remain).
  • Removed individual checks for ES6 features (could potentially improve performance based on your usage).

In all likelihood, going forward we will probably follow a similar pattern of introducing experimental features using ecmaFeatures and then rolling them into a larger ecmaVersion grouping later on.

Exporting VisitorKeys

Tools that use the ESTree format rely on knowledge of the syntax tree in order to work with it. This information tends to be spread amongst various tools. In order to make it easier to work with Espree, we are now exporting VisitorKeys, which contains the navigation information that tools like escope and estraverse use to navigate the tree. We’re hoping this makes it easier for other tools to understand what Espree is producing.

Breaking Changes

  • Breaking: move ecmaFeatures into ecmaVersion (fixes #222) (Nicholas C. Zakas)


  • New: Export VisitorKeys (fixes #220) (Nicholas C. Zakas)

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