Upgrade to Babel 7 (API)

Refer users to this document when upgrading to Babel 7.

Also check out the v7-migration guide for other user-level changes.

All Babel packages

NodeJS support

Support for Node.js 0.10 and 0.12 has been dropped as both of this versions are out of maintenance.

Export changes

Dropped use of add-module-exports plugin on Babel packages. This had to be used earlier to prevent a breaking change with our exports. If you import a Babel package in a library you may need to use .default when using require rather than import.

@babel/core

Changed ast to be false by default for performance (most tools aren't using it) babel/babel#7436.

The publicly exposed but undocumented Pipeline class has been removed. Best to use the transformation methods exposed from @babel/core directly babel/babel#5376.

The babel.util.* helper methods have been removed, and util.EXTENSIONS has been moved to babel.DEFAULT_EXTENSIONS babel/babel#5487.

Calls to babel.transform or any other transform function may return null if the file matched an ignore pattern or failed to match an only pattern babel/babel#5487.

The opts.basename option exposed on state.file.opts has been removed. If you need it, best to build it from opts.filename yourself babel/babel#5467.

Removed resolveModuleSource. We recommend using babel-plugin-module-resolver@3's 'resolvePath' options babel/babel#6343

Removed babel.analyse because it was just an alias for babel.transform

Removed path.mark() since we didn't use it and it can be implemented in your own plugin.

Removed babel.metadata since the generated plugin metadata is always included in the output result.

Removed path.hub.file.addImport. You can use the @babel/helper-module-imports module instead.

+  import { addDefault } from "@babel/helper-module-imports";
function importModule(pkgStore, name, path) {
-  return path.hub.file.addImport(resolvePath(pkgStore, name, path), 'default', name);
+  return addDefault(path, resolvePath(pkgStore, name, path), { nameHint: name });
}

Config changes

We've made some major changes to way config lookup works:

By default when searching for .babelrc files for a given file, stop at package.json.

For any particular file, Babel v6 keeps looking up the directory hierarchy until it finds a config file. This means your project might break because it uses a config file found outside the package root like in the home directory.

Adds support for a babel.config.js file along the lines of what Webpack does

Because this would break how a monorepo works (including Babel itself), we are introducing a new config file, that basically removes the hierarchical nature of configs.

Included is a root option that defaults to the current working directory for it to find the file. It is also not loaded relatively so it will handle symlinking correctly, whereas before you may have had hard-code the paths in webpack before.

Check the babel.config.js docs for more info: project-wide configuration

This file combined with the new overrides property and env lets you have a single config file that can work for all the files in a project vs. multiple config files per folder.

We also exclude node_modules by default and only look in the root unless you opt-in to setting an array of the .babelrcRoots option such as "babelrcRoots": [".", "node_modules/pkgA"]

Asserting Babel version #7450

Plugins can check that it's loaded with a certain version of Babel. The API will expose an assertVersion method, where you can pass in semver.

The declare helper is used to keep backwards compat with v6.

JavaScript

import { declare } from "@babel/helper-plugin-utils";

export default declare(api => {
  api.assertVersion(7);
  // ...
});

Babel plugins/presets

It currently takes it as the first parameter the babel object, and plugin/preset options, and the dirname

JavaScript

module.exports = function(api, options, dirname) {};

babel-parser (known as Babylon)

Removed the * plugin option #301

This was first added in v6.14.1 (Nov 17, 2016) so it's unlikely anyone was using this.

This catch-all option was removed; instead you should specifically decide which plugins you want to activate.

We thought it would be a good idea for tools so they wouldn't have to constantly update their config but it also means we can't easily make a breaking change.

Before:

JavaScript

babelParser.parse(code, {
  plugins: ["*"],
});

You can get the old behavior using:

JavaScript

babelParser.parse(code, {
  plugins: [
    "asyncGenerators",
    "classProperties",
    "decorators",
    "doExpressions",
    "dynamicImport",
    "exportExtensions",
    "flow",
    "functionBind",
    "functionSent",
    "jsx",
    "objectRestSpread",
  ],
});

See Babylon's plugin options.

Renamed decorators plugin to decorators-legacy

It has been renamed to align with the legacy option of @babel/plugin-proposal-decorators. A new decorators plugin has been implemented, which implements the new decorators proposal.

The two versions of the proposals have different syntaxes, so it is highly recommended to use decorators-legacy until the new semantics are implemented by Babel.

Removed classConstructorCall plugin #291

@babel/traverse

Remove support for flow bindings babel/babel#6528

The reason behind this change is that declare var foo doesn't introduce a new local binding, but it represents a global one.

getFunctionParent will no longer return Program, please use getProgramParent instead #5923.

It doesn't make sense that a function named getFunctionParent also returns the Program, so that was removed.

To get the equivalent behavior, you'll need to make a change like

- path.scope.getFunctionParent()
+ path.scope.getFunctionParent() || path.scope.getProgramParent()

Path replacement/removal APIs now return an array of new paths

For instance, using Path#insertBefore, or Path#replaceWith will now always return an array of the newly inserted/replaced paths.

JavaScript

const node = t.nullLiteral();
const [replaced] = path.replaceWith(node);
replace.node === node; // => true

This is especially useful when inserting several nodes into some higher-up scope, since you can immediately call the Path APIs on the node's new Path.

JavaScript

const parent = path.findParent(() => /* some selection criteria */);
const helperPaths = path.unshiftContainer("body", helpers);
// helperPaths can now be referenced, manipulated, etc.

AST changes

Add InterpreterDirective Node #7928

Babylon already parses "shebangs" (#!env node) but put's in a comment in the Program node. Now we are just creating an actual node for it.

Add a new interpreter field to the Program node.

JavaScript

extend interface Program {
  interpreter: InterpreterDirective;
}

Add the InterpreterDirective Node

JavaScript

interface InterpreterDirective <: Node {
    type: "InterpreterDirective";
    value: string;
}

JSX and TS node builders (from @babel/types package) renamed

The case has been changed: jsx and ts are now in lowercase.

- t.jSXIdentifier()
+ t.jsxIdentifier()

In general, we have differentiated the node types with TypeAnnotation for Flow and TSTypeAnnotation for TypeScript so for the shared type nodes, TypeScript has a TS prefix.

.expression field removed from ArrowFunctionExpression

The expression field was removed to eliminate two different sources of truth and needing plugins to manually keep them in sync. You can now simply check whether the function body is a BlockStatement or not:

  return {
    visitor: {
      ArrowFunctionExpression({ node }) {
-       if (node.expression) {
+       if (node.body.type !== "BlockStatement") {
          // () => foo;
        }
      }
    }
  };

Tokens removed

In previous versions tokens were always attached to the AST on the top-level. In the latest version of @babel/parser we removed this behavior and made it disabled by default to improve the performance of the parser. All usages in babel itself have been removed and @babel/generator is not using the tokens anymore for pretty printing.

If your babel plugin uses tokens at the moment, evaluate if it is still necessary and try to remove the usage if possible. If your plugin really depends on getting tokens you can reactivate it but please only consider this if there is no other way as this will hurt users performance.

To activate you need to set the tokens option of babylon to true. You can do this directly from your plugin.

JavaScript

export default function() {
  return {
    manipulateOptions(opts, parserOpts) {
      parserOpts.tokens = true;
    },
    ...
  };
}

Renamed

The following nodes have been renamed:

Name 6.x	Name 7.x	Example	PR
ExistentialTypeParam	ExistsTypeAnnotation	type A = B<*>;	#322
NumericLiteralTypeAnnotation	NumberLiteralTypeAnnotation	type T = 0;	#332

Besides the AST-Nodes also all the corresponding functions in @babel/types have been renamed.

 import * as t from "@babel/types";

 return {
-  ExistentialTypeParam(path) {
-    const parent = path.findParent((path) => path.isExistentialTypeParam());
-    t.isExistentialTypeParam(parent);
+  ExistsTypeAnnotation(path) {
+    const parent = path.findParent((path) => path.isExistsTypeAnnotation());
+    t.isExistsTypeAnnotation(parent);

-    return t.existentialTypeParam();
+    return t.existsTypeAnnotation();
   },
-  NumericLiteralTypeAnnotation(path) {
-    const parent = path.findParent((path) => path.isNumericLiteralTypeAnnotation());
-    t.isNumericLiteralTypeAnnotation(parent);
+  NumberLiteralTypeAnnotation(path) {
+    const parent = path.findParent((path) => path.isNumberLiteralTypeAnnotation());
+    t.isNumberLiteralTypeAnnotation(parent);

-    return t.numericLiteralTypeAnnotation();
+    return t.numberLiteralTypeAnnotation();
   }
 };

Replaced

On the following AST-Nodes the value of the field variance has been changed from a simple string value to be its own AST-Node called Variance. #333

The field is only available when enabling the flow plugin in babylon.

• ObjectProperty
• ObjectMethod
• AssignmentProperty
• ClassMethod
• ClassProperty
• Property

The type of the new Variance node looks like this:

JavaScript

type VarianceNode = {
  type: "Variance",
  kind: "plus" | "minus",
};

 return {
   Property({ node }) {
-    if (node.variance === "plus") {
+    if (node.variance.kind === "plus") {
       ...
-    } else if (node.variance === "minus") {
+    } else if (node.variance.kind === "minus") {
       ...
     }
   }
 };

Location changes

The location info of ObjectTypeIndexer has been changed to not include semicolons. This was done to align with the flow-parser and have the same location information. #228

Example:

JavaScript

var a: { [a: number]: string };

 {
   "type": "ObjectTypeIndexer",
   "start": 9,
-  "end": 29,
+  "end": 28,
   "loc": {
     "start": {
       "line": 1,
       "column": 9,
     },
     "end": {
       "line": 1,
-      "column": 29
+      "column": 28
     }
   }
 }

Removal

ForAwaitStatement

The AST-Node ForAwaitStatement has been removed and is replace with the field await in the ForOfStatement node #349

 interface ForOfStatement <: ForInStatement {
   type: "ForOfStatement";
+  await: boolean;
 }

 return {
-  ForAwaitStatement(path) {
-    ...
+  ForOfStatement(path) {
+    if (path.node.await) {
+      ...
+    }
   }
 };

RestProperty & SpreadProperty

The two AST-Nodes RestProperty and SpreadProperty have been removed in favor of reusing RestElement and SpreadElement #384

 return {
   SpreadElement(path) {
-    ...
-  },
-  SpreadProperty(path) {
-    ...
+    if (path.parentPath.isObjectExpression()) {
+      ...
+    } else if (path.parentPath.isArrayExpression()) {
+      ...
+    }
   },
   RestElement(path) {
-    ...
-  },
-  RestProperty(path) {
-    ...
+    if (path.parentPath.isObjectPattern()) {
+      ...
+    } else if (path.parentPath.isArrayPattern()) {
+      ...
+    }
   }
 };

See our upgrade PR for Babel and the Babylon AST spec for more information.