最佳实践

Traversing the AST is expensive, and it’s easy to accidentally traverse the AST more than necessary. This could be thousands if not tens of thousands of extra operations.

Babel optimizes this as much as possible, merging visitors together if it can in order to do everything in a single traversal.

When writing visitors, it may be tempting to call path.traverse in multiple places where they are logically necessary.

path.traverse({
  Identifier(path) {
    // ...
  }
});
path.traverse({
  BinaryExpression(path) {
    // ...
  }
});

However, it is far better to write these as a single visitor that only gets run once. Otherwise you are traversing the same tree multiple times for no reason.

path.traverse({
  Identifier(path) {
    // ...
  },
  BinaryExpression(path) {
    // ...
  }
});

It may also be tempting to call path.traverse when looking for a particular node type.

const nestedVisitor = {
  Identifier(path) {
    // ...
  }
};
const MyVisitor = {
  FunctionDeclaration(path) {
    path.get('params').traverse(nestedVisitor);
  }
};

However, if you are looking for something specific and shallow, there is a good chance you can manually lookup the nodes you need without performing a costly traversal.

const MyVisitor = {
  FunctionDeclaration(path) {
    path.node.params.forEach(function() {
      // ...
    });
  }
};

当您嵌套访问者（visitor）时，把它们嵌套在您的代码中可能是有意义的。

const MyVisitor = {
  FunctionDeclaration(path) {
    path.traverse({
      Identifier(path) {
        // ...
    });
  }
};

如果您在嵌套的访问者中需要一些状态，像这样：

const MyVisitor = {
  FunctionDeclaration(path) {
    var exampleState = path.node.params[0].name;
      Identifier(path) {
        if (path.node.name === exampleState) {
          // ...
        }
      }
    });
  }
};

您可以将它作为状态传递给traverse()</ 0>方法，并有权访问<code>this在访问者中。

const nestedVisitor = {
  Identifier(path) {
    if (path.node.name === this.exampleState) {
      // ...
    }
  }
};
const MyVisitor = {
  FunctionDeclaration(path) {
    var exampleState = path.node.params[0].name;
    path.traverse(nestedVisitor, { exampleState });
  }
};

有时候在考虑给定的转换时，可能会忘记给定的转换结构可以是嵌套的。

例如，想象一下，我们想要查找构造函数 ClassMethod Foo ClassDeclaration.

class Foo {
  constructor() {
    // ...
  }
}

const constructorVisitor = {
  ClassMethod(path) {
    if (path.node.name === 'constructor') {
      // ...
    }
  }
}
const MyVisitor = {
  ClassDeclaration(path) {
    if (path.node.id.name === 'Foo') {
      path.traverse(constructorVisitor);
    }
  }
}

我们忽略了类可以嵌套的事实，使用遍历的话，上面我们也会得到一个嵌套的`构造函数：

class Foo {
  constructor() {
      constructor() {
        // ...
    }
  }
}

有几种主要的方法来测试babel插件：快照测试，AST测试和执行测试。 对于这个例子，我们将使用 jest ，因为它支持盒外快照测试。 我们在这里创建的示例是托管在这个 repo.

首先我们需要一个babel插件，我们将把它放在src / index.js中。

// src/__tests__/index-test.js
const babel = require('babel-core');
const plugin = require('../');
var example = `
var foo = 1;
if (foo) console.log(foo);
`;
it('works', () => {
  const {code} = babel.transform(example, {plugins: [plugin]});
  expect(code).toMatchSnapshot();
});
 

这给了我们一个快照文件在 src / tests / snapshots / index-test.js.snap

exports[`test works 1`] = `
"
var bar = 1;
if (bar) console.log(bar);"
`;

如果我们在插件中将“bar”更改为“baz”并再次运行，则可以得到以下结果：

接收到的值与存储的快照1不匹配。
    - Snapshot
    + Received
    @@ -1,3 +1,3 @@
     "
    -var bar = 1;
    -if (bar) console.log(bar);"
    +var baz = 1;
    +if (baz) console.log(baz);"

我们看到我们对插件代码的改变如何影响了我们插件的输出 如果输出看起来不错，我们可以运行jest -u 来更新快照。

除了快照测试外，我们还可以手动检查AST。 这是一个简单但是脆弱的例子。 对于更多涉及的情况，您可能希望利用Babel-遍历。 它允许您用访问者键指定一个对象，就像您使用插件本身。

it(‘contains baz’, () => {
  const {ast} = babel.transform(example, {plugins: [plugin]});
  const program = ast.program;
  const declaration = program.body[0].declarations[0];
  assert.equal(declaration.id.name, ‘baz’);
  // or babelTraverse(program, {visitor: …})
});

it('foo is an alias to baz', () => {
  var input = `
    var foo = 1;
    // test that foo was renamed to baz
    var res = baz;
  `;
  var {code} = babel.transform(input, {plugins: [plugin]});
  var f = new Function(`
    ${code};
    return res;
  `);
  var res = f();
  assert(res === 1, 'res is 1');
});
 

Babel核心使用类似的方法</>去获取快照和执行测试。