babel-plugin-handbook

This document covers how to create Babel plugins.

Special thanks to @sebmck, @jdalton, @abraithwaite, @robey, and others for their amazing help on this handbook.

Node Packaged Manuscript

You can install this handbook with npm. Just do:

$ npm install -g babel-plugin-handbook

Now you will have a babel-plugin-handbook command that will open this readme file in your $PAGER. Otherwise, you may continue reading this document as you are presently doing.

Table of Contents

• Introduction
• Basics
  • ASTs
  • Stages of Babel
    • Parse
      • Lexical Analysis
      • Syntactic Analysis
    • Transform
    • Generate
  • Traversal
    • Visitors
    • Paths
      • Paths in Visitors
    • State
    • Scopes
      • Bindings
• API
  • babylon
  • babel-traverse
  • babel-types
    • Definitions
    • Builders
    • Validators
    • Converters
  • babel-generator
  • babel-template
• Writing your first Babel Plugin
• Transformation Operations
  • Visiting
    • Check if a node is a certain type
    • Check if an identifier is referenced
  • Manipulation
    • Replacing a node
    • Replacing a node with multiple nodes
    • Replacing a node with a source string
    • Inserting a sibling node
    • Removing a node
    • Replacing a parent
    • Removing a parent
  • Scope
    • Checking if a local variable is bound
    • Generating a UID
    • Pushing a variable declaration to a parent scope
    • Rename a binding and its references
• Best Practices

Introduction

Babel is a generic multi-purpose compiler for JavaScript. More than that it is a collection of modules that can be used for many different forms of static analysis.

Static analysis is the process of analyzing code without executing it. (Analysis of code while executing it is known as dynamic analysis). The purpose of static analysis varies greatly. It can be used for linting, compiling, code highlighting, code transformation, optimization, minification, and much more.

You can use Babel to build many different types of tools that can help you be more productive and write better programs.

Basics

Babel is a JavaScript compiler, specifically a source-to-source compiler, often called a "transpiler". This means that you give Babel some JavaScript code, Babel modifies the code, and generates the new code back out.

ASTs

Each of these steps involve creating or working with an Abstract Syntax Tree or AST.

function square(n) {
  return n * n;
}

This same program can be represented as a list like this:

- FunctionDeclaration:
  - id:
    - Identifier:
      - name: square
  - params [1]
    - Identifier
      - name: n
  - body:
    - BlockStatement
      - body [1]
        - ReturnStatement
          - argument
            - BinaryExpression
              - operator: *
              - left
                - Identifier
                  - name: n
              - right
                - Identifier
                  - name: n

Or as a JavaScript Object like this:

{
  type: "FunctionDeclaration",
  id: {
    type: "Identifier",
    name: "square"
  },
  params: [{
    type: "Identifier",
    name: "n"
  }],
  body: {
    type: "BlockStatement",
    body: [{
      type: "ReturnStatement",
      argument: {
        type: "BinaryExpression",
        operator: "*",
        left: {
          type: "Identifier",
          name: "n"
        },
        right: {
          type: "Identifier",
          name: "n"
        }
      }
    }]
  }
}

You'll notice that each level of the AST has a similar structure:

{
  type: "FunctionDeclaration",
  id: {...},
  params: [...],
  body: {...}
}

{
  type: "Identifier",
  name: ...
}

{
  type: "BinaryExpression",
  operator: ...,
  left: {...},
  right: {...}
}

Note: Some properties have been removed for simplicity.

Each of these are known as a Node. An AST can be made up of a single Node, or hundreds if not thousands of Nodes. Together they are able to describe the syntax of a program that can be used for static analysis.

Every Node has this interface:

interface Node {
  type: string;
}

The type field is a string representing the type of Node the object is (ie. "FunctionDeclaration", "Identifier", or"BinaryExpression"`). Each type of Node defines an additional set of properties that describe that particular node type.

There are additional properties on every Node that Babel generates which describe the position of the Node in the original source code.

{
  type: ...,
  start: 0,
  end: 38,
  loc: {
    start: {
      line: 1,
      column: 0
    },
    end: {
      line: 3,
      column: 1
    }
  },
  ...
}

These properties start, end, loc, appear in every single Node.

Stages of Babel

The three primary stages of Babel are parse, transform, generate.

Parse

The parse stage, takes code and outputs an AST. There are two phases of parsing in Babel: Lexical Analysis and Syntactic Analysis.

Lexical Analysis

Lexical Analysis will take a string of code and turn it into a stream of tokens.

You can think of tokens as a flat array of pieces of language syntax.

n * n;

[
  { type: { ... }, value: "n", start: 0, end: 1, loc: { ... } },
  { type: { ... }, value: "*", start: 2, end: 3, loc: { ... } },
  { type: { ... }, value: "n", start: 4, end: 5, loc: { ... } },
  ...
]

Each of the types here have a set of properties describing the token:

{
  type: {
    label: 'name',
    keyword: undefined,
    beforeExpr: false,
    startsExpr: true,
    rightAssociative: false,
    isLoop: false,
    isAssign: false,
    prefix: false,
    postfix: false,
    binop: null,
    updateContext: null
  },
  ...
}

Like AST nodes they also have a start, end, and loc.

Syntactic Analysis

Syntactic Analysis will take a stream of tokens and turn it into an AST representation. Using the information in the tokens, this phase will reformat them as an AST which represents the structure of the code in a way that makes it easier to work with.

Transform

The transform stage takes an AST a traverses through it, adding, updating, and removing nodes as it goes along. This is by far the most complex part of Babel or any compiler. This is where plugins operate and so it will be the subject of most of this handbook. So we won't dive too deep right now.

Generate

The code generation stage takes the final AST and turns in back into a string of code, also creating source maps.

Code generation is pretty simple, you traverse through the AST depth-first, building a string that represents the transformed code.

Traversal

When you want to transform an AST you have to traverse the tree recursively.

Say we have the type FunctionDeclaration, it has a few properties: id, params, and body. Each of them have nested nodes.

{
  type: "FunctionDeclaration",
  id: {
    type: "Identifier",
    name: "square"
  },
  params: [{
    type: "Identifier",
    name: "n"
  }],
  body: {
    type: "BlockStatement",
    body: [{
      type: "ReturnStatement",
      argument: {
        type: "BinaryExpression",
        operator: "*",
        left: {
          type: "Identifier",
          name: "n"
        },
        right: {
          type: "Identifier",
          name: "n"
        }
      }
    }]
  }
}

So we start at the FunctionDeclaration and we know its internal properties so we visit each of them and their children in order.

Next we go to id which is an Identifier. Identifiers don't have any child node properties so we move on.

After that is params which is an array of nodes so we visit each of them. In this case it's a single node which is also an Identifier so we move on.

Then we hit body which is a BlockStatement with a property body that is an array of Nodes so we go to each of them.

The only item here is a ReturnStatement node which has an argument, we go to the argument and find a BinaryExpression.

The BinaryExpression has an operator, a left, and a right. The operator isn't a node, just a value, so we don't go to it, and instead just visit left and right.

This traversal process happens throughout the Babel transform stage.

Visitors

When we talk about "going" to a node, we actually mean we are visiting them. The reason we use that term is because there is this concept of a visitor.

Visitors are a pattern used in AST traversal across languages. Simply put they are an object with methods defined for accepting particular node types in a tree. That's a bit abstract so let's look at an example.

const MyVisitor = {
  Identifier() {
    console.log("Called!");
  }
};

Note: Identifier() { ... } is shorthand for Identifier: { enter() { ... } }.

This is a basic visitor that when used during a traversal will call the Identifer() method for every Identifier in the tree.

So with this code the Identifier() method will be called three times with each Identifier.

function square(n) {
  return n * n;
}

Called!
Called!
Called!

These calls are all on node enter. However there is also the possibility of calling a visitor method when on exit.

Imagine we have this tree structure:

- FunctionDeclaration
  - Identifier (id)
  - Identifier (params[0])
  - BlockStatement (body)
    - ReturnStatement (body)
      - BinaryExpression (argument)
        - Identifier (left)
        - Identifier (right)

As we traverse down each branch of the tree we eventually hit dead ends where we need to traverse back up the tree to get to the next node. Going down the tree we enter each node, then going back up we exit each node.

Let's walk through what this process looks like for the above tree.

• Enter FunctionDeclaration
  • Enter Identfier (id)
    • Hit dead end
  • Exit Identifier (id)
  • Enter Identifier (params[0])
    • Hit dead end
  • Exit Identifier (params[0])
  • Enter BlockStatement (body)
    • Enter ReturnStatement (body)
      • Enter BinaryExpression (argument)
        • Enter Identifier (left)
          • Hit dead end
        • Exit Identifier (left)
        • Enter Identifier (right)
          • Hit dead end
        • Exit Identifier (right)
      • Exit BinaryExpression (argument)
    • Exit ReturnStatement (body)
  • Exit BlockStatement (body)
• Exit FunctionDeclaration

So when creating a visitor you have two opportunities to visit a node.

const MyVisitor = {
  Identifier: {
    enter() {
      console.log("Entered!");
    },
    exit() {
      console.log("Exited!");
    }
  }
};

Paths

An AST generally has many Nodes, but how do Nodes relate to one another? We could have one giant mutable object that you manipulate and have full access to, or we can simplify this with Paths.

A Path is an object representation of the link between two nodes.

For example if we take the following node and its child:

{
  type: "FunctionDeclaration",
  id: {
    type: "Identifier",
    name: "square"
  },
  ...
}

And represent the child Identifier as a path, it looks something like this:

{
  "parent": {
    "type": "FunctionDeclaration",
    "id": {...},
    ....
  },
  "node": {
    "type": "Identifier",
    "name": "square"
  }
}

It also has additional metadata about the path:

{
  "parent": {...},
  "node": {...},
  "hub": {...},
  "contexts": [],
  "data": {},
  "shouldSkip": false,
  "shouldStop": false,
  "removed": false,
  "state": null,
  "opts": null,
  "skipKeys": null,
  "parentPath": null,
  "context": null,
  "container": null,
  "listKey": null,
  "inList": false,
  "parentKey": null,
  "key": null,
  "scope": null,
  "type": null,
  "typeAnnotation": null
}

As well as tons and tons of methods related to adding, updating, moving, and removing nodes, but we'll get into those later.

In a sense, paths are a reactive representation of a node's position in the tree and all sorts of information about the node. Whenever you call a method that modifies the tree, this information is updated. Babel manages all of this for you to make working with nodes easy and as stateless as possible.

Paths in Visitors

When you have a visitor that has a Identifier() method, you're actually visiting the path instead of the node. This way you are mostly working with the reactive representation of a node instead of the node itself.

const MyVisitor = {
  Identifier(path) {
    console.log("Visiting: " + path.node.name);
  }
};

a + b + c;

Visiting: a
Visiting: b
Visiting: c

State

State is the enemy of AST transformation. State will bite you over and over again and your assumptions about state will almost always be proven wrong by some syntax that you didn't consider.

Take the following code:

function square(n) {
  return n * n;
}

Let's write a quick hacky visitor that will rename n to x.

let paramName;
 
const MyVisitor = {
  FunctionDeclaration(path) {
    const param = path.node.params[0];
    paramName = param.name;
    param.name = "x";
  },
 
  Identifier(path) {
    if (path.node.name === paramName) {
      path.node.name = "x";
    }
  }
};

This might work for the above code, but we can easily break that by doing this:

function square(n) {
  return n * n;
}
n;

The better way to deal with this is recursion. So let's make like a Christopher Nolan film and put a visitor inside of a visitor.

const updateParamNameVisitor = {
  Identfier(path) {
    if (path.node.name === this.paramName) {
      path.node.name = "x";
    }
  }
};
 
const MyVisitor = {
  FunctionDeclaration(path) {
    const param = path.node.params[0];
    const paramName = param.name;
    param.name = "x";
 
    path.traverse(updateParamNameVisitor, { paramName });
  }
};

Of course, this is a contrived example but it demonstrates how to eliminate global state from your visitors.

Scopes

Next let's introduce the concept of a scope. JavaScript has lexical scoping, which is a tree structure where blocks create new scope.

// global scope
 
function scopeOne() {
  // scope 1
 
  function scopeTwo() {
    // scope 2
  }
}

Whenever you create a reference in JavaScript, whether that be by a variable, function, class, param, import, label, etc., it belongs to the current scope.

var global = "I am in the global scope";
 
function scopeOne() {
  var one = "I am in the scope created by `scopeOne()`";
 
  function scopeTwo() {
    var two = "I am in the scope created by `scopeTwo()`";
  }
}

Code within a deeper scope may use a reference from a higher scope.

function scopeOne() {
  var one = "I am in the scope created by `scopeOne()`";
 
  function scopeTwo() {
    one = "I am updating the reference in `scopeOne` inside `scopeTwo`";
  }
}

A lower scope might also create a reference of the same name without modifying it.

function scopeOne() {
  var one = "I am in the scope created by `scopeOne()`";
 
  function scopeTwo() {
    var one = "I am creating a new `one` but leaving reference in `scopeOne()` alone.";
  }
}

When writing a transform, we want to be wary of scope. We need to make sure we don't break existing code while modifying different parts of it.

We may want to add new references and make sure they don't collide with existing ones. Or maybe we just want to find where a variable is referenced. We want to be able to track these references within a given scope.

A scope can be represented as:

{
  path: path,
  block: path.node,
  parentBlock: path.parent,
  parent: parentScope,
  bindings: [...]
}

When you create a new scope you do so by giving it a path and a parent scope. Then during the traversal process it collects all the references ("bindings") within that scope.

Once that's done, there's all sorts of methods you can use on scopes. We'll get into those later though.

Bindings

References all belong to a particular scope; this relationship is known as a binding.

function scopeOnce() {
  var ref = "This is a binding";
 
  ref; // This is a reference to a binding
 
  function scopeTwo() {
    ref; // This is a reference to a binding from a lower scope
  }
}

A single binding looks like this:

{
  identifier: node,
  scope: scope,
  path: path,
  kind: 'var',
 
  referenced: true,
  references: 3,
  referencePaths: [path, path, path],
 
  constant: false,
  constantViolations: [path]
}

With this information you can find all the references to a binding, see what type of binding it is (parameter, declaration, etc.), lookup what scope it belongs to, or get a copy of its identifier. You can even tell if it's constant and if not, see what paths are causing it to be non-constant.

Being able to tell if a binding is constant is useful for many purposes, the largest of which is minification.

function scopeOne() {
  var ref1 = "This is a constant binding";
 
  becauseNothingEverChangesTheValueOf(ref1);
 
  function scopeTwo() {
    var ref2 = "This is *not* a constant binding";
    ref2 = "Because this changes the value";
  }
}

---

API

Babel is actually a collection of modules. In this section we'll walk through the major ones, explaining what they do and how to use them.

Note: This is not a replacement for detailed API documentation which will be available elsewhere shortly.

babylon

Babylon is Babel's parser. Started as a fork of Acorn, it's fast, simple to use, has plugin-based architecture for non-standard features (as well as future standards).

First, let's install it.

$ npm install --save babylon

Let's start by simply parsing a string of code:

import babylon from "babylon";
 
const code = `function square(n) {
  return n * n;
}`;
 
babylon.parse(code);
// Node {
//   type: "File",
//   start: 0,
//   end: 38,
//   loc: SourceLocation {...},
//   program: Node {...},
//   comments: [],
//   tokens: [...]
// }

We can also pass options to parse() like so:

babylon.parse(code, {
  sourceType: "module",
  plugins: [
    "jsx"
  ]
});

sourceType can either be "module" or "script" which is the mode that Babylon should parse in. "module" will parse in strict mode and allow module declarations, "script" will not.

Since Babylon is built with a plugin-based architecture, there is also a plugins option which will enable the internal plugins. Note that Babylon has not yet opened this API to external plugins, although may do so in the future.

To see a full list of plugins, see the Babylon README.

babel-traverse

The Babel Traverse module maintains the overall tree state, and is responsible for replacing, removing, and adding nodes.

Install it by running:

$ npm install --save babel-traverse

We can use it alongside Babylon to traverse and update nodes:

import babylon from "babylon";
import traverse from "babel-traverse";
 
const code = `function square(n) {
  return n * n;
}`;
 
const ast = babylon.parse(code);
 
traverse(ast, {
  enter(path) {
    if (
      path.node.type === "Identifier" &&
      path.node.name === "n"
    ) {
      path.node.name = "x";
    }
  }
});

babel-types

Babel Types is a Lodash-esque utility library for AST nodes. It contains methods for building, validating, and converting AST nodes. It's useful for cleaning up AST logic with well thought out utility methods.

You can install it by running:

$ npm install --save babel-types

Then start using it:

import traverse from "babel-traverse";
import t from "babel-types";
 
traverse(ast, {
  enter(path) {
    if (t.isIdentifier(path.node, { name: "n" })) {
      path.node.name = "x";
    }
  }
});

Definitions

Babel Types has definitions for every single type of node, with information on what properties belong where, what values are valid, how to build that node, how the node should be traversed, and aliases of the Node.

A single node type definition looks like this:

defineType("BinaryExpression", {
  builder: ["operator", "left", "right"],
  fields: {
    operator: {
      validate: assertValueType("string")
    },
    left: {
      validate: assertNodeType("Expression")
    },
    right: {
      validate: assertNodeType("Expression")
    }
  },
  visitor: ["left", "right"],
  aliases: ["Binary", "Expression"]
});

Builders

You'll notice the above definition for BinaryExpression has a field for a builder.

builder: ["operator", "left", "right"]

This is because each node type gets a builder method, which when used looks like this:

t.binaryExpression("*", t.identifier("a"), t.identifier("b"));

Which creates an AST like this:

{
  type: "BinaryExpression",
  operator: "*",
  left: {
    type: "Identifier",
    name: "a"
  },
  right: {
    type: "Identifier",
    name: "b"
  }
}

Which when printed looks like this:

a * b

Builders will also validate the nodes they are creating and throw descriptive errors if used improperly. Which leads into the next type of method.

Validators

The definition for BinaryExpression also includes information on the fields of a node and how to validate them.

fields: {
  operator: {
    validate: assertValueType("string")
  },
  left: {
    validate: assertNodeType("Expression")
  },
  right: {
    validate: assertNodeType("Expression")
  }
}

This is used to create two types of validating methods. The first of which is isX.

t.isBinaryExpression(maybeBinaryExpressionNode);

This tests to make sure that the node is a binary expression, but you can also pass a second parameter to ensure that the node contains certain properties and values.

t.isBinaryExpression(maybeBinaryExpressionNode, { operator: "*" });

There is also the more, ehem, assertive version of these methods, which will throw errors instead of returning true or false.

t.assertBinaryExpression(maybeBinaryExpressionNode);
t.assertBinaryExpression(maybeBinaryExpressionNode, { operator: "*" });
// Error: Expected type "BinaryExpression" with option { "operator": "*" }

Converters

[WIP]

babel-generator

Babel Generator is the code generator for Babel. It takes an AST and turns it into code with sourcemaps.

Run the following to install it:

$ npm install --save babel-generator

Then use it

import babylon from "babylon";
import generate from "babel-generator";
 
const code = `function square(n) {
  return n * n;
}`;
 
const ast = babylon.parse(code);
 
generate(ast, null, code);
// {
//   code: "...",
//   map: "..."
// }

You can also pass options to generate().

generate(ast, {
  retainLines: false,
  compact: "auto",
  concise: false,
  quotes: "double",
  // ...
}, code);

babel-template

Babel Template is another tiny but incredibly useful module. It allows you to write strings of code with placeholders that you can use instead of manually building up a massive AST.

$ npm install --save babel-template

import template from "babel-template";
import generate from "babel-generator";
import * as t from "babel-types";
 
const buildRequire = template(`
  var IMPORT_NAME = require(SOURCE);
`);
 
const ast = buildRequire({
  IMPORT_NAME: t.identifier("myModule"),
  SOURCE: t.stringLiteral("my-module")
});
 
console.log(generate(ast).code);

var myModule = require("my-module");

Writing your first Babel Plugin

Now that you're familiar with all the basics of Babel, let's tie it together with the plugin API.

Start off with a function that gets passed the current babel object.

export default function(babel) {
  // plugin contents
}

Since you'll be using it so often, you'll likely want to grab just babel.types like so:

export default function({ types: t }) {
  // plugin contents
}

Then you return an object with a property visitor which is the primary visitor for the plugin.

export default function({ types: t }) {
  return {
    visitor: {
      // visitor contents
    }
  };
};

Let's write a quick plugin to show off how it works. Here's our source code:

foo === bar;

Or in AST form:

{
  type: "BinaryExpression",
  operator: "===",
  left: {
    type: "Identifier",
    name: "foo"
  },
  right: {
    type: "Identifier",
    name: "bar"
  }
}

We'll start off by adding a BinaryExpression visitor method.

export default function({ types: t }) {
  return {
    visitor: {
      BinaryExpression(path) {
        // ...
      }
    }
  };
}

Then let's narrow it down to just BinaryExpressions that are using the === operator.

visitor: {
  BinaryExpression(path) {
    if (path.node.operator !== "===") {
      return;
    }
 
    // ...
  }
}

Now let's replace the left property with a new identifier:

BinaryExpression(path) {
  if (path.node.operator !== "===") {
    return;
  }
 
  path.node.left = t.identifier("sebmck");
  // ...
}

Already if we run this plugin we would get:

sebmck === bar;

Now let's just replace the right property.

BinaryExpression(path) {
  if (path.node.operator !== "===") {
    return;
  }
 
  path.node.left = t.identifier("sebmck");
  path.node.right = t.identifier("dork");
}

And now for our final result:

sebmck === dork;

Awesome! Our very first Babel plugin.

---

Transformation Operations

Visiting

Check if a node is a certain type

If you want to check what the type of a node is, the preferred way to do so is:

BinaryExpression(path) {
  if (t.isIdentifier(path.node.left)) {
    // ...
  }
}

You can also do a shallow check for properties on that node:

BinaryExpression(path) {
  if (t.isIdentifier(path.node.left, { name: "n" })) {
    // ...
  }
}

This is functionally equivalent to:

BinaryExpression(path) {
  if (
    path.node.left != null &&
    path.node.left.type === "Identifier" &&
    path.node.left.name === "n"
  ) {
    // ...
  }
}

Check if an identifier is referenced

Identifier(path) {
  if (path.isReferencedIdentifier()) {
    // ...
  }
}

Alternatively:

Identifier(path) {
  if (t.isReferenced(path.node, path.parent)) {
    // ...
  }
}

Manipulation

Replacing a node

BinaryExpression(path) {
  path.replaceWith(
    t.binaryExpression("**", path.node.left, t.numberLiteral(2))
  );
}

  function square(n) {
-   return n * n;
+   return n ** 2;
  }

Replacing a node with multiple nodes

ReturnStatement(path) {
  path.replaceWithMultiple([
    t.expressionStatement(t.stringLiteral("Is this the real life?")),
    t.expressionStatement(t.stringLiteral("Is this just fantasy?")),
    t.expressionStatement(t.stringLiteral("(Enjoy singing the rest of the song in your head)")),
  ]);
}

  function square(n) {
-   return n * n;
+   "Is this the real life?";
+   "Is this just fantasy?";
+   "(Enjoy singing the rest of the song in your head)";
  }

Note: When replacing an expression with multiple nodes, they must be statements. This is because Babel uses heuristics extensively when replacing nodes which means that you can do some pretty crazy transformations that would be extremely verbose otherwise.

Replacing a node with a source string

FunctionDeclaration(path) {
  path.replaceWithSourceString(`function add(a, b) {
    return a + b;
  }`);
}

- function square(n) {
-   return n * n;
+ function add(a, b) {
+   return a + b;
  }

Note: It's not recommended to use this API unless you're dealing with dynamic source strings, otherwise it's more efficient to parse the code outside of the visitor.

Inserting a sibling node

FunctionDeclaration(path) {
  path.insertBefore(t.expressionStatement(t.stringLiteral("Because I'm easy come, easy go.")));
  path.insertAfter(t.expressionStatement(t.stringLiteral("A little high, little low.")));
}

+ "Because I'm easy come, easy go.";
  function square(n) {
    return n * n;
  }
+ "A little high, little low.";

Note: This should always be a statement or an array of statements. This uses the same heuristics mentioned in Replacing a node with multiple nodes.

Removing a node

FunctionDeclaration(path) {
  path.remove();
}

- function square(n) {
-   return n * n;
- }

Replacing a parent

BinaryExpression(path) {
  path.parentPath.replaceWith(
    t.expressionStatement(t.stringLiteral("Anyway the wind blows, doesn't really matter to me, to me."))
  );
}

  function square(n) {
-   return n * n;
+   "Anyway the wind blows, doesn't really matter to me, to me.";
  }

Removing a parent

BinaryExpression(path) {
  path.parentPath.remove();
}

  function square(n) {
-   return n * n;
  }

Scope

Checking if a local variable is bound

FunctionDeclaration(path) {
  if (path.scope.hasBinding("n")) {
    // ...
  }
}

This will walk up the scope tree and check for that particular binding.

You can also check if a scope has its own binding:

FunctionDeclaration(path) {
  if (path.scope.hasOwnBinding("n")) {
    // ...
  }
}

Generating a UID

This will generate an identifier that doesn't collide with any locally defined variables.

FunctionDeclaration(path) {
  path.scope.generateUidIdentifier("uid");
  // Node { type: "Identifier", name: "_uid" }
  path.scope.generateUidIdentifier("uid");
  // Node { type: "Identifier", name: "_uid2" }
}

Pushing a variable declaration to a parent scope

Sometimes you may want to push a VariableDeclaration so you can assign to it.

FunctionDeclaration(path) {
  const id = path.scope.generateUidIdentifierBasedOnNode(path.node.id);
  path.remove();
  scope.parent.push({ id, init: path.node });
}

- function square(n) {
+ var _square = function square(n) {
    return n * n;
- }
+ };

Rename a binding and its references

FunctionDeclaration(path) {
  path.scope.rename("n", "x");
}

- function square(n) {
-   return n * n;
+ function square(x) {
+   return x * x;
  }

Alternatively, you can rename a binding to a generated unique identifier:

FunctionDeclaration(path) {
  path.scope.rename("n");
}

- function square(n) {
-   return n * n;
+ function square(_n) {
+   return _n * _n;
  }

---

Best Practices

I'll be working on this section over the coming weeks.