Course overview

Introduction

What is an Abstract Syntax Tree? Visualizing Code Like a Compiler

How to View Abstract Syntax Tree Code With AST Explorer

The Best JavaScript AST Tools - ESLint, Babel, Terser, and More

Environment setup

Understanding Abstract Syntax Trees (AST)

How to Generate a JavaScript AST With Babel Plugins

Traversing an AST With Babel Traverse

Add Type Safety and Prevent Runtime Errors With AST

Working with Abstract Syntax Trees

Practical code audits

How to Audit Your Code With AST Programming

How to Add a Component and Update an AST

When to Use Abstract Syntax Tree Tooling to Refactor at Scale

Code Audits

How to Mutate an AST and Automatically Replace Code Components

How to Create Codemods Using jscodeshift

How to Unit Test jscodeshift Transforms With ts-jest

Codemods

How to Ensure Codebase is Up to Date With Linting Rules

How to Use ESLint to Create AST Rules With Babel Traverse

How to Test Custom ESLint Linting Rules With AST

Linting

node.js

In this course, we'll start with the fundamentals of abstract syntax trees (ASTs) and learn the basic mental models. This general AST knowledge can be translated to almost any tool that works with ASTs.

## Why this course?

Understanding and using ASTs unlocks the ability to make sweeping changes in a safe and reliable way in any size codebase.

## Course topics

Throughout this course, we'll have converted source code into ASTs, traversed, mutated, and generated ASTs. With these concepts we'll then explore several practical applications including things like code audits (static analysis), code transformations (codemods), and linting.

### Module 1

We'll learn the fundamentals of abstract syntax trees. 

- What is an AST?
- How to explore an AST
- Examples of JavaScript tools that work with ASTs

### Module 2

We'll learn how to work with ASTs.

- How to turn code into an AST
- How to programmatically navigate any AST 
- How to leverage TypeScript to prevent runtime errors

### Module 3

We'll learn how to statically analyze, or "audit" code to understand the state of the codebase using abstract syntax trees.

- An introduction to an example codebase and refactor
- Understanding the state of the current codebase
- When to use an AST-based tool versus doing something manually

### Module 4

We'll learn how to transform, or "codemod" code from one state to another using abstract syntax trees.

- How to make changes to an AST
- How to change ASTs with [jscodeshift](https://github.com/facebook/jscodeshift)
- How to test a code transform

### Module 5

We'll learn how to write rules, or "lint" code using abstract syntax trees.

- How to create rules for code
- How to create custom rules with [ESLint](https://eslint.org)
- How to test a rule

In this course, you'll learn the fundamentals of abstract syntax trees, what they are, how they work, and dive into several practical use cases of abstract syntax trees to maintain a JavaScript codebase.

Practical Abstract Syntax Trees

JavaScript tools that work with abstract syntax trees

How to parse, traverse, and generate abstract syntax trees

Practical skill set for maintaining large JavaScript codebases

Manually exploring an AST is a helpful exercise in understanding the relevant subtree and its nodes which represent a given piece of code. Understanding the relevant nodes is the first step to being able to automatically or programmatically traverse an AST.

 is one of the simplest ways to begin exploring real ASTs. It provides a UI on top of many tools, and a way to interactively explore ASTs. This helps make ASTs more digestible, by highlighting the relationship between the source code and specific nodes of the AST.

 and paste in the example snippet of code in the code pane on the left.

There are a few views and settings AST Explorer provides. First, there are the 

 settings. For this course, the language can always be set to 

 because all of the example code we're parsing is JavaScript.

 option will need to be changed throughout the course depending on the exact tool being looked at. For now, using 

 is a good default, and many of the first lessons will use this as the parser. This setting controls which parser is used to generate the AST. The final AST can differ depending on which parser is selected.

---
title: How to View Abstract Syntax Tree Code With AST Explorer
description: Looking at an example code snippet and how it relates to the resulting AST
privateVideoUrl: "https://fullstack.wistia.com/medias/bfyntxhslo"
---

# Exploring an Abstract Syntax Tree

Manually exploring an AST is a helpful exercise in understanding the relevant subtree and its nodes which represent a given piece of code. Understanding the relevant nodes is the first step to being able to automatically or programmatically traverse an AST.

## AST Explorer

[AST Explorer](https://astexplorer.net) is one of the simplest ways to begin exploring real ASTs. It provides a UI on top of many tools, and a way to interactively explore ASTs. This helps make ASTs more digestible, by highlighting the relationship between the source code and specific nodes of the AST.

![Screenshot of astexplorer.net](https://d2uusema5elisf.cloudfront.net/courses/practical-abstract-syntax-trees/module_01/lesson_01.02/public/assets/ast-explorer-net.png)

To give this a try, go to [AST Explorer](https://astexplorer.net) and paste in the example snippet of code in the code pane on the left.

```javascript { actualFilename=./protected/src/example.js endChar=12 endLine=1 startChar=0 startLine=1 statedFilename=example-code.js url=https&#x3A;//gitlab.com/fullstackio/books/practical-ast-and-codemods/module_01/lesson_01.02/protected/src/example.js }
2 + (4 * 10)
```

There are a few views and settings AST Explorer provides. First, there are the **language** and **parser** settings. For this course, the language can always be set to **JavaScript** because all of the example code we're parsing is JavaScript.

![Screenshot of astexplorer.net language and parser](https://d2uusema5elisf.cloudfront.net/courses/practical-abstract-syntax-trees/module_01/lesson_01.02/public/assets/ast-explorer-net-language-parser.png)

The **parser** option will need to be changed throughout the course depending on the exact tool being looked at. For now, using **`@babel/parser`** is a good default, and many of the first lessons will use this as the parser. This setting controls which parser is used to generate the AST. The final AST can differ depending on which parser is selected.

![Screenshot of astexplorer.net parsers](https://d2uusema5elisf.cloudfront.net/courses/practical-abstract-syntax-trees/module_01/lesson_01.02/public/assets/ast-explorer-net-parsers.png)

Looking at an example code snippet and how it relates to the resulting AST

Learn

The newline Guide to Building Your First GraphQL Server with Node and TypeScript

Teach

Amelia Wattenberger

Author of Fullstack D3

Community

Practical Abstract Syntax Trees

Module 1: Introduction

Introduction:

Start Here

Introduction: (2:27)

Course overview

Module 2: Understanding Abstract Syntax Trees (AST)

Lesson 1: (5:55)

What is an Abstract Syntax Tree? Visualizing Code Like a Compiler

Lesson 2: (4:21)

How to View Abstract Syntax Tree Code With AST Explorer

Lesson 3: (1:46)

The Best JavaScript AST Tools - ESLint, Babel, Terser, and More

Lesson 4: (1:03)

Environment setup

Module 3: Working with Abstract Syntax Trees

Lesson 1: (6:29)

How to Generate a JavaScript AST With Babel Plugins

Lesson 2: (7:33)

Traversing an AST With Babel Traverse

Lesson 3: (9:31)

Add Type Safety and Prevent Runtime Errors With AST

Module 4: Code Audits

Lesson 1: (2:35)

Practical code audits

Lesson 2: (15:29)

How to Audit Your Code With AST Programming

Lesson 3: (2:32)

How to Add a Component and Update an AST

Lesson 4: (1:43)

When to Use Abstract Syntax Tree Tooling to Refactor at Scale

Module 5: Codemods

Lesson 1: (21:13)

How to Mutate an AST and Automatically Replace Code Components

Lesson 2: (10:03)

How to Create Codemods Using jscodeshift

Lesson 3: (5:55)

How to Unit Test jscodeshift Transforms With ts-jest

Module 6: Linting

Lesson 1: (8:44)

How to Ensure Codebase is Up to Date With Linting Rules

Lesson 2: (5:31)

How to Use ESLint to Create AST Rules With Babel Traverse

Lesson 3: (4:31)

How to Test Custom ESLint Linting Rules With AST

How to View Abstract Syntax Tree Code With AST Explorer