Beginner's Guide to ASTs

Kent C. Dodds

Utah

1 wife, 3.5 kids

PayPal, Inc.

@kentcdodds

Please Stand...

... if you're physically able

What this talk is

  • Basic introduction to the concept of ASTs
  • (sorta) Practical demo of how to use ASTs with Babel and ESLint
  • Something to get you totally psyched about using ASTs

What this talk is not

  • A deep dive into parsing, tokenizing, and lexing
  • Everything you need to know to be productive with ASTs

Let's
Get
STARTED!

Why

Should I care?

ESNext

ESNow

ESLint

eslint-plugin-import

Codemods

better than find/replace

no matter how good your regex skills are...

What

Is an AST?

Code for humans

const uniqueRandomArray = require('unique-random-array')

const transformersNames = require('./transformers-names.json')
const getRandomItem = uniqueRandomArray(transformersNames)

module.exports = {
  all: transformersNames,
  random: random,
}

function random(number) {
  if (number === undefined) {
    return getRandomItem()
  } else {
    const randomItems = []
    for (let i = 0; i < number; i++) {
      randomItems.push(getRandomItem())
    }
    return randomItems
  }
}

Code for computers

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    {
      "type": {
        "label": "}",
        "beforeExpr": false,
        "startsExpr": false,
        "rightAssociative": false,
        "isLoop": false,
        "isAssign": false,
        "prefix": false,
        "postfix": false,
        "binop": null
      },
      "start": 478,
      "end": 479,
      "loc": {
        "start": {
          "line": 20,
          "column": 2
        },
        "end": {
          "line": 20,
          "column": 3
        }
      }
    },
    {
      "type": {
        "label": "}",
        "beforeExpr": false,
        "startsExpr": false,
        "rightAssociative": false,
        "isLoop": false,
        "isAssign": false,
        "prefix": false,
        "postfix": false,
        "binop": null
      },
      "start": 480,
      "end": 481,
      "loc": {
        "start": {
          "line": 21,
          "column": 0
        },
        "end": {
          "line": 21,
          "column": 1
        }
      }
    },
    {
      "type": {
        "label": "eof",
        "beforeExpr": false,
        "startsExpr": false,
        "rightAssociative": false,
        "isLoop": false,
        "isAssign": false,
        "prefix": false,
        "postfix": false,
        "binop": null,
        "updateContext": null
      },
      "start": 482,
      "end": 482,
      "loc": {
        "start": {
          "line": 22,
          "column": 0
        },
        "end": {
          "line": 22,
          "column": 0
        }
      }
    }
  ]
}

Code for computers

For humans...

Exploring ASTs

For humans, for computers, for humans

How

Do I use them?

Let's try it out!

  1. ESLint Plugin: setup / complete
  2. Babel Plugin: setup / complete
  3. Codemod: setup / complete

Resources

Thank you!

A beginner's guide to ASTs

By Kent C. Dodds

A beginner's guide to ASTs

The Abstract Syntax Tree. It sounds a lot worse than it is. It’s actually quite simple and enables some powerful tools. BabelJS uses it to transform your code from ES.Next to ES5. ESLint uses it to lint your code. And with a knowledge of how it works, you can extend these and other tools to do some mind bustingly powerful things. Prepare to be amazed by ASTs!

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