abstract-syntax-tree

abstract-syntax-tree

A library for working with abstract syntax trees.

Table of Contents

• Background
• Install
• Usage
• API
• REPL
• Maintainers
• Contributing
• License

Background

An abstract syntax tree is a way to represent the source code. In case of this library it is represented in the estree format.

For example, the following source code:

const answer = 42

Has the following representation:

{
  "type": "Program",
  "body": [
    {
      "type": "VariableDeclaration",
      "declarations": [
        {
          "type": "VariableDeclarator",
          "id": {
            "type": "Identifier",
            "name": "answer"
          },
          "init": {
            "type": "Literal",
            "value": 42
          }
        }
      ],
      "kind": "const"
    }
  ]
}

The goal of this library is to consolidate common abstract syntax tree operations in one place. It uses a variety of libriaries under the hood based on their performance and flexibility, e.g. meriyah for parsing and astring for source code generation.

The library exposes a set of utility methods that can be useful for analysis or transformation of abstract syntax trees. It supports functional and object-oriented programming style.

Install

npm install abstract-syntax-tree

Usage

const { parse, find } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(find(tree, 'Literal')) // [ { type: 'Literal', value: 42 } ]

const AbstractSyntaxTree = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = new AbstractSyntaxTree(source)
console.log(tree.find('Literal')) // [ { type: 'Literal', value: 42 } ]

API

Static Methods

parse

The library uses meriyah to create an estree compatible abstract syntax tree. All meriyah parsing options can be passed to the parse method.

const { parse } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(tree) // { type: 'Program', body: [ ... ] }

const { parse } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source, {
  loc: true,
  ranges: true
})
console.log(tree) // { type: 'Program', body: [ ... ], loc: {...} }

generate

The library uses astring to generate the source code. All astring generate options can be passed to the generate method.

const { parse, generate } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(generate(tree)) // 'const answer = 42;'

walk

Walk method is a thin layer over estraverse.

const { parse, walk } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
walk(tree, (node, parent) => {
  console.log(node)
  console.log(parent)
})

find

Find supports two traversal methods. You can pass an esquery compatible selector or pass an object that will be compared to every node in the tree. The method returns an array of nodes.

const { parse, find } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(find(tree, 'VariableDeclaration')) // [ { type: 'VariableDeclaration', ... } ]
console.log(find(tree, { type: 'VariableDeclaration' })) // [ { type: 'VariableDeclaration', ... } ]

traverse

Traverse method accepts a configuration object with enter and leave callbacks. It allows multiple transformations in one traversal.

const { parse, traverse } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
traverse(tree, {
  enter (node) {},
  leave (node) {}
})

replace

Replace extends estraverse by handling replacement of give node with multiple nodes. It will also remove given node if null is returned.

const { parse, replace } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
replace(tree, node => {
  if (node.type === 'VariableDeclaration') {
    node.kind = 'let'
  }
  return node
})

remove

Remove uses estraverse and ensures that no useless nodes are left in the tree. It accepts a string, object or callback as the matching strategy.

const { parse, remove, generate } = require('abstract-syntax-tree')
const source = '"use strict"; const b = 4;'
const tree = parse(source)
remove(tree, 'Literal[value="use strict"]')

// or
// remove(tree, { type: 'Literal', value: 'use strict' })

// or
// remove(tree, (node) => {
//   if (node.type === 'Literal' && node.value === 'use strict') return null
//   return node 
// })

console.log(generate(tree)) // 'const b = 4;'

each

const { parse, each } = require('abstract-syntax-tree')
const source = 'const foo = 1; const bar = 2;'
const tree = parse(source)
each(tree, 'VariableDeclaration', node => {
  console.log(node)
})

first

const { parse, first } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(first(tree, 'VariableDeclaration')) // { type: 'VariableDeclaration', ... }

last

const { parse, last } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(last(tree, 'VariableDeclaration')) // { type: 'VariableDeclaration', ... }

reduce

const { parse, reduce } = require('abstract-syntax-tree')
const source = 'const a = 1, b = 2'
const tree = parse(source)
const value = reduce(tree, (sum, node) => {
  if (node.type === 'Literal') {
    sum += node.value
  }
  return sum
}, 0)
console.log(value) // 3

has

const { parse, has } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(has(tree, 'VariableDeclaration')) // true
console.log(has(tree, { type: 'VariableDeclaration' })) // true

count

const { parse, count } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
console.log(count(tree, 'VariableDeclaration')) // 1
console.log(count(tree, { type: 'VariableDeclaration' })) // 1

append

Append pushes nodes to the body of the abstract syntax tree. It accepts estree nodes as input.

const { parse, append } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
append(tree, {
  type: 'ExpressionStatement',
  expression:  {
    type: "CallExpression",
    callee: {
      type: 'MemberExpression',
      object: {
        type: 'Identifier',
        name: 'console'
      },
      property: {
        type: 'Identifier',
        name: 'log'
      },
      computed: false
    },
    arguments: [
      {
        type: 'Identifier',
        name: 'answer'
      }
    ]
  }
})

Strings will be converted into abstract syntax tree under the hood. Please note that this approach might make the code run a bit slower due to an extra interpretation step.

const { parse, append } = require('abstract-syntax-tree')
const source = 'const answer = 42'
const tree = parse(source)
append(tree, 'console.log(answer)')

prepend

Prepend unshifts nodes to the body of the abstract syntax tree. Accepts estree nodes or strings as input, same as append.

const { parse, prepend } = require('abstract-syntax-tree')
const source = 'const a = 1;'
const tree = parse(source)
prepend(tree, {
  type: 'ExpressionStatement',
  expression: {
    type: 'Literal',
    value: 'use strict'
  }
})

equal

const { equal } = require('abstract-syntax-tree')
console.log(equal({ type: 'Literal', value: 42 }, { type: 'Literal', value: 42 })) // true
console.log(equal({ type: 'Literal', value: 41 }, { type: 'Literal', value: 42 })) // false

template

const { template } = require('abstract-syntax-tree')
const literal = template(42)
const nodes = template('const foo = <%= bar %>;', { bar: { type: 'Literal', value: 1 } })

Instance Methods

Almost all of the static methods (excluding parse, generate, template and equal) have their instance equivalents. There are few extra instance methods:

mark

const AbstractSyntaxTree = require('abstract-syntax-tree')
const tree = new AbstractSyntaxTree('const a = 1')
tree.mark()
console.log(tree.first('Program').cid) // 1
console.log(tree.first('VariableDeclaration').cid) // 2

wrap

const AbstractSyntaxTree = require('abstract-syntax-tree')
const source = 'const a = 1'
const tree = new AbstractSyntaxTree(source)
tree.wrap(body => {
    return [
      {
        type: 'ExpressionStatement',
        expression: {
          type: 'CallExpression',
          callee: {
            type: 'FunctionExpression',
            params: [],
            body: {
              type: 'BlockStatement',
              body
            }
          },
          arguments: []
        }
      }
    ]
})

unwrap

const AbstractSyntaxTree = require('abstract-syntax-tree')
const source = '(function () { console.log(1); }())'
const tree = new AbstractSyntaxTree(source)
tree.unwrap()
console.log(tree.source) // console.log(1);

Getters

body

Gives the body of the root node.

source

Gives access to the source code representation of the abstract syntax tree.

const AbstractSyntaxTree = require('abstract-syntax-tree')
const source = 'const foo = "bar";'
const tree = new AbstractSyntaxTree(source)
console.log(tree.source) // const foo = "bar";

map

Gives the source map of the source code.

Setters

body

Sets the body of the root node.

Maintainers

@emilos.

Contributing

All contributions are highly appreciated! Open an issue or a submit PR.

The lib follows the tdd approach and is expected to have a high code coverage. Please follow the Contributor Covenant Code of Conduct.

License

MIT © buxlabs