abstract syntax tree
npm install abstract-syntax-tree 
> A library for working with abstract syntax trees.
- Background
- Install
- Usage
- API
- REPL
- Nodes
- Optimizations
- Maintainers
- Contributing
- License
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:
``js`
const answer = 42
Has the following representation:
`json`
{
"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 libraries 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.
`bash`
npm install abstract-syntax-tree
`js`
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 } ]
`js`
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 } ]
#### parse
The library uses meriyah to create an estree compatible abstract syntax tree. All meriyah parsing options can be passed to the parse method.
`js`
const { parse } = require("abstract-syntax-tree")
const source = "const answer = 42"
const tree = parse(source)
console.log(tree) // { type: 'Program', body: [ ... ] }
`js`
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.
`js`
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.
`js`
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 a string selector or pass an object that will be compared to every node in the tree. The method returns an array of nodes.
The following selectors are supported:
- node type (Identifier)[name="foo"]
- node attribute ()[name]
- node attribute existence ()*
- wildcard ()
`js`
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', ... } ]
#### serialize
Serialize can transform nodes into values. Works for: Array, Boolean, Error, Infinity, Map, NaN, Number, Object, RegExp, Set, String, Symbol, WeakMap, WeakSet, null and undefined.
`js`
const { serialize } = require("abstract-syntax-tree")
const node = {
type: "ArrayExpression",
elements: [
{ type: "Literal", value: 1 },
{ type: "Literal", value: 2 },
{ type: "Literal", value: 3 },
{ type: "Literal", value: 4 },
{ type: "Literal", value: 5 },
],
}
const array = serialize(node) // [1, 2, 3, 4, 5]
#### traverse
Traverse method accepts a configuration object with enter and leave callbacks. It allows multiple transformations in one traversal.
`js`
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.
`js`
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.
`js
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
`js`
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
`js`
const { parse, first } = require("abstract-syntax-tree")
const source = "const answer = 42"
const tree = parse(source)
console.log(first(tree, "VariableDeclaration")) // { type: 'VariableDeclaration', ... }
#### last
`js`
const { parse, last } = require("abstract-syntax-tree")
const source = "const answer = 42"
const tree = parse(source)
console.log(last(tree, "VariableDeclaration")) // { type: 'VariableDeclaration', ... }
#### reduce
`js`
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
`js`
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
`js`
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.
`js`
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.
`js`
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.
`js`
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
`js`
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
#### match
`js`
const { match } = require("abstract-syntax-tree")
console.log(match({ type: "Literal", value: 42 }, "Literal[value=42]")) // true
console.log(match({ type: "Literal", value: 41 }, "Literal[value=42]")) // false
#### template
The function converts the input to an equivalent abstract syntax tree representation. The function uses a <%= %> syntax to insert nodes.
You can also use standard javascript types and they're going to be transformed automatically.
`js`
const { template } = require("abstract-syntax-tree")
const literal = template(42)
const nodes = template("const foo = <%= bar %>;", {
bar: { type: "Literal", value: 1 },
})
`js`
const { template } = require("abstract-syntax-tree")
const nodes = template("function foo(<%= bar %>) {}", {
bar: [
{ type: "Identifier", name: "baz" },
{ type: "Identifier", name: "qux" },
],
})
`js`
const { template } = require("abstract-syntax-tree")
const literal = template(42)
const nodes = template("const foo = <%= bar %>;", {
bar: 1,
})
#### program
Creates an abstract syntax tree with a blank program.
`js`
const { program } = require("abstract-syntax-tree")
const tree = program() // { type: 'Program', sourceType: 'module', body: [] }
#### iife
Creates an abstract syntax tree for an immediately invoked function expression.
`js`
const { iife } = require("abstract-syntax-tree")
const node = iife() // { type: 'ExpressionStatement', expression: { ... } }
Almost all of the static methods (excluding parse, generate, template and match) have their instance equivalents. There are few extra instance methods:
#### mark
`js`
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
`js`
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
`js`
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);
#### body
Gives the body of the root node.
#### source
Gives access to the source code representation of the abstract syntax tree.
`js`
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.
#### body
Sets the body of the root node.
#### toBinaryExpression
`js`
const { toBinaryExpression } = require("abstract-syntax-tree")
const expression = {
type: "ArrayExpression",
elements: [
{ type: "Literal", value: "foo" },
{ type: "Literal", value: "bar" },
{ type: "Literal", value: "baz" },
],
}
console.log(toBinaryExpression(expression)) // { type: 'BinaryExpression', ... }
You can also use classes to create nodes.
`js`
const { ArrayExpression, Literal } = require("abstract-syntax-tree")
const expression = new ArrayExpression([
new Literal("foo"),
new Literal("bar"),
new Literal("baz"),
])
Here's a list of all available nodes, with examples.
| Type | Example |
| ------------------------ | :--------------------------------------------------------------: |
| ArrayExpression | const foo = [] |
| ArrayPattern | const [foo, bar] = bar |
| ArrowFunctionExpression | (() => {}) |
| AssignmentExpression | foo = bar |
| AssignmentOperator | |
| AssignmentPattern | function foo(bar = baz) {} |
| AwaitExpression | (async () => { await foo() })() |
| BigIntLiteral | |
| BinaryExpression | foo + bar |
| BinaryOperator | |
| BlockStatement | { console.log(foo) } |
| BreakStatement | for (foo in bar) break |
| CallExpression | foo() |
| CatchClause | try {} catch (error) {} |
| ChainElement | |
| ChainExpression | foo?.() |
| Class | |
| ClassBody | class Foo {} |
| ClassDeclaration | class Foo {} |
| ClassExpression | (class {}) |
| ConditionalExpression | foo ? bar : baz |
| ContinueStatement | while(true) { continue } |
| DebuggerStatement | debugger |
| Declaration | |
| Directive | |
| DoWhileStatement | do {} while (true) {} |
| EmptyStatement | ; |
| ExportAllDeclaration | export \* from "foo" |
| ExportDefaultDeclaration | export default foo |
| ExportNamedDeclaration | export { foo as bar } |
| ExportSpecifier | export { foo } |
| Expression | |
| ExpressionStatement | foo |
| ForInStatement | for (foo in bar) {} |
| ForOfStatement | for (foo of bar) {} |
| ForStatement | for (let i = 0; i < 10; i ++) {} |
| Function | |
| FunctionBody | |
| FunctionDeclaration | function foo () {} |
| FunctionExpression | (function () {}) |
| Identifier | foo |
| IfStatement | if (foo) {} |
| ImportDeclaration | import "foo" |
| ImportDefaultSpecifier | import foo from "bar" |
| ImportExpression | import(foo).then(bar) |
| ImportNamespaceSpecifier | import \* as foo from "bar" |
| ImportSpecifier | import { foo } from "bar" |
| LabeledStatement | label: foo |
| Literal | 42 |
| LogicalExpression | true && false |
| LogicalOperator | |
| MemberExpression | foo.bar |
| MetaProperty | function foo () { new.target } |
| MethodDefinition | class Foo { bar() {} } |
| ModuleDeclaration | |
| ModuleSpecifier | |
| NewExpression | new Foo() |
| Node | |
| ObjectExpression | ({}) |
| ObjectPattern | function foo ({}) {} |
| Pattern | |
| Position | |
| Program | 42 |
| Property | |
| RegExpLiteral | |
| RestElement | function foo (...bar) {} |
| ReturnStatement | function foo () { return bar } |
| SequenceExpression | foo, bar |
| SourceLocation | |
| SpreadElement | |
| Statement | |
| Super | class Foo extends Bar { constructor() { super() } } |
| SwitchCase | switch (foo) { case 'bar': } |
| SwitchStatement | switch(foo) {} |
| TaggedTemplateExpression | css.foo { color: red; } |
| TemplateLiteral | css.foo { color: red; } |
| ThisExpression | this.foo = 'bar' |
| ThrowStatement | throw new Error("foo") |
| TryStatement | try { foo() } catch (exception) { bar() } |
| UnaryExpression | !foo |
| UnaryOperator | |
| UpdateExpression | foo++ |
| UpdateOperator | |
| VariableDeclaration | const answer = 42 |
| VariableDeclarator | const foo = 'bar' |
| WhileStatement | while (true) {} |
| WithStatement | with (foo) {} |
| YieldExpression | function\* foo() { yield bar } |
Abstract syntax tree is a tree-like structure that represents your program. The program is interpreted at some point, e.g. in your browser. Everything takes time, and the same applies to the interpretation. Some of the operations, e.g. adding numbers can be done at compile time, so that the interpreter has less work to do. Having less work to do means that your program will run faster.
`js`
const { binaryExpressionReduction } = require("abstract-syntax-tree")
#### binaryExpressionReduction
`js`
const number = 2 + 2
In the example above we have added two numbers. We could optimize the code by:
`js`
const number = 4
The tree would be translated from:
`json`
{
"type": "BinaryExpression",
"operator": "+",
"left": { "type": "Literal", "value": 2 },
"right": { "type": "Literal", "value": 2 }
}
to
`json`
{ "type": "Literal", "value": 4 }
#### ifStatementRemoval
`js`
if (true) {
console.log("foo")
} else {
console.log("bar")
}
It seems that we'll only enter the true path. We can simplify the code to:
`js`
console.log("foo")
The tree would be translated from:
`json`
{
"type": "IfStatement",
"test": {
"type": "Literal",
"value": true
},
"consequent": {
"type": "BlockStatement",
"body": [
{
"type": "ExpressionStatement",
"expression": {
"type": "CallExpression",
"callee": {
"type": "MemberExpression",
"object": {
"type": "Identifier",
"name": "console"
},
"property": {
"type": "Identifier",
"name": "log"
},
"computed": false
},
"arguments": [
{
"type": "Literal",
"value": "foo"
}
]
}
}
]
},
"alternate": {
"type": "BlockStatement",
"body": [
{
"type": "ExpressionStatement",
"expression": {
"type": "CallExpression",
"callee": {
"type": "MemberExpression",
"object": {
"type": "Identifier",
"name": "console"
},
"property": {
"type": "Identifier",
"name": "log"
},
"computed": false
},
"arguments": [
{
"type": "Literal",
"value": "bar"
}
]
}
}
]
}
}
to:
`js`
{
"type": "CallExpression",
"callee": {
"type": "MemberExpression",
"object": {
"type": "Identifier",
"name": "console"
},
"property": {
"type": "Identifier",
"name": "log"
},
"computed": false
},
"arguments": [
{
"type": "Literal",
"value": "foo"
}
]
}
#### negationOperatorRemoval
`js`
if (!(foo === bar)) {
console.log("foo")
}
It seems that our negation operator could be a part of the condition inside the brackets.
`js`
if (foo !== bar) {
console.log("foo")
}
The tree would be translated from:
`json`
{
"type": "UnaryExpression",
"operator": "!",
"prefix": true,
"argument": {
"type": "BinaryExpression",
"left": {
"type": "Identifier",
"name": "foo"
},
"operator": "===",
"right": {
"type": "Identifier",
"name": "bar"
}
}
}
to
`json`
{
"type": "BinaryExpression",
"left": {
"type": "Identifier",
"name": "foo"
},
"operator": "!==",
"right": {
"type": "Identifier",
"name": "bar"
}
}
#### logicalExpressionReduction
`js`
const foo = "bar" || "baz"
The first value is truthy so it's safe to simplify the code.
`js`
const foo = "bar"
The tree would be translated from:
`json`
{
"type": "LogicalExpression",
"left": {
"type": "Literal",
"value": "bar"
},
"operator": "||",
"right": {
"type": "Literal",
"value": "baz"
}
}
To:
`json`
{
"type": "Literal",
"value": "bar"
}
#### ternaryOperatorReduction
`js`
const foo = true ? "bar" : "baz"
Given a known value of the conditional expression it's possible to get the right value immediately.
`js`
const foo = "bar"
The tree would be translated from:
`json`
{
"type": "ConditionalExpression",
"test": {
"type": "Literal",
"value": true
},
"consequent": {
"type": "Literal",
"value": "bar"
},
"alternate": {
"type": "Literal",
"value": "baz"
}
}
To:
`json`
{
"type": "Literal",
"value": "bar"
}
#### typeofOperatorReduction
`js`
const foo = typeof "bar"
It's possible to determine the type of some variables during analysis.
`js`
const foo = "string"
The tree would be translated from:
`json`
{
"type": "UnaryExpression",
"operator": "typeof",
"prefix": true,
"argument": {
"type": "Literal",
"value": "foo"
}
}
To:
`json`
{
"type": "Literal",
"value": "string"
}
#### memberExpressionReduction
`js`
const foo = { bar: "baz" }.bar
Given an inlined object expression it's possible to retrieve the value immediately.
`js`
const foo = "baz"
The tree would be translated from:
`json`
{
"type": "MemberExpression",
"object": {
"type": "ObjectExpression",
"properties": [
{
"type": "Property",
"method": false,
"shorthand": false,
"computed": false,
"key": {
"type": "Identifier",
"name": "bar"
},
"value": {
"type": "Literal",
"value": "baz"
},
"kind": "init"
}
]
},
"property": {
"type": "Identifier",
"name": "baz"
},
"computed": false
}
To:
`json``
{
"type": "Literal",
"value": "baz"
}
The library is not intended to work inside of a browser. This might change in the future, but it's a bigger lift, pretty time consuming. For now, consider exposing and using an API endpoint instead.
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.
MIT © buxlabs