Hiking Through the JavaScript Forest

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An AST makes it easier to inspect and manipulate your code with conﬁdence.

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An abstract syntax tree is basically a DOM for your code.

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Syntax Highlighting Code Completion Static Analysis Code Coverage • Miniﬁcation • JIT Compilation in the Browser • Source Maps • Transpilers (CoffeeScript, Babel) Interesting Things

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Getting Started with ASTs Parser     Tree Code

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var seattle = rain + trees;

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var acorn = require(“acorn”) acorn.parse(“var seattle = rain + trees”) Could also be esprima, espree, babylon, etc.

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Program Variable Declaration Variable Declarator Binary Expression

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Program Variable Declaration Variable Declarator Binary Expression var seattle rain + trees

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{ "type": "Program", "body": [{ "type": “VariableDeclaration", "kind": "var" "declarations": [{ "type": "VariableDeclarator", "id": { "type": “Identifier", "name": "seattle" }, "init": { "type": "BinaryExpression", "left": { "type": “Identifier", "name": “rain" }, "operator": "+", "right": { "type": “Identifier", "name": “trees" } } }] }] }

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Fun With Trees Static Analysis Codemods Transpilers

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Tree Wrangling 1. Use ESLint to perform static analysis 2. Use jscodeshift for one-time code migration (codemods) 3. Use Babel for build-time transformation

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Types of AST Tools Most use something called the visitor pattern Some tools prefer a “jQuery-like” API

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Visitor Pattern You’re climbing the tree, but you stop and visit certain parts that seem really nice.

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Visitor Pattern {  “VariableDeclaration”: function(node) {  console.log(node);  }  }

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{ "type": “VariableDeclaration", "kind": "var" “declarations": [ … ] } One Branch of the Tree

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Static Analysis

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It’s not just about formatting.

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Fix a bug. Add a unit test. Fix a similar bug…

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Fix a bug. Write an ESLint Rule. Prevent that entire class of bugs from entering your codebase ever again.

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var seattle = rain + trees;

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const seattle = rain + trees;

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Visitor Pattern (Revisited) {  “VariableDeclaration”: function(node) {  console.log(node);  }  }

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{ "type": “VariableDeclaration", "kind": "var" “declarations": [ … ] } One Branch of the Tree

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ESLint Rules! module.exports = function(context) {  // …  return vistor;  }

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ESLint Rules! {  “VariableDeclaration”: function(node) {  if (node.kind === “var”)  context.report(node, “You’re var-y bad!”);  }  }

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ESLint Rules! module.exports = function(context) { return {  “VariableDeclaration”: function(node) {  if (node.kind === “var”) {  context.report(node, “You’re var-y bad!”);  }  }  }}

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Transforms

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Basic AST Flow Chart Parser     Tree Code

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Reverse AST Flow Chart Generator     Tree Code

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Reverse AST Flow Chart Generator     Tree Code

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Program Variable Declaration Variable Declarator Binary Expression

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Program Variable Declaration Variable Declarator Binary Expression

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Tools like jscodeshift and babel give you an API to manipulate an AST and then convert that back into source code.

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Build a Simple Babel Plugin

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const seattle = rain + trees;

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Program Variable Declaration Variable Declarator Binary Expression

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Babel Plugins module.exports = function(babel) {  // var visitor = …  return { visitor: visitor };  }

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Babel Plugins {  “VariableDeclaration”: function(path) {  if (path.node.kind === “const”) {  path.node.kind = “var”;  }  }  }

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Babel Plugins module.exports = function(babel) {  return {  visitor: {  “VariableDeclaration”: function(path){  if (path.node.kind === “const”) {  path.node.kind = “var”;  }  }  }  }}

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AST Explorer

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export default function(file, api) { const j = api.jscodeshift; return j(file.source) .find(j.VariableDeclaration) .replaceWith( p => j.variableDeclaration(“const”, p.value.declarations) ) .toSource(); }; JSCodeshift Transform