Don't Be Afraid of Abstract Syntax Trees

Don’t Be Afraid of
ASTs
Jamund Ferguson

Our Basic Plan
1. High-level overview
2. Static Analysis with ASTs
3. Transforming and refactoring
4. A quick look at the Mozilla Parser API
(de-facto standard AST format)

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

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

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

Things Built On ASTs
• Syntax Highlighting
• Code Completion
• Static Analysis (aka
JSLint, etc.)
• Code Coverage
• Minification
• JIT Compilation
• Source Maps
• Compile to JS
Languages
So much more…

It’s not just about
formatting.

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

Write some really
solid static analysis.
Never write that same
type of bug again.

function loadUser(req, res, next) {
User.loadUser(function(err, user) {
req.session.user = user;
next();
});
}
Bad Example
We forgot to handle the error!

handle-callback-err
1. Each time a function is declared check
if there is an error* parameter
If so set a count to 0;
Increment count when error is used
At the end of the function warn when
count is empty
* the parameter name can be defined by the user

Static Analysis
• Complexity Analysis
• Catching Mistakes
• Consistent Style

History Lesson
• 1995: JavaScript
• 2002: JSLint started by Douglas Crockford
• 2011: JSHint comes out as a fork of JSLint.
Esprima AST parser released.
• 2012: plato, escomplex, complexity-report
• 2013: Nicholoas Zakas releases ESLint. Marat
Dulin releases JSCS.

My static analysis tool
of choice is ESLint.

JSHint
Mixes the rule engine with the parser

no-console
return {
"MemberExpression": function(node) {
if (node.object.name === "console") {
context.report(node, "Unexpected console statement.”);
}
}
};
https://github.com/eslint/eslint/blob/master/lib/rules/no-console.js

no-loop-func
function checkForLoops(node) {
var ancestors = context.getAncestors();
if (ancestors.some(function(ancestor) {
return ancestor.type === "ForStatement" ||
ancestor.type === "WhileStatement" ||
ancestor.type === "DoWhileStatement";
})) {
context.report(node, "Don't make functions within a loop");
}
}
return {
"FunctionExpression": checkForLoops,
"FunctionDeclaration": checkForLoops
};

max-params
var numParams = context.options[0] || 3;
function checkParams(node) {
if (node.params.length > numParams) {
context.report(node, "This function has too many parameters
({{count}}). Maximum allowed is {{max}}.", {
count: node.params.length,
max: numParams
});
}
}
return {
“FunctionDeclaration”: checkParams,
“FunctionExpression”: checkParams
}

no-jquery
function isjQuery(name) {
return name === '$' || name === 'jquery' || name === 'jQuery';
}
return {
“CallExpression”: function(node) {
var name = node.callee && node.callee.name;
if (isjQuery(name)) {
context.report(node, 'Please avoid using jQuery here.’);
}
}
}

More Complex Rules
• indent
• no-extend-native
• no-next-next
• security
• internationalization

Other Areas for Static
Analysis
Code complexity and visualization is another
area where static analysis is really useful. Plato is
an exciting start, but I believe there are tons of
more interesting things that can be done in this
area.

Recap
• Static Analysis can help you catch real bugs and keep
your code maintainable
• ESLint and JSCS both use ASTs for inspecting your
code to make it easy to cleanly to add new rules
• Static analysis can also help you manage your code
complexity as well
• What exactly does a for loop sound like?

Sometimes you want to step
into the future, but something
is keeping you in the past.

Maybe it’s Internet
Explorer

Maybe it’s the size of
your code base

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

Two Types of AST
Transformations
Regenerative
Regenerate the full file from the AST. Often losing
comments and non-essential formatting. Fine for code
not read by humans (i.e. browserify transforms).
Partial-source transformation
Regenerate only the parts of the source that have
changed based on the AST modifications. Nicer for
one-time changes in source.

Build a Simple
Browserify Transform

var vfaurl lfsutlalcskt a=c kn o=d en o+d ber o+w sueir;ify;

4 Steps
1. Buffer up the stream of source code
2. Convert the source into an AST
3. Transform the AST
4. Re-generate and output the source

Step 1
Use through to grab the source code
var through = require(‘through');
var buffer = [];
return through(function write(data) {
buffer.push(data);
}, function end () {
var source = buffer.join(‘’);
});

Step 2
Use falafel to transform create an AST
var falafel = require(‘falafel’);
function end () {
var out = falafel(source, parse).toString();
}

Step 3
function parse(node) {
if (node.type === 'Identifier' &&
node.value === ‘ui’) {
node.update('browserify');
}
}
Use falafel to transform the AST

Step 4
Stream the source with through and close the stream
function end () {
this.queue(out);
this.queue(null); // end the stream
}

It Works!
browserify -t ./ui-to-browserify.js code.js
(function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="var fullstack = node + browserify;
},{}]},{},[1]);

Lots of code to do
something simple?

Probably, but…
It will do exactly what is
expected 100% of the time.

And it’s a building block
for building a bunch of
cooler things.

V8 doesn’t do it, but
there’s nothing stopping
you*.

A Basic Map/Filter
var a = [1, 2, 3];
var b = a.filter(function(n) {
return n > 1;
}).map(function(k) {
return k * 2;
});

Faster Like This
var a = [1, 2, 3];
var b = [];
for (var i = 0; i < a.length; i++) {
if (a[i] > 1) {
b.push(a[i] * 2);
}
}

A Basic Recast Script
var recast = require(‘recast’);
var code = fs.readFileSync(‘code.js', 'utf-8');
var ast = recast.parse(code);
var faster = transform(ast);
var output = recast.print(faster).code;

How Does it Work?
1. Move the right side of the b declaration into a for loop
2. Set b = []
3. Place the .filter() contents inside of an if statement
4. Unwrap the .map contents and .push() them into b
5. Replace all of the local counters with a[_i]

And Voila….
var a = [1, 2, 3];
var b = [];
for (var i = 0; i < a.length; i++) {
if (a[i] > 1) {
b.push(a[i] * 2);
}
}

The most well-read documentation
for how to engineer your app is the
current codebase.

If you change your code,
you can change the
future.

Knowledge
What is an AST and what does it look like?

Parser
1. Read your raw JavaScript source.
2. Parse out every single thing that’s happening.
3. Return an AST that represents your code

Esprima is a very popular*
parser that converts your code
into an abstract syntax tree.
*FB recently forked it to add support for ES6 and JSX

Parsers
narcissus
ZeParser
Treehugger
Uglify-JS
Esprima
Acorn

Esprima follows the Mozilla Parser
API which is a well documented AST
format used internally by Mozilla (and
now by basically everyone else*)

Node Types
SwitchCase (1)
Property (1)
Literal (1)
Identifier (1)
Declaration (3)
Expression (14)
Statement (18)

Expression Types
• FunctionExpression
• MemberExpression
• CallExpression
• NewExpression
• ConditionalExpression
• LogicalExpression
• UpdateExpression
• AssignmentExpression
• BinaryExpression
• UnaryExpression
• SequenceExpression
• ObjectExpression
• ArrayExpression
• ThisExpression

Statement Types
• DebuggerStatement
• ForInStatement
• ForStatement
• DoWhileStatement
• WhileStatement
• CatchClause
• TryStatement
• ThrowStatement
• ReturnStatement
• SwitchStatement
• WithStatement
• ContinueStatement
• BreakStatement
• LabeledStatement
• IfStatement
• ExpressionStatement
• BlockStatement
• EmptyStatement

• When debugging console.log(ast) will not print a
large nested AST properly. Instead you can use
util.inspect:
var util = require('util');
var tree = util.inspect(ast, { depth: null });
console.log(tree);
• When transforming code start with the AST you
want and then work backward.
• Often this means pasting code using the Esprima
online visualization tool or just outputting the trees
into JS files and manually diffing them.

Oftentimes it helps to print out the code
representation of a single node.
In recast you can do:
var source = recast.prettyPrint(ast, {
tabWidth: 2 }).code;
In ESLint you can get the current node with:
var source = context.getSource(node)

ASTs can turn your
code into play-dough

Related
- http://pegjs.majda.cz/
- https://www.kickstarter.com/projects/michaelficarra/make-a-better-coffeescript-compiler
- http://coffeescript.org/documentation/docs/grammar.html
- https://github.com/padolsey/parsers-built-with-js
- https://github.com/zaach/jison
- https://github.com/substack/node-falafel - eprima-based code modifier
- https://github.com/substack/node-burrito - uglify-based AST walking code-modifier
- https://github.com/jscs-dev/node-jscs/blob/e745ceb23c5f1587c3e43c0a9cfb05f5ad86b5ac/lib/js-file.js - JSCS’s way of walking the AST
- https://www.npmjs.org/package/escodegen - converts an AST into real code again
- https://www.npmjs.org/package/ast-types - esprima-ish parser
- http://esprima.org/demo/parse.html - the most helpful tool
https://github.com/RReverser/estemplate - AST-based search and replace
https://www.npmjs.org/package/aster - build system thing
Technical Papers
http://aosd.net/2013/escodegen.html
Videos / Slides
http://slidedeck.io/benjamn/fluent2014-talk
http://vimeo.com/93749422
https://speakerdeck.com/michaelficarra/spidermonkey-parser-api-a-standard-for-structured-js-representations
https://www.youtube.com/watch?v=fF_jZ7ErwUY
https://speakerdeck.com/ariya/bringing-javascript-code-analysis-to-the-next-level
Just in Time Compilers
http://blogs.msdn.com/b/ie/archive/2012/06/13/advances-in-javascript-performance-in-ie10-and-windows-8.aspx
https://blog.mozilla.org/luke/2014/01/14/asm-js-aot-compilation-and-startup-performance/
https://blog.indutny.com/4.how-to-start-jitting
Podcasts
http://javascriptjabber.com/082-jsj-jshint-with-anton-kovalyov/
http://javascriptjabber.com/054-jsj-javascript-parsing-asts-and-language-grammar-w-david-herman-and-ariya-hidayat/

Static analysis tools like ESLint
and JSCS provide an API to let you
inspect an AST to make sure it’s
following certain patterns.

function isEmptyObject( obj ) {
for ( var name in obj ) {
return false;
}
return true;
}

static analysis > unit testing > functional testing

function loadUser(req, res, next) {
User.loadUser(function(err, user) {
if (err) {
next(err);
}
req.session.user = user;
next();
});
}
Another Example

Program
VariableDeclarator
VariableDeclaration
FunctionExpression
ExpressionStatement
Identifier
Identifier
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