JavaScript parser / mangler / compressor / beautifier toolkit
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Tl;dr — I want to make UglifyJS2 faster, better, easier to maintain and more useful than version 1. If you enjoy using UglifyJS v1, I can promise you that you will love its successor.

Please help me make this happen by funding the development!

Click here to lend your support to: Funding development of UglifyJS 2.0 and make a donation at !

UglifyJS v2

UglifyJS is a popular JavaScript parser/compressor/beautifier and it's itself written in JavaScript. Version 1 is battle-tested and used in many production systems. The parser is included in WebKit. In two years UglifyJS got over 3000 stars at Github and hundreds of bugs have been identified and fixed, thanks to a great and expanding community.

I'd say version 1 is rock stable. However, its architecture can't be stretched much further. Some features are hard to add, such as source maps or keeping comments in the compressed AST. I started work on version 2 in May, but I gave up quickly because I lacked time. What prompted me to resume it was investigating the difficulty of adding source maps (an increasingly popular feature request).

Status and goals

In short, the goals for v2 are:

  • better modularity, cleaner and more maintainable code; (✓ it's better already)
  • parser generates objects instead of arrays for nodes; (✓ done)
  • store location information in all nodes; (✓ done)
  • better scope representation and mangler; (✓ done)
  • better code generator; (✓ done)
  • compression options at least as good as in v1; (⌛ in progress)
  • support for generating source maps;
  • better regression tests; (⌛ in progress)
  • ability to keep certain comments;
  • command-line utility compatible with UglifyJS v1;
  • documentation for the AST node hierarchy and the API.

Longer term goals—beyond compressing JavaScript:

  • provide a linter; (started)
  • feature to dump an AST in a simple JSON format, along with information that could be useful for an editor (such as Emacs);
  • write a minor JS mode for Emacs to highlight obvious errors, locate symbol definition or warn about accidental globals;
  • support type annotations like Closure does (though I'm thinking of a syntax different from comments; no big plans for this yet).

Objects for nodes

Version 1 uses arrays to represent AST nodes. This model worked well for most operations, but adding additional information in nodes could only be done with hacks I don't really like (you can add properties to an array just as if it were an object, but that's just a dirty hack; also, such properties were not propagated in the compressor).

In v2 I switched to a more “object oriented” approach. Nodes are objects and there's also an inheritance tree that aims to be useful in practice. For example in v1 in order to see if a node is an aborting statement, we might do something like this:

if (node[0] == "return"
    || node[0] == "throw"
    || node[0] == "break"
    || node[0] == "continue") aborts();

In v2 they all inherit from the base class AST_Jump, so I can say:

if (node instanceof AST_Jump) aborts();

The parser was heavily modified to support the new node types, however you can still find the same code layout as in v1, and I trust it's just as stable. Except for the parser, all other parts of UglifyJS are rewritten from scratch.

The parser itself got a bit slower (430ms instead of 330ms on my usual 650K test file).

A word about Esprima

UPDATE: A discussion in my commit suggests that Esprima is not as slow as I thought even when requesting location information. YMMV. In any case, we're going to keep the battle-tested parser in UglifyJS.

Esprima is a really nice JavaScript parser. It supports EcmaScript 5.1 and it claims to be “up to 3x faster than UglifyJS's parse-js”. I thought that's quite cool and I considered using Esprima in UglifyJS v2, but then I did some tests.

On my 650K test file, UglifyJS v1's parser takes 330ms and Esprima about 250ms. That's not exactly “3x faster” but very good indeed! However, I noticed that in the default configuration Esprima does not keep location information in the nodes. Enabled that, and parse time grew to 680ms.

Some would claim it's a fair comparison, because UglifyJS doesn't keep location information either, but that's not entirely accurate. It's true that the parse() function will not propagate location into the AST unless you set embed_tokens, but the lexer always stores it in the tokens.

Enabling embed_tokens makes UglifyJS do it in 400ms, which is still a lot better than Esprima's 680ms.

In version 2 we always maintain location info and comments in the AST nodes, which is why the parser in v2 takes about 430ms on that file (some milliseconds get lost because it's more work to create object nodes than arrays). I might try to speed it up, though I'm not sure it's worth the trouble (parsing 650K in 430ms (on my rather outdated machine) to get an objectual AST with full location/range info and comments seems good enough for me).

The code generator, V2 vs. V1

The code generator in v1 is a big function that takes a node and applies various walkers on it in order to generate code. The code was returned from each walker function, and finally assembled into a big string by concatenation or array.join, and further returned. It is impossible there to know what's the current line/column of the output, which would be necessary for source maps. For the same reason, v1 required an additional step to split very long lines (that includes an additional run of the tokenizer). It's slow.

The rules for inserting parentheses in v1 are an unholy mess; we know at least one case where it inserts unnecessary parens (non-trivial to fix), and I just discovered one case where it generates invalid code—UglifyJS can properly parse the following (valid) statement:

for (var a = ("foo" in bar), i = 0; i < 5; ++i);

however, the code generator in version 1 will break it by not including the parens (the in operator is not allowed in a for initializer, unless it's parenthesized).

The codegen in V2 is a thing of beauty. Since I now use objects for AST nodes, I defined a "print" method on each object type. This method takes an object (an OutputStream) and instead of returning the source code for the node, it prints it in the output stream. The stream object keeps track of current line/colum in the output and provides helper functions to insert semicolons, to indent etc. The code is somewhat bigger than the gen_code in v1, but it's much easier to understand, it's faster and does not require an additional pass for splitting long lines. Also the rules for inserting parens are nicely separated from the print method definitions.

More aggressive compressing

As I blogged a few days ago, it seems to me that the squeezer works really hard for not too much benefit. On my test file, passing --no-squeeze to UglifyJS v1 adds only 500 bytes after gzip, that is 0.68% of the gzipped file size; every byte counts, but to be frank, that's not a very big deal either.

Beyond doing what V1 does, I'd like to make it smarter in certain situations, for example:

function foo() {
    var something = compute_something();
    var something_else = compute_something_else(something);
    return something_else;

I sometimes write this kind of code because it's cleaner, it nests less and it avoids the need to add explanatory comments. It could safely compress into:

function foo() {
    return compute_something_else(compute_something());

which makes it a single statement (further compressable into sequences and allowing to drop brackets in other cases) and it avoids the var declarations. That's one tricky optimization to do in V1, but I feel with the new architecture is doable, at least for the simple cases.

Currently the compressor in V2 is far from complete (where by “complete” I mean as good as V1), and I'll actually put it on hold to add support for generating source maps first. However the mangler is complete (seems to be working properly) as well as the code generator, so V2 is already usable for achieving pretty good compression.

Better regression test suite

The existing test suite in UglifyJS v1 has been contributed (thanks!). Unfortunately it's not great because it employs all the compression techniques in each test. Eventually I'd like to port all existing tests to v2, but for now I started it from scratch.

Tests broke many times for no good reason as I added new features; for example the feature that transforms consecutive simple statements into sequences:

INPUT  → function f(){ if (x) { foo(); bar(); baz(); }}
OUTPUT → function f(){ x && foo(), bar(), baz() }

It's an useful technique; without meshing consecutive statements into an AST_Seq we would have to keep the if and the brackets.

Having a test only for this feature is fine; but if the feature is applied to all tests, then tests where the “expected” file contains consecutive statements will break, although the output is perfectly fine.

In v2 I started a new test suite (I actually took the “test driven development” approach: I'm progressing on both compressor and test suite at once; for each new compressor option I add a test case). Tests look like this:

keep_debugger: {
    options = {
        drop_debugger: false
    input: {
    expect: {

drop_debugger: {
    options = {
        drop_debugger: true
    input: {
        if (foo) debugger;
    expect: {
        if (foo);

That might look funny, but it's syntactically valid JS. A test file consists of a sequence of labeled block statements. Each label names a test in that file. In each block you can assign to the options variable to override compressor options (for the purpose of running the tests, all compression options are turned off, so you just enable the stuff you test). Then you include two other labeled statements: input and expect. The compressor test suite simply parses these statements to get two AST-s. It applies the compressor on the input AST, then the codegen on the compressed AST. It applies the codegen to the expect AST (without compressing it). Then it compares the results and if they match, the test passes.

I expect this model to give a lot less false negatives, and it would work quite well for the name mangling too (no tests for that yet).

For the code generator we'll need something more fine-tuned, since we care exactly how the output is going to look like. I don't yet have any plans about code generator tests.

Play with it

We don't yet have a nice command line utility, but there's a test script for NodeJS in tmp/test-node.js. To play with UglifyJS v2 just clone the repository anywhere you like and run tmp/test-node.js script.js (script.js being the script that you'd like to compress). Take a look at the source of test-node.js to see how the API looks like, to enable/disable steps or compressor options.

To run the existing tests, run test/run-tests.js

Status of UglifyJS v1

We didn't have any significant new features in the last few months; most commits are about bug fixes. I plan to continue to fix show-stopper bugs in v1 for a while, depending on how time permits, but there won't be any new development.

Help me complete the new version

I've put a lot of energy already into this project and I think it comes out nicely. It's based on all my previous experience from working on version 1 and I'm working carefully, trying not to introduce bugs that were already fixed, trying to keep it fast and clean. If you'd like to help me dedicate more time to it, please consider making a donation!

Click here to
lend your support to: Funding development of UglifyJS 2.0 and make a
donation at !