XUtils

Uglify

parser / mangler / compressor / beautifier toolkit


CLI source map options

UglifyJS can generate a source map file, which is highly useful for debugging your compressed JavaScript. To get a source map, pass --source-map --output output.js (source map will be written out to output.js.map).

Additional options:

  • --source-map "filename='<NAME>'" to specify the name of the source map. The value of filename is only used to set file attribute (see the spec) in source map file.

  • --source-map "root='<URL>'" to pass the URL where the original files can be found.

  • --source-map "names=false" to omit symbol names if you want to reduce size of the source map file.

  • --source-map "url='<URL>'" to specify the URL where the source map can be found. Otherwise UglifyJS assumes HTTP X-SourceMap is being used and will omit the //# sourceMappingURL= directive.

For example:

uglifyjs js/file1.js js/file2.js \
         -o foo.min.js -c -m \
         --source-map "root='http://foo.com/src',url='foo.min.js.map'"

The above will compress and mangle file1.js and file2.js, will drop the output in foo.min.js and the source map in foo.min.js.map. The source mapping will refer to http://foo.com/src/js/file1.js and http://foo.com/src/js/file2.js (in fact it will list http://foo.com/src as the source map root, and the original files as js/file1.js and js/file2.js).

Composed source map

When you’re compressing JS code that was output by a compiler such as CoffeeScript, mapping to the JS code won’t be too helpful. Instead, you’d like to map back to the original code (i.e. CoffeeScript). UglifyJS has an option to take an input source map. Assuming you have a mapping from CoffeeScript → compiled JS, UglifyJS can generate a map from CoffeeScript → compressed JS by mapping every token in the compiled JS to its original location.

To use this feature pass --source-map "content='/path/to/input/source.map'" or --source-map "content=inline" if the source map is included inline with the sources.

CLI compress options

You need to pass --compress (-c) to enable the compressor. Optionally you can pass a comma-separated list of compress options.

Options are in the form foo=bar, or just foo (the latter implies a boolean option that you want to set true; it’s effectively a shortcut for foo=true).

Example:

uglifyjs file.js -c toplevel,sequences=false

CLI mangling property names (--mangle-props)

Note: THIS WILL PROBABLY BREAK YOUR CODE. Mangling property names is a separate step, different from variable name mangling. Pass --mangle-props to enable it. It will mangle all properties in the input code with the exception of built in DOM properties and properties in core JavaScript classes. For example:

// example.js
var x = {
    baz_: 0,
    foo_: 1,
    calc: function() {
        return this.foo_ + this.baz_;
    }
};
x.bar_ = 2;
x["baz_"] = 3;
console.log(x.calc());

Mangle all properties (except for JavaScript builtins):

$ uglifyjs example.js -c -m --mangle-props
var x={o:0,_:1,l:function(){return this._+this.o}};x.t=2,x.o=3,console.log(x.l());

Mangle all properties except for reserved properties:

$ uglifyjs example.js -c -m --mangle-props reserved=[foo_,bar_]
var x={o:0,foo_:1,_:function(){return this.foo_+this.o}};x.bar_=2,x.o=3,console.log(x._());

Mangle all properties matching a regex:

$ uglifyjs example.js -c -m --mangle-props regex=/_$/
var x={o:0,_:1,calc:function(){return this._+this.o}};x.l=2,x.o=3,console.log(x.calc());

Combining mangle properties options:

$ uglifyjs example.js -c -m --mangle-props regex=/_$/,reserved=[bar_]
var x={o:0,_:1,calc:function(){return this._+this.o}};x.bar_=2,x.o=3,console.log(x.calc());

In order for this to be of any use, we avoid mangling standard JS names by default (--mangle-props builtins to override).

A default exclusion file is provided in tools/domprops.json which should cover most standard JS and DOM properties defined in various browsers. Pass --mangle-props domprops to disable this feature.

A regular expression can be used to define which property names should be mangled. For example, --mangle-props regex=/^_/ will only mangle property names that start with an underscore.

When you compress multiple files using this option, in order for them to work together in the end we need to ensure somehow that one property gets mangled to the same name in all of them. For this, pass --name-cache filename.json and UglifyJS will maintain these mappings in a file which can then be reused. It should be initially empty. Example:

$ rm -f /tmp/cache.json  # start fresh
$ uglifyjs file1.js file2.js --mangle-props --name-cache /tmp/cache.json -o part1.js
$ uglifyjs file3.js file4.js --mangle-props --name-cache /tmp/cache.json -o part2.js

Now, part1.js and part2.js will be consistent with each other in terms of mangled property names.

Using the name cache is not necessary if you compress all your files in a single call to UglifyJS.

Mangling unquoted names (--mangle-props keep_quoted)

Using quoted property name (o["foo"]) reserves the property name (foo) so that it is not mangled throughout the entire script even when used in an unquoted style (o.foo). Example:

// stuff.js
var o = {
    "foo": 1,
    bar: 3,
};
o.foo += o.bar;
console.log(o.foo);
$ uglifyjs stuff.js --mangle-props keep_quoted -c -m
var o={foo:1,o:3};o.foo+=o.o,console.log(o.foo);

If the minified output will be processed again by UglifyJS, consider specifying keep_quoted_props so the same property names are preserved:

$ uglifyjs stuff.js --mangle-props keep_quoted -c -m -O keep_quoted_props
var o={"foo":1,o:3};o.foo+=o.o,console.log(o.foo);

Debugging property name mangling

You can also pass --mangle-props debug in order to mangle property names without completely obscuring them. For example the property o.foo would mangle to o._$foo$_ with this option. This allows property mangling of a large codebase while still being able to debug the code and identify where mangling is breaking things.

$ uglifyjs stuff.js --mangle-props debug -c -m
var o={_$foo$_:1,_$bar$_:3};o._$foo$_+=o._$bar$_,console.log(o._$foo$_);

You can also pass a custom suffix using --mangle-props debug=XYZ. This would then mangle o.foo to o._$foo$XYZ_. You can change this each time you compile a script to identify how a property got mangled. One technique is to pass a random number on every compile to simulate mangling changing with different inputs (e.g. as you update the input script with new properties), and to help identify mistakes like writing mangled keys to storage.

Minify options structure

{
    parse: {
        // parse options
    },
    compress: {
        // compress options
    },
    mangle: {
        // mangle options

        properties: {
            // mangle property options
        }
    },
    output: {
        // output options
    },
    sourceMap: {
        // source map options
    },
    nameCache: null, // or specify a name cache object
    toplevel: false,
    warnings: false,
}

Source map options

To generate a source map:

var result = UglifyJS.minify({"file1.js": "var a = function() {};"}, {
    sourceMap: {
        filename: "out.js",
        url: "out.js.map"
    }
});
console.log(result.code); // minified output
console.log(result.map);  // source map

Note that the source map is not saved in a file, it’s just returned in result.map. The value passed for sourceMap.url is only used to set //# sourceMappingURL=out.js.map in result.code. The value of filename is only used to set file attribute (see the spec) in source map file.

You can set option sourceMap.url to be "inline" and source map will be appended to code.

You can also specify sourceRoot property to be included in source map:

var result = UglifyJS.minify({"file1.js": "var a = function() {};"}, {
    sourceMap: {
        root: "http://example.com/src",
        url: "out.js.map"
    }
});

If you’re compressing compiled JavaScript and have a source map for it, you can use sourceMap.content:

var result = UglifyJS.minify({"compiled.js": "compiled code"}, {
    sourceMap: {
        content: "content from compiled.js.map",
        url: "minified.js.map"
    }
});
// same as before, it returns `code` and `map`

If you’re using the X-SourceMap header instead, you can just omit sourceMap.url.

If you wish to reduce file size of the source map, set option sourceMap.names to be false and all symbol names will be omitted.

Compress options

  • annotations (default: true) — Pass false to disable potentially dropping functions marked as “pure”. A function call is marked as “pure” if a comment annotation /*@__PURE__*/ or /*#__PURE__*/ immediately precedes the call. For example: /*@__PURE__*/foo();

  • arguments (default: true) — replace arguments[index] with function parameter name whenever possible.

  • arrows (default: true) — apply optimizations to arrow functions

  • assignments (default: true) — apply optimizations to assignment expressions

  • awaits (default: true) — apply optimizations to await expressions

  • booleans (default: true) — various optimizations for boolean context, for example !!a ? b : c → a ? b : c

  • collapse_vars (default: true) — Collapse single-use non-constant variables, side effects permitting.

  • comparisons (default: true) — apply certain optimizations to binary nodes, e.g. !(a <= b) → a > b, attempts to negate binary nodes, e.g. a = !b && !c && !d && !e → a=!(b||c||d||e) etc.

  • conditionals (default: true) — apply optimizations for if-s and conditional expressions

  • dead_code (default: true) — remove unreachable code

  • default_values (default: true) — drop overshadowed default values

  • directives (default: true) — remove redundant or non-standard directives

  • drop_console (default: false) — Pass true to discard calls to console.* functions. If you wish to drop a specific function call such as console.info and/or retain side effects from function arguments after dropping the function call then use pure_funcs instead.

  • drop_debugger (default: true) — remove debugger; statements

  • evaluate (default: true) — Evaluate expression for shorter constant representation. Pass "eager" to always replace function calls whenever possible, or a positive integer to specify an upper bound for each individual evaluation in number of characters.

  • expression (default: false) — Pass true to preserve completion values from terminal statements without return, e.g. in bookmarklets.

  • functions (default: true) — convert declarations from var to function whenever possible.

  • global_defs (default: {}) — see conditional compilation

  • hoist_exports (default: true) — hoist export statements to facilitate various compress and mangle optimizations.

  • hoist_funs (default: false) — hoist function declarations

  • hoist_props (default: true) — hoist properties from constant object and array literals into regular variables subject to a set of constraints. For example: var o={p:1, q:2}; f(o.p, o.q); is converted to f(1, 2);. Note: hoist_props works best with toplevel and mangle enabled, alongside with compress option passes set to 2 or higher.

  • hoist_vars (default: false) — hoist var declarations (this is false by default because it seems to increase the size of the output in general)

  • if_return (default: true) — optimizations for if/return and if/continue

  • imports (default: true) — drop unreferenced import symbols when used with unused

  • inline (default: true) — inline calls to function with simple/return statement:

    • false — same as 0
    • 0 — disabled inlining
    • 1 — inline simple functions
    • 2 — inline functions with arguments
    • 3 — inline functions with arguments and variables
    • 4 — inline functions with arguments, variables and statements
    • true — same as 4
  • join_vars (default: true) — join consecutive var statements

  • keep_fargs (default: false) — discard unused function arguments except when unsafe to do so, e.g. code which relies on Function.prototype.length. Pass true to always retain function arguments.

  • keep_infinity (default: false) — Pass true to prevent Infinity from being compressed into 1/0, which may cause performance issues on Chrome.

  • loops (default: true) — optimizations for do, while and for loops when we can statically determine the condition.

  • merge_vars (default: true) — combine and reuse variables.

  • module (default: false) — set to true if you wish to process input as ES module, i.e. implicit "use strict";.

  • negate_iife (default: true) — negate “Immediately-Called Function Expressions” where the return value is discarded, to avoid the parentheses that the code generator would insert.

  • objects (default: true) — compact duplicate keys in object literals.

  • passes (default: 1) — The maximum number of times to run compress. In some cases more than one pass leads to further compressed code. Keep in mind more passes will take more time.

  • properties (default: true) — rewrite property access using the dot notation, for example foo["bar"] → foo.bar

  • pure_funcs (default: null) — You can pass an array of names and UglifyJS will assume that those functions do not produce side effects. DANGER: will not check if the name is redefined in scope. An example case here, for instance var q = Math.floor(a/b). If variable q is not used elsewhere, UglifyJS will drop it, but will still keep the Math.floor(a/b), not knowing what it does. You can pass pure_funcs: [ 'Math.floor' ] to let it know that this function won’t produce any side effect, in which case the whole statement would get discarded. The current implementation adds some overhead (compression will be slower). Make sure symbols under pure_funcs are also under mangle.reserved to avoid mangling.

  • pure_getters (default: "strict") — Pass true for UglifyJS to assume that object property access (e.g. foo.bar or a[42]) does not throw exception or alter program states via getter function. Pass "strict" to allow dropping or reordering foo.bar only if foo is not null or undefined and is safe to access as a variable. Pass false to retain all property accesses.

  • reduce_funcs (default: true) — Allows single-use functions to be inlined as function expressions when permissible allowing further optimization. Enabled by default. Option depends on reduce_vars being enabled. Some code runs faster in the Chrome V8 engine if this option is disabled. Does not negatively impact other major browsers.

  • reduce_vars (default: true) — Improve optimization on variables assigned with and used as constant values.

  • rests (default: true) — apply optimizations to rest parameters

  • sequences (default: true) — join consecutive simple statements using the comma operator. May be set to a positive integer to specify the maximum number of consecutive comma sequences that will be generated. If this option is set to true then the default sequences limit is 200. Set option to false or 0 to disable. The smallest sequences length is 2. A sequences value of 1 is grandfathered to be equivalent to true and as such means 200. On rare occasions the default sequences limit leads to very slow compress times in which case a value of 20 or less is recommended.

  • side_effects (default: true) — drop extraneous code which does not affect outcome of runtime execution.

  • spreads (default: true) — flatten spread expressions.

  • strings (default: true) — compact string concatenations.

  • switches (default: true) — de-duplicate and remove unreachable switch branches

  • templates (default: true) — compact template literals by embedding expressions and/or converting to string literals, e.g. `foo ${42}` → "foo 42"

  • top_retain (default: null) — prevent specific toplevel functions and variables from unused removal (can be array, comma-separated, RegExp or function. Implies toplevel)

  • toplevel (default: false) — drop unreferenced functions ("funcs") and/or variables ("vars") in the top level scope (false by default, true to drop both unreferenced functions and variables)

  • typeofs (default: true) — compress typeof expressions, e.g. typeof foo == "undefined" → void 0 === foo

  • unsafe (default: false) — apply “unsafe” transformations (discussion below)

  • unsafe_comps (default: false) — assume operands cannot be (coerced to) NaN in numeric comparisons, e.g. a <= b. In addition, expressions involving in or instanceof would never throw.

  • unsafe_Function (default: false) — compress and mangle Function(args, code) when both args and code are string literals.

  • unsafe_math (default: false) — optimize numerical expressions like 2 * x * 3 into 6 * x, which may give imprecise floating point results.

  • unsafe_proto (default: false) — optimize expressions like Array.prototype.slice.call(a) into [].slice.call(a)

  • unsafe_regexp (default: false) — enable substitutions of variables with RegExp values the same way as if they are constants.

  • unsafe_undefined (default: false) — substitute void 0 if there is a variable named undefined in scope (variable name will be mangled, typically reduced to a single character)

  • unused (default: true) — drop unreferenced functions and variables (simple direct variable assignments do not count as references unless set to "keep_assign")

  • varify (default: true) — convert block-scoped declarations into var whenever safe to do so

  • yields (default: true) — apply optimizations to yield expressions

Mangle options

  • eval (default: false) — Pass true to mangle names visible in scopes where eval or with are used.

  • reserved (default: []) — Pass an array of identifiers that should be excluded from mangling. Example: ["foo", "bar"].

  • toplevel (default: false) — Pass true to mangle names declared in the top level scope.

Examples:

// test.js
var globalVar;
function funcName(firstLongName, anotherLongName) {
    var myVariable = firstLongName +  anotherLongName;
}
var code = fs.readFileSync("test.js", "utf8");

UglifyJS.minify(code).code;
// 'function funcName(a,n){}var globalVar;'

UglifyJS.minify(code, { mangle: { reserved: ['firstLongName'] } }).code;
// 'function funcName(firstLongName,a){}var globalVar;'

UglifyJS.minify(code, { mangle: { toplevel: true } }).code;
// 'function n(n,a){}var a;'

Mangle properties options

  • builtins (default: false) — Use true to allow the mangling of built-in properties of JavaScript API. Not recommended to override this setting.

  • debug (default: false) — Mangle names with the original name still present. Pass an empty string "" to enable, or a non-empty string to set the debug suffix.

  • domprops (default: false) — Use true to allow the mangling of properties commonly found in Document Object Model. Not recommended to override this setting.

  • keep_fargs (default: false) — Use true to prevent mangling of function arguments.

  • keep_quoted (default: false) — Only mangle unquoted property names.

  • regex (default: null) — Pass a RegExp literal to only mangle property names matching the regular expression.

  • reserved (default: []) — Do not mangle property names listed in the reserved array.

Miscellaneous

The unsafe compress option

It enables some transformations that might break code logic in certain contrived cases, but should be fine for most code. You might want to try it on your own code, it should reduce the minified size. Here’s what happens when this flag is on:

  • new Array(1, 2, 3) or Array(1, 2, 3)[ 1, 2, 3 ]
  • new Object(){}
  • String(exp) or exp.toString()"" + exp
  • new Object/RegExp/Function/Error/Array (...) → we discard the new

Conditional compilation API

You can also use conditional compilation via the programmatic API. With the difference that the property name is global_defs and is a compressor property:

var result = UglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
    compress: {
        dead_code: true,
        global_defs: {
            DEBUG: false
        }
    }
});

To replace an identifier with an arbitrary non-constant expression it is necessary to prefix the global_defs key with "@" to instruct UglifyJS to parse the value as an expression:

UglifyJS.minify("alert('hello');", {
    compress: {
        global_defs: {
            "@alert": "console.log"
        }
    }
}).code;
// returns: 'console.log("hello");'

Otherwise it would be replaced as string literal:

UglifyJS.minify("alert('hello');", {
    compress: {
        global_defs: {
            "alert": "console.log"
        }
    }
}).code;
// returns: '"console.log"("hello");'

Using native Uglify AST with minify()

// example: parse only, produce native Uglify AST

var result = UglifyJS.minify(code, {
    parse: {},
    compress: false,
    mangle: false,
    output: {
        ast: true,
        code: false  // optional - faster if false
    }
});

// result.ast contains native Uglify AST
// example: accept native Uglify AST input and then compress and mangle
//          to produce both code and native AST.

var result = UglifyJS.minify(ast, {
    compress: {},
    mangle: {},
    output: {
        ast: true,
        code: true  // optional - faster if false
    }
});

// result.ast contains native Uglify AST
// result.code contains the minified code in string form.

Working with Uglify AST

Transversal and transformation of the native AST can be performed through TreeWalker and TreeTransformer respectively.

ESTree / SpiderMonkey AST

UglifyJS has its own abstract syntax tree format; for practical reasons we can’t easily change to using the SpiderMonkey AST internally. However, UglifyJS now has a converter which can import a SpiderMonkey AST.

For example Acorn is a super-fast parser that produces a SpiderMonkey AST. It has a small CLI utility that parses one file and dumps the AST in JSON on the standard output. To use UglifyJS to mangle and compress that:

acorn file.js | uglifyjs -p spidermonkey -m -c

The -p spidermonkey option tells UglifyJS that all input files are not JavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore we don’t use our own parser in this case, but just transform that AST into our internal AST.

Use Acorn for parsing

More for fun, I added the -p acorn option which will use Acorn to do all the parsing. If you pass this option, UglifyJS will require("acorn").

Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), but converting the SpiderMonkey tree that Acorn produces takes another 150ms so in total it’s a bit more than just using UglifyJS’s own parser.

Uglify Fast Minify Mode

It’s not well known, but whitespace removal and symbol mangling accounts for 95% of the size reduction in minified code for most JavaScript - not elaborate code transforms. One can simply disable compress to speed up Uglify builds by 3 to 5 times.

d3.js minify size gzip size minify time (seconds)
original 511,371 119,932 -
uglify-js@3.13.0 mangle=false, compress=false 363,988 95,695 0.56
uglify-js@3.13.0 mangle=true, compress=false 253,305 81,281 0.99
uglify-js@3.13.0 mangle=true, compress=true 244,436 79,854 5.30

To enable fast minify mode from the CLI use:

uglifyjs file.js -m

To enable fast minify mode with the API use:

UglifyJS.minify(code, { compress: false, mangle: true });

Source maps and debugging

Various compress transforms that simplify, rearrange, inline and remove code are known to have an adverse effect on debugging with source maps. This is expected as code is optimized and mappings are often simply not possible as some code no longer exists. For highest fidelity in source map debugging disable the Uglify compress option and just use mangle.

Compiler assumptions

To allow for better optimizations, the compiler makes various assumptions:

  • The code does not rely on preserving its runtime performance characteristics. Typically uglified code will run faster due to less instructions and easier inlining, but may be slower on rare occasions for a specific platform, e.g. see reduce_funcs.
  • .toString() and .valueOf() don’t have side effects, and for built-in objects they have not been overridden.
  • undefined, NaN and Infinity have not been externally redefined.
  • arguments.callee, arguments.caller and Function.prototype.caller are not used.
  • The code doesn’t expect the contents of Function.prototype.toString() or Error.prototype.stack to be anything in particular.
  • Getting and setting properties on a plain object does not cause other side effects (using .watch() or Proxy).
  • Object properties can be added, removed and modified (not prevented with Object.defineProperty(), Object.defineProperties(), Object.freeze(), Object.preventExtensions() or Object.seal()).
  • If array destructuring is present, index-like properties in Array.prototype have not been overridden:
    
    Object.prototype[0] = 42;
    var [ a ] = [];
    var { 0: b } = {};
    // 42 undefined
    console.log([][0], a);
    // 42 42
    console.log({}[0], b);
    
  • Earlier versions of JavaScript will throw SyntaxError with the following:
    
    ({
      p: 42,
      get p() {},
    });
    // SyntaxError: Object literal may not have data and accessor property with
    //              the same name
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Iteration order of keys over an object which contains spread syntax in later versions of Chrome and Node.js may be altered.
  • When toplevel is enabled, UglifyJS effectively assumes input code is wrapped within function(){ ... }, thus forbids aliasing of declared global variables:
    
    A = "FAIL";
    var B = "FAIL";
    // can be `global`, `self`, `window` etc.
    var top = function() {
      return this;
    }();
    // "PASS"
    top.A = "PASS";
    console.log(A);
    // "FAIL" after compress and/or mangle
    top.B = "PASS";
    console.log(B);
    
  • Use of arguments alongside destructuring as function parameters, e.g. function({}, arguments) {} will result in SyntaxError in earlier versions of Chrome and Node.js - UglifyJS may modify the input which in turn may suppress those errors.
  • Earlier versions of Chrome and Node.js will throw ReferenceError with the following:
    
    var a;
    try {
      throw 42;
    } catch ({
      [a]: b,
      // ReferenceError: a is not defined
    }) {
      let a;
    }
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of JavaScript will throw SyntaxError with the following:
    
    a => {
      let a;
    };
    // SyntaxError: Identifier 'a' has already been declared
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of JavaScript will throw SyntaxError with the following:
    
    try {
      // ...
    } catch ({ message: a }) {
      var a;
    }
    // SyntaxError: Identifier 'a' has already been declared
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some versions of Chrome and Node.js will throw ReferenceError with the following:
    
    console.log(((a, b = function() {
      return a;
      // ReferenceError: a is not defined
    }()) => b)());
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some arithmetic operations with BigInt may throw TypeError:
    
    1n + 1;
    // TypeError: can't convert BigInt to number
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some versions of JavaScript will throw SyntaxError with the following:
    
    console.log(String.raw`\uFo`);
    // SyntaxError: Invalid Unicode escape sequence
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some versions of JavaScript will throw SyntaxError with the following:
    
    try {} catch (e) {
      for (var e of []);
    }
    // SyntaxError: Identifier 'e' has already been declared
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some versions of Chrome and Node.js will give incorrect results with the following:
    
    console.log({
      ...{
          set 42(v) {},
          42: "PASS",
      },
    });
    // Expected: { '42': 'PASS' }
    // Actual:   { '42': undefined }
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of JavaScript will throw SyntaxError with the following:
    
    var await;
    class A {
      static p = await;
    }
    // SyntaxError: Unexpected reserved word
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of JavaScript will throw SyntaxError with the following:
    
    var async;
    for (async of []);
    // SyntaxError: The left-hand side of a for-of loop may not be 'async'.
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some versions of Chrome and Node.js will give incorrect results with the following:
    
    console.log({
      ...console,
      get 42() {
          return "FAIL";
      },
      [42]: "PASS",
    }[42], {
      ...console,
      get 42() {
          return "FAIL";
      },
      42: "PASS",
    }[42]);
    // Expected: "PASS PASS"
    // Actual:   "PASS FAIL"
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Earlier versions of JavaScript will throw TypeError with the following:
    
    (function() {
      {
          const a = "foo";
      }
      {
          const a = "bar";
      }
    })();
    // TypeError: const 'a' has already been declared
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of Chrome and Node.js will give incorrect results with the following:
    
    try {
      class A {
          static 42;
          static get 42() {}
      }
      console.log("PASS");
    } catch (e) {
      console.log("FAIL");
    }
    // Expected: "PASS"
    // Actual:   "FAIL"
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Some versions of Chrome and Node.js will give incorrect results with the following:
    
    (async function(a) {
      (function() {
          var b = await => console.log("PASS");
          b();
      })();
    })().catch(console.error);
    // Expected: "PASS"
    // Actual:   SyntaxError: Unexpected reserved word
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of Chrome and Node.js will give incorrect results with the following:
    
    try {
      f();
      function f() {
          throw 42;
      }
    } catch (e) {
      console.log(typeof f, e);
    }
    // Expected: "function 42"
    // Actual:   "undefined 42"
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Later versions of JavaScript will throw SyntaxError with the following:
    
    "use strict";
    console.log(function f() {
      return f = "PASS";
    }());
    // Expected: "PASS"
    // Actual:   TypeError: invalid assignment to const 'f'
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Adobe ExtendScript will give incorrect results with the following:
    
    alert(true ? "PASS" : false ? "FAIL" : null);
    // Expected: "PASS"
    // Actual:   "FAIL"
    
    UglifyJS may modify the input which in turn may suppress those errors.
  • Adobe ExtendScript will give incorrect results with the following:
    
    alert(42 ? null ? "FAIL" : "PASS" : "FAIL");
    // Expected: "PASS"
    // Actual:   SyntaxError: Expected: :
    
    UglifyJS may modify the input which in turn may suppress those errors.

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