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 offilename
is only used to setfile
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 HTTPX-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
) — Passfalse
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
) — replacearguments[index]
with function parameter name whenever possible.arrows
(default:true
) — apply optimizations to arrow functionsassignments
(default:true
) — apply optimizations to assignment expressionsawaits
(default:true
) — apply optimizations toawait
expressionsbooleans
(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 forif
-s and conditional expressionsdead_code
(default:true
) — remove unreachable codedefault_values
(default:true
) — drop overshadowed default valuesdirectives
(default:true
) — remove redundant or non-standard directivesdrop_console
(default:false
) — Passtrue
to discard calls toconsole.*
functions. If you wish to drop a specific function call such asconsole.info
and/or retain side effects from function arguments after dropping the function call then usepure_funcs
instead.drop_debugger
(default:true
) — removedebugger;
statementsevaluate
(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
) — Passtrue
to preserve completion values from terminal statements withoutreturn
, e.g. in bookmarklets.functions
(default:true
) — convert declarations fromvar
tofunction
whenever possible.global_defs
(default:{}
) — see conditional compilationhoist_exports
(default:true
) — hoistexport
statements to facilitate variouscompress
andmangle
optimizations.hoist_funs
(default:false
) — hoist function declarationshoist_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 tof(1, 2);
. Note:hoist_props
works best withtoplevel
andmangle
enabled, alongside withcompress
optionpasses
set to2
or higher.hoist_vars
(default:false
) — hoistvar
declarations (this isfalse
by default because it seems to increase the size of the output in general)if_return
(default:true
) — optimizations for if/return and if/continueimports
(default:true
) — drop unreferenced import symbols when used withunused
inline
(default:true
) — inline calls to function with simple/return
statement:false
— same as0
0
— disabled inlining1
— inline simple functions2
— inline functions with arguments3
— inline functions with arguments and variables4
— inline functions with arguments, variables and statementstrue
— same as4
join_vars
(default:true
) — join consecutivevar
statementskeep_fargs
(default:false
) — discard unused function arguments except when unsafe to do so, e.g. code which relies onFunction.prototype.length
. Passtrue
to always retain function arguments.keep_infinity
(default:false
) — Passtrue
to preventInfinity
from being compressed into1/0
, which may cause performance issues on Chrome.loops
(default:true
) — optimizations fordo
,while
andfor
loops when we can statically determine the condition.merge_vars
(default:true
) — combine and reuse variables.module
(default:false
) — set totrue
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 examplefoo["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 instancevar q = Math.floor(a/b)
. If variableq
is not used elsewhere, UglifyJS will drop it, but will still keep theMath.floor(a/b)
, not knowing what it does. You can passpure_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 underpure_funcs
are also undermangle.reserved
to avoid mangling.pure_getters
(default:"strict"
) — Passtrue
for UglifyJS to assume that object property access (e.g.foo.bar
ora[42]
) does not throw exception or alter program states via getter function. Pass"strict"
to allow dropping or reorderingfoo.bar
only iffoo
is notnull
orundefined
and is safe to access as a variable. Passfalse
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 onreduce_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 parameterssequences
(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 totrue
then the defaultsequences
limit is200
. Set option tofalse
or0
to disable. The smallestsequences
length is2
. Asequences
value of1
is grandfathered to be equivalent totrue
and as such means200
. On rare occasions the default sequences limit leads to very slow compress times in which case a value of20
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 unreachableswitch
branchestemplates
(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 fromunused
removal (can be array, comma-separated, RegExp or function. Impliestoplevel
)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
) — compresstypeof
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 involvingin
orinstanceof
would never throw.unsafe_Function
(default:false
) — compress and mangleFunction(args, code)
when bothargs
andcode
are string literals.unsafe_math
(default:false
) — optimize numerical expressions like2 * x * 3
into6 * x
, which may give imprecise floating point results.unsafe_proto
(default:false
) — optimize expressions likeArray.prototype.slice.call(a)
into[].slice.call(a)
unsafe_regexp
(default:false
) — enable substitutions of variables withRegExp
values the same way as if they are constants.unsafe_undefined
(default:false
) — substitutevoid 0
if there is a variable namedundefined
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 intovar
whenever safe to do soyields
(default:true
) — apply optimizations toyield
expressions
Mangle options
eval
(default:false
) — Passtrue
to mangle names visible in scopes whereeval
orwith
are used.reserved
(default:[]
) — Pass an array of identifiers that should be excluded from mangling. Example:["foo", "bar"]
.toplevel
(default:false
) — Passtrue
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
) — Usetrue
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
) — Usetrue
to allow the mangling of properties commonly found in Document Object Model. Not recommended to override this setting.keep_fargs
(default:false
) — Usetrue
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 thereserved
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)
orArray(1, 2, 3)
→[ 1, 2, 3 ]
new Object()
→{}
String(exp)
orexp.toString()
→"" + exp
new Object/RegExp/Function/Error/Array (...)
→ we discard thenew
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
andInfinity
have not been externally redefined.arguments.callee
,arguments.caller
andFunction.prototype.caller
are not used.- The code doesn’t expect the contents of
Function.prototype.toString()
orError.prototype.stack
to be anything in particular. - Getting and setting properties on a plain object does not cause other side effects
(using
.watch()
orProxy
). - Object properties can be added, removed and modified (not prevented with
Object.defineProperty()
,Object.defineProperties()
,Object.freeze()
,Object.preventExtensions()
orObject.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:
UglifyJS may modify the input which in turn may suppress those errors.({ p: 42, get p() {}, }); // SyntaxError: Object literal may not have data and accessor property with // the same name
- 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 withinfunction(){ ... }
, 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 inSyntaxError
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:
UglifyJS may modify the input which in turn may suppress those errors.var a; try { throw 42; } catch ({ [a]: b, // ReferenceError: a is not defined }) { let a; }
- Later versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.a => { let a; }; // SyntaxError: Identifier 'a' has already been declared
- Later versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.try { // ... } catch ({ message: a }) { var a; } // SyntaxError: Identifier 'a' has already been declared
- Some versions of Chrome and Node.js will throw
ReferenceError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.console.log(((a, b = function() { return a; // ReferenceError: a is not defined }()) => b)());
- Some arithmetic operations with
BigInt
may throwTypeError
:
UglifyJS may modify the input which in turn may suppress those errors.1n + 1; // TypeError: can't convert BigInt to number
- Some versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.console.log(String.raw`\uFo`); // SyntaxError: Invalid Unicode escape sequence
- Some versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.try {} catch (e) { for (var e of []); } // SyntaxError: Identifier 'e' has already been declared
- Some versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.console.log({ ...{ set 42(v) {}, 42: "PASS", }, }); // Expected: { '42': 'PASS' } // Actual: { '42': undefined }
- Later versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.var await; class A { static p = await; } // SyntaxError: Unexpected reserved word
- Later versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.var async; for (async of []); // SyntaxError: The left-hand side of a for-of loop may not be 'async'.
- Some versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.console.log({ ...console, get 42() { return "FAIL"; }, [42]: "PASS", }[42], { ...console, get 42() { return "FAIL"; }, 42: "PASS", }[42]); // Expected: "PASS PASS" // Actual: "PASS FAIL"
- Earlier versions of JavaScript will throw
TypeError
with the following:
UglifyJS may modify the input which in turn may suppress those errors.(function() { { const a = "foo"; } { const a = "bar"; } })(); // TypeError: const 'a' has already been declared
- Later versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.try { class A { static 42; static get 42() {} } console.log("PASS"); } catch (e) { console.log("FAIL"); } // Expected: "PASS" // Actual: "FAIL"
- Some versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.(async function(a) { (function() { var b = await => console.log("PASS"); b(); })(); })().catch(console.error); // Expected: "PASS" // Actual: SyntaxError: Unexpected reserved word
- Later versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.try { f(); function f() { throw 42; } } catch (e) { console.log(typeof f, e); } // Expected: "function 42" // Actual: "undefined 42"
- Later versions of JavaScript will throw
SyntaxError
with the following:
UglifyJS may modify the input which in turn may suppress those errors."use strict"; console.log(function f() { return f = "PASS"; }()); // Expected: "PASS" // Actual: TypeError: invalid assignment to const 'f'
- Adobe ExtendScript will give incorrect results with the following:
UglifyJS may modify the input which in turn may suppress those errors.alert(true ? "PASS" : false ? "FAIL" : null); // Expected: "PASS" // Actual: "FAIL"
- Adobe ExtendScript will give incorrect results with the following:
UglifyJS may modify the input which in turn may suppress those errors.alert(42 ? null ? "FAIL" : "PASS" : "FAIL"); // Expected: "PASS" // Actual: SyntaxError: Expected: :