supported-mutators.md

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Supported mutators	https://github.com/stryker-mutator/mutation-testing-elements/edit/master/docs/supported-mutators.md

All Stryker versions support a variety of different mutators. We've aligned on a standard naming scheme so it is easy to switch and compare implementations. The difference in support is listed below.

Support

Mutator	StrykerJS	Stryker.NET	Stryker4s
Arithmetic Operator	✅	✅	❌
Array Declaration	✅	✅	❌
Assignment Expression	❌	✅	n/a
Block Statement	✅	✅	❌
Boolean Literal	✅	✅	️✅
Checked Statement	n/a	✅	n/a
Conditional Expression	✅	✅	✅
Equality Operator	✅	✅	✅
Logical Operator	✅	✅	✅
Method Expression	✅	✅	✅
Object literal	✅	n/a	n/a
Optional chaining	✅	❌	n/a
Regex	✅	✅	✅
String Literal	✅	✅	✅
Unary Operator	✅	✅	❌
Update Operator	✅	✅	n/a

Arithmetic Operator

Mutant operator	Original	Mutated
AdditionNegation	a + b	a - b
SubtractionNegation	a - b	a + b
MultiplicationNegation	a * b	a / b
DivisionNegation	a / b	a * b
RemainderToMultiplication	a % b	a * b

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Array Declaration

Mutant operator	Original	Mutated
ArrayConstructorItemsRemoval	new Array(1, 2, 3, 4)	new Array()
ArrayLiteralItemsRemoval	[1, 2, 3, 4]	[ ]

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Assignment Expression

Mutant operator	Original	Mutated
AdditionAssignmentNegation	+=	-=
SubtractionAssignmentNegation	-=	+=
MultiplicationAssignmentNegation	*=	/=
DivisionAssignmentNegation	/=	*=
RemainderAssignmentToMultiplicationAssignment	%=	*=
LeftShiftAssignmentNegation	<<=	>>=, >>>=²
RightShiftAssignmentNegation	>>=	<<=, >>>=²
UnsignedRightShiftAssignmentNegation²	>>>=	<<=, >>=
BitwiseAndAssignmentNegation	&=	|=
BitwiseOrAssignmentNegation	|=	&=
NullCoalescingAssignmentToAndAssignment	??=	&&=¹

• ¹: Only supported by Stryker-JS
• ²: Only supported by Stryker.NET

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Block Statement

This group has one mutant operator, the BlockRemoval mutant operator. This mutant operator removes the content of every block statement. For example the code:

function saySomething() {
  console.log('Hello world!');
}

becomes:

function saySomething() {}

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Boolean Literal

Mutant operator	Original	Mutated
TrueNegation	true	false
FalseNegation	false	true
NotRemoval	!(a == b)	a == b ¹

• ¹: Not supported by Stryker4s

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Checked Statement

Stryker.NET specific mutator

Mutant operator	Original	Mutated
CheckedRemoval	checked(2 + 4)	2 + 4

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Conditional Expression

Mutant Operator	Original	Mutated
LessThanToFalse	for (var i = 0; i < 10; i++) { }	for (var i = 0; false; i++) { } ¹
GreaterThanToFalse	while (a > b) { }	while (false) { }
GreaterThanToFalse	do { } while (a > b);	do { } while (false);
GreaterThanToTrue	if (a > b) { }	if (true) { }
GreaterThanToFalse	if (a > b) { }	if (false) { }
GreaterThanToTrue	var x = a > b ? 1 : 2;	var x = true ? 1 : 2; ¹
GreaterThanToFalse	var x = a > b ? 1 : 2;	var x = false ? 1 : 2; ¹

• ¹: Not supported by Stryker4s

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Equality Operator

Mutant operator	Original	Mutated
LessThanBoundary	a < b	a <= b
LessThanNegation	a < b	a >= b
LessThanEqualBoundary	a <= b	a < b
LessThanEqualNegation	a <= b	a > b
GreaterThanBoundary	a > b	a >= b
GreaterThanNegation	a > b	a <= b
GreaterThanEqualBoundary	a >= b	a > b
GreaterThanEqualNegation	a >= b	a < b
EqualityNegation	a == b	a != b
InequalityNegation	a != b	a == b
StrictEqualityNegation	a === b	a !== b ¹
StrictInequalityNegation	a !== b	a === b ¹

• ¹: Only supported on StrykerJS and Stryker4s

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Logical Operator

Mutant operator	Original	Mutated
AndNegation	a && b	a || b
OrNegation	a || b	a && b
NullCoalescingToAnd	a ?? b	a && b

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Method Expression

Due to differences in language syntax, method expressions are implemented differently in each Stryker framework:

StrykerJS

Mutant operator	Original	Mutated
EndsWithToStartsWith	endsWith()	startsWith()
StartsWithToEndsWith	startsWith()	endsWith()
TrimToTrimEnd	trim()	trimEnd()
TrimToTrimStart	trimEnd()	trimStart()
TrimToTrimEnd	trimStart()	trimEnd()
SubstrRemoval	substr()
SubstringRemoval	substring()
UpperCaseToLowerCase	toUpperCase()	toLowerCase()
LowerCaseToUpperCase	toLowerCase()	toUpperCase()
LocalLowerCaseToLocalUpperCase	toLocalLowerCase()	toLocalUpperCase()
LocalUpperCaseToLocalLowerCase	toLocalUpperCase()	toLocalLowerCase()
SortRemoval	sort()
SomeToEvery	some()	every()
EveryToSome	every()	some()
ReverseRemoval	reverse()
FilterRemoval	filter()
SliceRemoval	slice()
CharAtRemoval	charAt()
MinToMax	min()	max()
MaxToMin	max()	min()

Stryker.NET

Mutant operator	Original	Mutated
DistinctRemoval	Distinct()
ReverseRemoval	Reverse()
OrderByRemoval	OrderBy()
OrderByDescendingRemoval	OrderByDescending()
SingleOrDefaultToFirstOrDefault	SingleOrDefault()	FirstOrDefault()
FirstOrDefaultToSingleOrDefault	FirstOrDefault()	SingleOrDefault()
FirstToLast	First()	Last()
LastToFirst	Last()	First()
AllToAny	All()	Any()
AnyToAll	Any()	All()
SkipToTake	Skip()	Take()
TakeToSkip	Take()	Skip()
SkipWhileToTakeWhile	SkipWhile()	TakeWhile()
TakeWhileToSkipWhile	TakeWhile()	SkipWhile()
MinToMax	Min()	Max()
MaxToMin	Max()	Min()
SumToCount	Sum()	Count()
CountToSum	Count()	Sum()
Append	Append()	Prepend()
Prepend	Prepend()	Append()

Stryker4s

Mutant operator	Original	Mutated
FilterNegation	a.filter(b)	a.filterNot(b)
FilterNotNegation	a.filterNot(b)	a.filter(b)
ExistsToForAll	a.exists(b)	a.forall(b)
ForAllToExists	a.forall(b)	a.exists(b)
TakeToDrop	a.take(b)	a.drop(b)
DropToTake	a.drop(b)	a.take(b)
TakeRightNegation	a.takeRight(b)	a.dropRight(b)
DropRightNegation	a.dropRight(b)	a.takeRight(b)
TakeWhileToDropWile	a.takeWhile(b)	a.dropWhile(b)
DropWhileToTakeWhile	a.dropWhile(b)	a.takeWhile(b)
IsEmptyNegation	a.isEmpty	a.nonEmpty
IsNonEmptyNegation	a.nonEmpty	a.isEmpty
IndexOfToLastIndexOf	a.indexOf	a.lastIndexOf(b)
LastIndexOfToIndexOf	a.lastIndexOf(b)	a.indexOf(b)
MaxToMin	a.max	a.min
MinToMax	a.min	a.max
MaxByToMinBy	a.maxBy(b)	a.minBy(b)
MinByToMaxBy	a.minBy(b)	a.maxBy(b)

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Object literal

Mutant operator	Original	Mutated
ObjectPropertiesRemoval	{ foo: 'bar' }	{ }

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Optional chaining

StrykerJS specific mutator

Mutant operator	Original	Mutated
OptionalMemberToRequired	foo?.bar	foo.bar
OptionalComputedToRequired	foo?.[1]	foo[1]
OptionalCallToRequired	foo?.()	foo()

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Regex

Regular expressions are parsed and mutated separately. This is done by recognizing new Regex("...") call signatures in each language. Scala and JavaScript also have shorthand syntax, /regex/ and "regex".r respectively, which are mutated as well.

StrykerJS and Stryker4s use the awesome ⚔ weapon-regex to mutate their regular expressions. All Level 1 mutations are generated.

Strings and literals identified to a regex are mutated in the following way:

Original	Mutated
^abc	abc
abc$	abc
[abc]	[^abc]
[^abc]	[abc]
\d	\D
\D	\d
\s	\S
\S	\s
\w	\W
\W	\w
a?	a
a*	a
a+	a
a{1,3}	a
a*?	a
a+?	a
a{1,3}?	a
a?+	a
a*+	a
a++	a
a{1,3}+	a
(?=abc)	(?!abc)
(?!abc)	(?=abc)
(?<=abc)	(?<!abc)
(?<!abc)	(?<=abc)
\p{Alpha}	\P{Alpha} ¹
\P{Alpha}	\p{Alpha} ¹
\P{Alpha}	\p{Alpha} ¹

¹ JVM only.

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String Literal

Mutant operator	Original	Mutated
FilledStringToEmpty	"foo" (filled string)	"" (empty string)
EmptyStringToFilled	"" (empty string)	"Stryker was here!"
FilledInterpolatedStringToEmpty	s"foo ${bar}" (string interpolation)	s"" ¹
FilledInterpolatedStringToEmpty	`foo ${bar}`	``

¹ For Stryker4s, only works with string interpolation and not other types of interpolation (like Scalameta quasiquotes) to avoid compile errors

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Unary Operator

Mutant operator	Original	Mutated
PlusNegation	+a	-a
MinusNegation	-a	+a

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Update Operator

Mutant operator	Original	Mutated
PostfixIncrementNegation	a++	a--
PostfixDecrementNegation	a--	a++
PrefixIncrementNegation	++a	--a
PrefixDecrementNegation	--a	++a

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