supported-mutators.md

title	Supported mutators
custom_edit_url	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
Math Methods	❌	✅	❌

Arithmetic Operator

Original	Mutated
a + b	a - b
a - b	a + b
a * b	a / b
a / b	a * b
a % b	a * b

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Array Declaration

Original	Mutated
new Array(1, 2, 3, 4)	new Array()
[1, 2, 3, 4]	[ ]

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Assignment Expression

Original	Mutated
+=	-=
-=	+=
*=	/=
/=	*=
%=	*=
<<=	>>=
>>=	<<=
&=	|=
|=	&=
??=	&&=¹

• ¹: Only supported by Stryker-JS

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Block Statement

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

Original	Mutated
true	false
false	true
!(a == b)	a == b ¹

• ¹: Not supported by Stryker4s

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Checked Statement

Stryker.NET specific mutator

Original	Mutated
checked(2 + 4)	2 + 4

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Conditional Expression

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

• ¹: Not supported by Stryker4s

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Equality Operator

Original	Mutated
a < b	a <= b
a < b	a >= b
a <= b	a < b
a <= b	a > b
a > b	a >= b
a > b	a <= b
a >= b	a > b
a >= b	a < b
a == b	a != b
a != b	a == b
a === b	a !== b ¹
a !== b	a === b ¹

• ¹: Only supported on StrykerJS and Stryker4s

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Logical Operator

Original	Mutated
a && b	a || b
a || b	a && b
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

Original	Mutated
endsWith()	startsWith()
startsWith()	endsWith()
trim()
trimEnd()	trimStart()
trimStart()	trimEnd()
substr()
substring()
toUpperCase()	toLowerCase()
toLowerCase()	toUpperCase()
toLocalLowerCase()	toLocalUpperCase()
toLocalUpperCase()	toLocalLowerCase()
sort()
some()	every()
every()	some()
reverse()
filter()
slice()
charAt()

Stryker.NET

Original	Mutated
Distinct()
Reverse()
OrderBy()
OrderByDescending()
SingleOrDefault()	FirstOrDefault()
FirstOrDefault()	SingleOrDefault()
First()	Last()
Last()	First()
All()	Any()
Any()	All()
Skip()	Take()
Take()	Skip()
SkipWhile()	TakeWhile()
TakeWhile()	SkipWhile()
Min()	Max()
Max()	Min()
Sum()	Count()
Count()	Sum()

Stryker4s

Original	Mutated
a.filter(b)	a.filterNot(b)
a.filterNot(b)	a.filter(b)
a.exists(b)	a.forall(b)
a.forall(b)	a.exists(b)
a.take(b)	a.drop(b)
a.drop(b)	a.take(b)
a.takeRight(b)	a.dropRight(b)
a.dropRight(b)	a.takeRight(b)
a.takeWhile(b)	a.dropWhile(b)
a.dropWhile(b)	a.takeWhile(b)
a.isEmpty	a.nonEmpty
a.nonEmpty	a.isEmpty
a.indexOf	a.lastIndexOf(b)
a.lastIndexOf(b)	a.indexOf(b)
a.max	a.min
a.min	a.max
a.maxBy(b)	a.minBy(b)
a.minBy(b)	a.maxBy(b)

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Object literal

Original	Mutated
{ foo: 'bar' }	{ }

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Optional chaining

Original	Mutated
foo?.bar	foo.bar
foo?.[1]	foo[1]
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

Original	Mutated
"foo" (non-empty string)	"" (empty string)
"" (empty string)	"Stryker was here!"
s"foo ${bar}" (string interpolation)	s"" ¹

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

Original	Mutated
+a	-a
-a	+a

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Update Operator

Original	Mutated
a++	a--
a--	a++
++a	--a
--a	++a

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Math Methods

Original	Mutated
Acos()	Acosh()
Acos()	Asin()
Acos()	Atan()
Acosh()	Acos()
Acosh()	Asinh()
Acosh()	Atanh()
Asin()	Asinh()
Asin()	Acos()
Asin()	Atan()
Asinh()	Asin()
Asinh()	Acosh()
Asinh()	Atanh()
Atan()	Atanh()
Atan()	Acos()
Atan()	Asin()
Atanh()	Atan()
Atanh()	Acosh()
Atanh()	Asinh()
BitDecrement()	BitIncrement()
BitIncrement()	BitDecrement()
Ceiling()	Floor()
Cos()	Cosh()
Cos()	Sin()
Cos()	Tan()
Cosh()	Cos()
Cosh()	Sinh()
Cosh()	Tanh()
Exp()	Log()
Floor()	Ceiling()
Log()	Exp()
Log()	Pow()
MaxMagnitude()	MinMagnitude()
MinMagnitude()	MaxMagnitude()
Pow()	Log()
ReciprocalEstimate()	ReciprocalSqrtEstimate()
ReciprocalSqrtEstimate()	ReciprocalEstimate()
ReciprocalSqrtEstimate()	Sqrt()
Sin()	Sinh()
Sin()	Cos()
Sin()	Tan()
Sinh()	Sin()
Sinh()	Cosh()
Sinh()	Tanh()
Tan()	Tanh()
Tan()	Cos()
Tan()	Sin()
Tanh()	Tan()
Tanh()	Cosh()
Tanh()	Sinh()