mutator-types.md

Supported mutators

All Stryker versions support a variety of different mutators. The difference in support is listed below.

Support

Mutator	Stryker	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	❌	✅	✅
String Literal	✅	✅	✅
Unary Operator	✅	✅	❌
Update Operator	✅	✅	n/a

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
+=	-=
-=	+=
*=	/=
/=	*=
%=	*=
<<=	>>=
>>=	<<=
&=	|=
|=	&=

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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

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Method Expression

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

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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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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