mutator-types.md

Supported mutators

Stryker supports a variety of mutators, which are listed below.

Support

Mutator	Stryker	Stryker.NET	Stryker4s
Binary Operators	✅	✅	ℹ️¹
Boolean Substitutions	✅	✅	ℹ️²
Logical operators	✅	✅	✅
Unary operators	✅	✅	❌
Update operators	✅	✅	n/a
Remove conditionals	✅	❌	❌
Assignment mutator	❌	✅	n/a
Array declarator	✅	❌	❌
String mutator	✅	❌	✅
Block statement	✅	❌	❌
Checked mutator	n/a	✅	n/a
Method mutator	n/a	❌	✅

• ¹: Stryker4s does not support (+,-,*,/ and % operators)
• ²: Stryker4s does not support boolean substitutions with !

Binary operators

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
a >= b	a > b
a >= b	a < b
a == b	a != b
a != b	a == b
a === b	a !== b
a !== b	a === b

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Boolean Substitutions

Original	Mutated
true	false
false	true
!

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Logical operators

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

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Unary operators

Original	Mutated
+a	-a
-a	+a

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Update operators

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

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Remove conditionals

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

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Assignment mutator

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

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Array Declarator

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

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String mutator

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

¹ Only works with string interpolation and not others (like Scalameta quasiquotes) to avoid compile errors

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Block statement

This mutator removes the content of every block statement. For example the code:

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

becomes:

function saySomething() {
}

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Checked mutator

Stryker.NET specific mutator

Original	Mutated
checked(2 + 4)	2 + 4

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Method mutator

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.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

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