Golang / GoLang System Architecture and Testing Interview Questions

What is mutation testing and how does it evaluate test suite quality beyond coverage?

Mutation testing evaluates whether your tests actually catch bugs. It automatically introduces small changes (mutations) to the source code — like changing > to >= — and checks if at least one test fails. Tests that don't catch any mutation are weak.

// The function under test
func isEligible(age int, hasLicense bool) bool {
    return age >= 18 && hasLicense
}

// Weak test â 100% coverage but misses mutations
func TestIsEligible_Weak(t *testing.T) {
    if !isEligible(20, true) {
        t.Error("expected eligible")
    }
    if isEligible(15, true) {
        t.Error("expected ineligible")
    }
}
// Mutation: change age >= 18 to age > 18
// isEligible(18, true) should return true, but weak test doesn't check 18
// â mutation SURVIVES â test is weak at the boundary

// Strong test â catches boundary mutations
func TestIsEligible_Strong(t *testing.T) {
    tests := []struct {
        age     int
        license bool
        want    bool
    }{
        {20, true,  true},   // clearly eligible
        {17, true,  false},  // underage
        {18, true,  true},   // boundary: exactly eligible
        {18, false, false},  // boundary: no license
        {19, false, false},  // old enough but no license
        {0,  false, false},  // both missing
    }
    for _, tt := range tests {
        t.Run(fmt.Sprintf("age=%d,license=%v", tt.age, tt.license), func(t *testing.T) {
            if got := isEligible(tt.age, tt.license); got != tt.want {
                t.Errorf("isEligible(%d, %v) = %v, want %v",
                    tt.age, tt.license, got, tt.want)
            }
        })
    }
}

// Go mutation testing tools:
// - go-mutesting (zimmski/go-mutesting)
// - gremlins (singularity-code/gremlins)

Take quiz

What does it mean when a mutation 'survives' in mutation testing?The mutated code compiles successfully

✗ Try again.

No test in the suite detected the mutation (caught a failure) — the mutant code behaves differently but all tests still pass, indicating a gap in test coverage

✓ Correct! Well done.

The mutation improved the code's performance

✗ Try again.

The mutation was identical to the original code

✗ Try again.

Why should boundary values always be included in table-driven tests?Go test coverage tools require boundary cases

✗ Try again.

Boundary values are where off-by-one errors, > vs >= mistakes, and edge case bugs typically hide — testing only typical values misses the mutations most likely to be real bugs

✓ Correct! Well done.

Boundary values run faster in benchmarks

✗ Try again.

The -race detector only activates for boundary value inputs

✗ Try again.

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Golang / GoLang System Architecture and Testing Interview Questions

What is mutation testing and how does it evaluate test suite quality beyond coverage?

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