Static Analysis — Catching Bugs at Write Time

Overview

Static analysis catches bugs before code runs by examining source code for type errors, style violations, security vulnerabilities, and formatting issues. It sits at the base of the Test Trophy because it provides free confidence — zero runtime cost, instant feedback, and it prevents entire classes of bugs from ever reaching tests.

Categories of Static Analysis

Type Checking

TypeScript

TypeScript provides compile-time type safety for JavaScript codebases.

// tsconfig.json — strict configuration
{
  "compilerOptions": {
    "strict": true,                    // Enable all strict checks
    "noUncheckedIndexedAccess": true,  // Array/object index returns T | undefined
    "noImplicitOverride": true,        // Require 'override' keyword
    "exactOptionalPropertyTypes": true,// Distinguish undefined from missing
    "noFallthroughCasesInSwitch": true,
    "forceConsistentCasingInFileNames": true,
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "declaration": true,
    "sourceMap": true,
    "outDir": "./dist"
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist"]
}

Run type checking:

# Check without emitting files
npx tsc --noEmit

# Watch mode during development
npx tsc --noEmit --watch

mypy (Python)

# mypy.ini
[mypy]
python_version = 3.12
strict = True
warn_return_any = True
warn_unused_configs = True
disallow_untyped_defs = True
disallow_incomplete_defs = True
check_untyped_defs = True
no_implicit_optional = True
warn_redundant_casts = True
warn_unused_ignores = True

[mypy-tests.*]
disallow_untyped_defs = False

# Run mypy
mypy src/
mypy src/ --html-report reports/mypy

Pyright (Python)

// pyrightconfig.json
{
  "include": ["src"],
  "exclude": ["**/node_modules", "**/__pycache__"],
  "typeCheckingMode": "strict",
  "reportMissingImports": true,
  "reportMissingTypeStubs": false,
  "pythonVersion": "3.12"
}

# Run Pyright
npx pyright
pyright src/

C# Strong Typing

C# provides strong typing out of the box. Enable nullable reference types and treat warnings as errors for maximum safety.

<!-- In .csproj -->
<PropertyGroup>
  <Nullable>enable</Nullable>
  <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
  <WarningLevel>9999</WarningLevel>
  <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
  <AnalysisLevel>latest-recommended</AnalysisLevel>
</PropertyGroup>

Linting

ESLint (JavaScript / TypeScript)

// eslint.config.mjs (flat config — ESLint v9+)
import js from "@eslint/js";
import tseslint from "typescript-eslint";

export default tseslint.config(
  js.configs.recommended,
  ...tseslint.configs.strictTypeChecked,
  ...tseslint.configs.stylisticTypeChecked,
  {
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
    rules: {
      "@typescript-eslint/no-unused-vars": ["error", {
        argsIgnorePattern: "^_",
        varsIgnorePattern: "^_",
      }],
      "@typescript-eslint/no-explicit-any": "error",
      "@typescript-eslint/explicit-function-return-type": "warn",
      "no-console": ["warn", { allow: ["warn", "error"] }],
      "prefer-const": "error",
      eqeqeq: ["error", "always"],
    },
  },
  {
    ignores: ["dist/", "node_modules/", "coverage/"],
  }
);

# Run ESLint
npx eslint .
npx eslint . --fix          # Auto-fix fixable issues
npx eslint . --format json  # Machine-readable output

Biome (JavaScript / TypeScript — fast alternative to ESLint + Prettier)

// biome.json
{
  "$schema": "https://biomejs.dev/schemas/1.9.0/schema.json",
  "organizeImports": { "enabled": true },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "complexity": { "noExcessiveCognitiveComplexity": "warn" },
      "suspicious": { "noExplicitAny": "error" },
      "style": { "useConst": "error" }
    }
  },
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2
  }
}

npx @biomejs/biome check .
npx @biomejs/biome check . --fix

Ruff (Python — fast linter + formatter)

# pyproject.toml
[tool.ruff]
target-version = "py312"
line-length = 88

[tool.ruff.lint]
select = [
  "E",   # pycodestyle errors
  "W",   # pycodestyle warnings
  "F",   # pyflakes
  "I",   # isort
  "N",   # pep8-naming
  "UP",  # pyupgrade
  "B",   # flake8-bugbear
  "S",   # flake8-bandit (security)
  "A",   # flake8-builtins
  "C4",  # flake8-comprehensions
  "SIM", # flake8-simplify
  "RUF", # ruff-specific rules
]
ignore = ["E501"]  # line length handled by formatter

[tool.ruff.lint.per-file-ignores]
"tests/**" = ["S101"]  # allow assert in tests

[tool.ruff.format]
quote-style = "double"

ruff check .
ruff check . --fix
ruff format .

Pylint (Python)

# .pylintrc
[MAIN]
load-plugins=pylint.extensions.docparams
jobs=0

[MESSAGES CONTROL]
disable=C0114,C0115,C0116  # missing docstrings (optional)

[FORMAT]
max-line-length=88

[DESIGN]
max-args=6
max-locals=15

pylint src/
pylint src/ --output-format=json

Roslyn Analyzers (C#)

<!-- Directory.Build.props (applies to all projects in solution) -->
<Project>
  <PropertyGroup>
    <Nullable>enable</Nullable>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
    <AnalysisLevel>latest-recommended</AnalysisLevel>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="9.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
    <PackageReference Include="StyleCop.Analyzers" Version="1.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
    <PackageReference Include="Roslynator.Analyzers" Version="4.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
  </ItemGroup>
</Project>

dotnet build /p:TreatWarningsAsErrors=true
dotnet format --verify-no-changes

Security Scanning (SAST)

Semgrep

Semgrep is a multi-language static analysis tool for finding security vulnerabilities, bugs, and enforcing code standards.

# .semgrep.yml — custom rules
rules:
  - id: no-hardcoded-secrets
    patterns:
      - pattern: |
          $KEY = "..."
      - metavariable-regex:
          metavariable: $KEY
          regex: (password|secret|api_key|token|private_key)
    message: Do not hardcode secrets — use environment variables or a secrets manager
    languages: [python, javascript, typescript, java, csharp]
    severity: ERROR

  - id: no-eval
    pattern: eval(...)
    message: Never use eval() — it enables code injection attacks
    languages: [python, javascript, typescript]
    severity: ERROR

  - id: no-sql-string-concat
    patterns:
      - pattern: |
          $QUERY = "..." + $INPUT + "..."
      - metavariable-regex:
          metavariable: $QUERY
          regex: .*(SELECT|INSERT|UPDATE|DELETE).*
    message: Use parameterized queries instead of string concatenation
    languages: [python, javascript, typescript, java, csharp]
    severity: ERROR

# Run Semgrep with community rules
semgrep --config auto .

# Run Semgrep with specific rulesets
semgrep --config p/owasp-top-ten .
semgrep --config p/javascript .
semgrep --config p/python .
semgrep --config p/csharp .

# Run with custom rules
semgrep --config .semgrep.yml .

# CI-friendly output
semgrep --config auto --json --output results.json .

CodeQL

# .github/codeql/codeql-config.yml
name: "CodeQL Config"
queries:
  - uses: security-extended
  - uses: security-and-quality
paths:
  - src
paths-ignore:
  - tests
  - "**/*.test.ts"

Bandit (Python security)

# .bandit
[bandit]
exclude = tests,docs
skips = B101  # skip assert warnings (used in tests)

bandit -r src/ -f json -o bandit-report.json

SonarQube

# sonar-project.properties
sonar.projectKey=my-project
sonar.sources=src
sonar.tests=tests
sonar.exclusions=**/node_modules/**,**/dist/**
sonar.typescript.lcov.reportPaths=coverage/lcov.info
sonar.python.coverage.reportPaths=coverage.xml

Formatting

Prettier (JavaScript / TypeScript / CSS / HTML / JSON / Markdown)

// .prettierrc
{
  "semi": true,
  "singleQuote": true,
  "trailingComma": "all",
  "printWidth": 100,
  "tabWidth": 2,
  "arrowParens": "always",
  "endOfLine": "lf"
}

npx prettier --check .
npx prettier --write .

Black (Python)

# pyproject.toml
[tool.black]
line-length = 88
target-version = ["py312"]

black --check .
black .

dotnet format (C#)

dotnet format --verify-no-changes  # Check only
dotnet format                       # Auto-fix
dotnet format whitespace            # Whitespace only
dotnet format style                 # Code style only
dotnet format analyzers             # Analyzer-backed fixes

.editorconfig (Cross-platform)

# .editorconfig
root = true

[*]
indent_style = space
indent_size = 2
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true

[*.md]
trim_trailing_whitespace = false

[*.{cs,csx,vb,vbx}]
indent_size = 4

[*.py]
indent_size = 4

[*.go]
indent_style = tab
indent_size = 4

[*.{json,yml,yaml}]
indent_size = 2

[Makefile]
indent_style = tab

CI Integration Patterns

GitHub Actions — Full Static Analysis Pipeline

# .github/workflows/static-analysis.yml
name: Static Analysis

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  lint-and-type-check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - uses: actions/setup-node@v4
        with:
          node-version: "22"
          cache: "npm"

      - run: npm ci

      - name: Type Check
        run: npx tsc --noEmit

      - name: Lint
        run: npx eslint . --format json --output-file eslint-report.json
        continue-on-error: true

      - name: Format Check
        run: npx prettier --check .

  security-scan:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Semgrep
        uses: semgrep/semgrep-action@v1
        with:
          config: >-
            p/owasp-top-ten
            p/javascript
            p/typescript

  codeql:
    runs-on: ubuntu-latest
    permissions:
      security-events: write
    steps:
      - uses: actions/checkout@v4

      - name: Initialize CodeQL
        uses: github/codeql-action/init@v3
        with:
          languages: javascript-typescript

      - name: Perform CodeQL Analysis
        uses: github/codeql-action/analyze@v3

Pre-commit Hooks

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.6.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-json
      - id: detect-private-key

  - repo: https://github.com/pre-commit/mirrors-eslint
    rev: v9.10.0
    hooks:
      - id: eslint
        files: \.[jt]sx?$
        additional_dependencies:
          - eslint@9
          - typescript-eslint@8

  - repo: https://github.com/pre-commit/mirrors-prettier
    rev: v4.0.0
    hooks:
      - id: prettier

  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.7.0
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

  - repo: https://github.com/semgrep/semgrep
    rev: v1.90.0
    hooks:
      - id: semgrep
        args: ["--config", "auto", "--error"]

Azure DevOps Pipeline

# azure-pipelines.yml (static analysis stage)
stages:
  - stage: StaticAnalysis
    jobs:
      - job: Lint
        pool:
          vmImage: "ubuntu-latest"
        steps:
          - task: NodeTool@0
            inputs:
              versionSpec: "22.x"
          - script: npm ci
          - script: npx tsc --noEmit
            displayName: "Type Check"
          - script: npx eslint . --format json --output-file $(Build.ArtifactStagingDirectory)/eslint.json
            displayName: "Lint"
          - script: npx prettier --check .
            displayName: "Format Check"

      - job: SecurityScan
        pool:
          vmImage: "ubuntu-latest"
        steps:
          - script: |
              pip install semgrep
              semgrep --config auto --json --output $(Build.ArtifactStagingDirectory)/semgrep.json .
            displayName: "Semgrep SAST"

Cross-Platform Tool Summary

Best Practices

• Run static analysis on every keystroke (IDE integration) and every commit (CI) for instant feedback.
• Enable strict mode in type checkers (strict: true in tsconfig, strict = True in mypy).
• Treat warnings as errors in CI to prevent gradual quality decay.
• Use .editorconfig to enforce consistent formatting across all editors and languages.
• Layer tools: type checker + linter + formatter + SAST for comprehensive coverage.
• Start with recommended/strict rulesets, then customize — do not build from an empty config.
• Use pre-commit hooks to catch issues before they reach CI.
• Pin tool versions in CI to avoid surprise breakages from upstream updates.
• Separate formatting from linting — formatters handle style, linters handle logic.
• Prefer Biome or Ruff when speed matters — they are 10-100x faster than ESLint/Pylint.
• Run security scanning (Semgrep, CodeQL) on every PR, not just periodically.
• Store all configuration in the repository so every developer gets the same rules.