Opinionated biome.js config. Works well for both frontend and backend services.

biome.json

{
  "$schema": "https://biomejs.dev/schemas/1.8.3/schema.json",
  "organizeImports": {
    "enabled": true
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "complexity": {
        "recommended": true,
        "noExtraBooleanCast": "warn",
        "noStaticOnlyClass": "off",
        "noThisInStatic": "error",
        "noUselessCatch": "error",
        "noUselessSwitchCase": "warn",
        "noUselessTernary": "warn",
        "useArrowFunction": "error",
        "useLiteralKeys": "warn",
        "useSimplifiedLogicExpression": "warn"
      },
      "correctness": {
        "recommended": true,
        "noNodejsModules": "off",
        "useIsNan": "error",
        "useYield": "error"
      },
      "performance": {
        "recommended": true,
        "noAccumulatingSpread": "error"
      },
      "style": {
        "recommended": true,
        "noDefaultExport": "off",
        "noNegationElse": "error",
        "noNonNullAssertion": "warn",
        "noShoutyConstants": "error",
        "noUnusedTemplateLiteral": "error",
        "noVar": "error",
        "noUselessElse": "error",
        "useConsistentArrayType": "error",
        "useConst": "error",
        "useExportType": "error",
        "useFilenamingConvention": "error",
        "useImportType": "error",
        "useNamingConvention": "off",
        "useNodejsImportProtocol": "error",
        "useNumberNamespace": "error",
        "useShorthandArrayType": "error",
        "useShorthandAssign": "error",
        "useShorthandFunctionType": "error"
      },
      "suspicious": {
        "recommended": true,
        "noCatchAssign": "error",
        "noDoubleEquals": "error",
        "noExplicitAny": "warn",
        "noFallthroughSwitchClause": "off",
        "noGlobalIsFinite": "error",
        "useAwait": "warn",
        "useIsArray": "error",
        "useDefaultSwitchClauseLast": "error"
      },
      "nursery": {
        "recommended": true,
        "noSubstr": "error",
        "useDeprecatedReason": "error"
      }
    }
  },
  "formatter": {
    "enabled": true,
    "lineEnding": "lf",
    "indentStyle": "space",
    "lineWidth": 100
  },
  "javascript": {
    "formatter": {
      "enabled": true,
      "trailingCommas": "all",
      "semicolons": "always",
      "quoteStyle": "double",
      "bracketSpacing": true,
      "arrowParentheses": "asNeeded"
    },
    "linter": {
      "enabled": true
    }
  },
  "json": {
    "formatter": {
      "enabled": true,
      "trailingCommas": "none"
    },
    "linter": {
      "enabled": true
    },
    "parser": {
      "allowComments": true
    }
  },
  "vcs": {
    "enabled": true,
    "clientKind": "git",
    "useIgnoreFile": true
  }
}

settings.json

{
  "files.exclude": {
    "**/*.tsbuildinfo": true,
    "**/.git": true
  },
  "search.exclude": {
    "**/node_modules": true,
    "**/*.tsbuildinfo": true,
    "**/*.code-search": true
  },
  "editor.formatOnType": false,
  "editor.formatOnPaste": true,
  "editor.formatOnSave": true,
  "editor.formatOnSaveMode": "file",
  "files.autoSave": "onFocusChange",
  "editor.codeActionsOnSave": {
    "quickfix.biome": "explicit",
    "source.organizeImports.biome": "always",
    "source.fixAll.biome": "explicit"
  },
  "[json]": {
    "editor.quickSuggestions": {
      "strings": true
    },
    "editor.suggest.insertMode": "replace",
    "editor.defaultFormatter": "biomejs.biome"
  },
  "[typescript]": {
    "editor.defaultFormatter": "biomejs.biome"
  }
}

If there are two instances of biomejs running, it might be related to the Dev Containers extension, disabling that will probably fix the issue.