no-unrendered-properties

This rule is introduced in LogTape 2.4.0.

Find properties that are not referenced by a log message template.

Severity	Fixable	Category
off	no	no-unrendered-properties

This rule is opt-in and is not part of the recommended preset. Use warn when enabling it in ESLint or Oxlint.

Rationale

Some sinks and log bridges forward only the rendered message. A property passed alongside that message can then disappear from the output:

logger.error("{operation} failed", { operation, error });

Here, error is available in the log record but is not part of the message. If the sink only writes the message, the error details may be lost. Include what you need to read in the template:

logger.error("{operation} failed: {error.message}", { operation, error });

Unreferenced properties are valid structured logging data. JSON Lines, logfmt, and other sinks or formatters can retain them separately from the message. Leave this rule disabled if that matches your project's convention. The rule does not inspect your sink configuration or guarantee that a value will be printed usefully.

This rule was inspired by cmdr's no-unrendered-log-fields rule, which caught error details omitted by a frontend log bridge.

Examples

With this rule enabled, these calls produce a diagnostic:

logger.info("Started", { id });
logger.error("{operation} failed", () => ({ operation, error }));
logger.info("{{id}}", { id }); // Escaped braces do not reference id.

These calls reference their properties:

logger.info("Started {id}", { id });
logger.error("{operation} failed: {error.message}", () => ({ operation, error }));
logger.info("First item: {items[0].name}", { items });
logger.info("User: {user?.name}", { user });
logger.info("Details: {*}", { id, operation });

{*} references the whole properties bag unless a property named "*" overrides it. If a wildcard placeholder contains whitespace, a key matching that whitespace takes precedence over "*".

{error.message} counts as a reference to error; it does not mean that the whole error, including its stack or cause, appears in the message. Choose the fields your output needs. There is no automatic fix because adding a placeholder or removing a property requires that judgment.

Analysis boundaries

The rule checks static string literals and template literals without ${} interpolation. Properties must be an object literal or an expression-bodied arrow function returning one, including an async arrow. TypeScript assertions and satisfies wrappers are supported.

Static string and numeric keys are checked, including computed literal keys. An object containing a spread or an unknown key is skipped. Variables, function calls, block-bodied callbacks, and general function expressions are not followed. Dynamic messages, properties-only calls, Error overloads, first-argument callbacks, and tagged templates are outside this rule's scope.

Only fields supplied at the call site are checked. Fields inherited from logger.with() or implicit contexts need not appear in every message. The rule uses the same logger recognition as the other LogTape rules, including import aliases and child loggers; custom logger factories are not tracked.

The analysis favors avoiding false positives. A direct key such as "error.message" and the nested root error both count as referenced when the template uses {error.message}. The rule does not evaluate which lookup will succeed or validate the remainder of a nested path. A "*" property hidden in a context can also override {*} without this rule detecting the resulting omission.

Configuration

ESLint v9 flat config:

import logtape from "@logtape/lint/eslint";

export default [
  logtape.configs.recommended,
  {
    rules: {
      "logtape/no-unrendered-properties": "warn",
    },
  },
];

Oxlint (.oxlintrc.json):

{
  "jsPlugins": [
    { "name": "logtape", "specifier": "@logtape/lint/eslint" }
  ],
  "rules": {
    "logtape/no-unrendered-properties": "warn"
  }
}

Deno Lint (deno.json), enabling this rule alongside the recommended rules:

{
  "lint": {
    "plugins": ["jsr:@logtape/lint/deno/strict"],
    "rules": {
      "exclude": ["logtape/no-dynamic-message"]
    }
  }
}

The default /deno entry point omits this rule. /deno/strict enables all LogTape rules, including both opt-in rules. Remove the exclude setting to use both. To keep only no-dynamic-message alongside the recommended rules, exclude logtape/no-unrendered-properties instead. An include list does not disable other plugin rules.

Deno Lint does not provide ESLint-style warning severity for this rule; violations make deno lint exit unsuccessfully. Use the host linter's ignore directives for intentional exceptions.