`prefer-lazy-evaluation`

Prefer lazy evaluation callbacks over eager property objects in LogTape log calls.

Severity	Fixable	Category
warn	yes (auto-fix)	`prefer-lazy-evaluation`

Rationale

LogTape log methods accept a second argument that provides structured properties for the message. This argument can be either an object literal or a function that returns an object:

// Eager: properties object is always evaluated
logger.debug("User data: {userData}.", { userData: fetchUserData(userId) });

// Lazy: callback is only called when the log level is active
logger.debug("User data: {userData}.", () => ({
  userData: fetchUserData(userId),
}));

With an eager object, fetchUserData(userId) runs regardless of whether the debug level is enabled. With a lazy callback, the expensive call is skipped entirely when debug logging is inactive. This rule flags any property value that contains a function call (CallExpression) anywhere in its subtree.

The auto-fix wraps the object literal in an arrow function: { key: fn() } → () => ({ key: fn() }).

Examples

Incorrect

import { getLogger } from "@logtape/logtape";
const logger = getLogger(["my-app"]);

// Any function call anywhere in a property value triggers the rule:
logger.debug("Fetched data: {data}.", { data: fetchData() });
logger.debug("Value: {x}.", { x: compute() });
logger.info("Result: {r}.", { r: a.method() });

Correct

import { getLogger } from "@logtape/logtape";
const logger = getLogger(["my-app"]);

// Wrap the object in an arrow function:
logger.debug("Fetched data: {data}.", () => ({ data: fetchData() }));
logger.debug("Value: {x}.", () => ({ x: compute() }));

// Plain property values without calls are fine:
logger.info("Hello {name}.", { name: "world" });
logger.info("Count: {n}.", { n: items.length });

Configuration

ESLint v9 flat config:

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

export default [
  {
    plugins: { "@logtape": logtape },
    rules: {
      "@logtape/prefer-lazy-evaluation": "warn",
    },
  },
];

Deno Lint (deno.json):

{
  "unstable": ["lint"],
  "lint": {
    "plugins": ["jsr:@logtape/lint/deno"],
    "rules": {
      "include": ["logtape/prefer-lazy-evaluation"]
    }
  }
}

prefer-lazy-evaluation ​

Rationale ​

Examples ​

Incorrect ​