no-message-interpolation

Disallow template literal interpolation (${}) in LogTape log message arguments.

Severity	Fixable	Category
error	no	no-message-interpolation

Rationale

LogTape supports structured logging through message templates with named placeholders:

logger.info("User {userId} logged in.", { userId });

Template literal interpolation bypasses this structured logging system. Interpolated values are embedded directly into the string and cannot be filtered, indexed, or forwarded as structured properties by log sinks.

For example, with template literal interpolation the log record would be:

// Bad — userId is baked into the string; sinks cannot see it as a field
logger.info(`User ${userId} logged in.`);
// → { message: "User 42 logged in." }

With a structured template, the value is available as a separate property:

// Good — userId is available as a structured field
logger.info("User {userId} logged in.", { userId });
// → { message: "User {userId} logged in.", userId: 42 }

Examples

Incorrect

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

logger.info(`User ${userId} logged in.`);       // ← error
logger.error(`Request ${reqId} failed.`);       // ← error
logger.debug(`Processing ${items.length} items.`); // ← error

Correct

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

logger.info("User {userId} logged in.", { userId });
logger.error("Request {reqId} failed.", { reqId });
logger.debug("Processing {count} items.", { count: items.length });

// Plain backtick strings without ${} are allowed
logger.info(`User logged in.`);

// Tagged template literals (LogTape template syntax) are allowed
logger.info`User ${userId} logged in.`;

Configuration

ESLint v9 flat config:

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

export default [
  {
    plugins: { "@logtape": logtape },
    rules: {
      "@logtape/no-message-interpolation": "error",
    },
  },
];

Deno Lint (deno.json):

{
  "unstable": ["lint"],
  "lint": {
    "plugins": ["jsr:@logtape/lint/deno"],
    "rules": {
      "include": ["logtape/no-message-interpolation"]
    }
  }
}