discourse/discourse · error · Error

on.cleanup can only be called once

Error message

on.cleanup can only be called once

What it means

A helper defined via helperFn attempted to register a second cleanup callback within a single recomputation. The `on` object passed to the helper's callback exposes `on.cleanup(fn)` for scheduling teardown, and the class stores at most one such function (this.cleanupFn); calling it again before compute() runs again trips this guard. Almost always means the helper body calls on.cleanup inside a loop or in multiple branches that both execute.

Solutions

1. Restructure the helper so on.cleanup is called exactly once per compute invocation; combine multiple teardown steps into a single function passed to one on.cleanup call.
2. If you need to clean up several resources, collect them in a local array and pass one cleanup that iterates it.
3. If you intended a different lifecycle, register a custom destructor with @ember/destroyable (registerDestructor/unregisterDestructor) instead of on.cleanup.

When it happens

Trigger: Thrown at frontend/discourse/app/helpers/helper-fn.js:35 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of discourse/discourse@1b2d7253e8 (2026-08-26). Data as JSON: /api/errors/79554fee2e60dc99. Report an issue: GitHub.

Appendix: source

Thrown at frontend/discourse/app/helpers/helper-fn.js:35

    constructor() {
      super(...arguments);
      registerDestructor(this, this.cleanup);
    }

    compute(positional, named) {
      if (positional.length) {
        throw new Error(
          "Positional arguments are not permitted for helperFn-defined helpers. Use named arguments instead."
        );
      }

      this.cleanup();

      const on = {
        cleanup: (fn) => {
          if (this.cleanupFn) {
            throw new Error("on.cleanup can only be called once");
          }
          this.cleanupFn = fn;
        },
      };

      return callback(named, on);
    }

    @bind
    cleanup() {
      this.cleanupFn?.();
      this.cleanupFn = null;
    }
  };
}

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