What causes "failed to evaluate expression" in denoland/deno?

Re-run the expression — it is usually transient

denoland/deno · error · anyhow::Error

failed to evaluate expression

Error message

failed to evaluate expression

What it means

In the REPL, each evaluated expression is stringified through the DevTools inspector: deno takes the RemoteObject's `value` or falls back to `description`. If the inspector response carries neither field, deno cannot render a result and throws `failed to evaluate expression`. This is a degenerate protocol response (empty payload or race), not a syntax error in the user's expression.

Source

Thrown at cli/tools/repl/session.rs:779

    let response = self
      .call_function_on_repl_internal_obj(
        r#"function (object) {
          try {
            return this.inspectArgs(["%o", object], { colors: !this.noColor });
          } catch (err) {
            return this.inspectArgs(["%o", err]);
          }
        }"#
          .to_string(),
        std::slice::from_ref(evaluate_result),
      )
      .await?;
    let s = response
      .result
      .value
      .map(|v| v.as_str().unwrap().to_string())
      .or(response.result.description)
      .ok_or_else(|| anyhow!("failed to evaluate expression"))?;

    Ok(s)
  }

  async fn evaluate_ts_expression(
    &mut self,
    expression: &str,
  ) -> Result<TsEvaluateResponse, AnyError> {
    let tsx_result =
      parse_source_as(expression.to_string(), deno_ast::MediaType::Tsx);
    // Prefer a clean `.tsx` parse. Fall back to parsing as TypeScript when the
    // `.tsx` parse fails outright, or when it only recovered by inserting an
    // `Invalid` placeholder node (e.g. a TypeScript type assertion like
    // `<string>x` is misread as JSX in `.tsx`).
    let needs_ts_fallback = match &tsx_result {
      Ok(parsed) => {
        deno_resolver::emit::invalid_syntax_parse_diagnostics(parsed).is_some()
      }

View on GitHub (pinned to 89f33cbef2)

Solutions

Re-run the expression — it is usually transient
Render the result a different way: console.log(expr) or Deno.inspect(value)
Restart the REPL session if it keeps recurring
If reproducible on the latest Deno, minimize the expression and open an issue

Example fix

// before — bare evaluation of a result the inspector cannot stringify
>>> myObject
// after — inspect explicitly
>>> console.log(myObject)
>>> Deno.inspect(myObject, { depth: 4 })

Defensive patterns

Strategy: retry

Prevention

Prefer explicit output (console.log / Deno.inspect) for values you need to see reliably
Restart the REPL when inspector responses degrade in long sessions
Report reproducible cases with the exact expression to the Deno repo

When it happens

Trigger: Evaluating an expression whose inspector RemoteObject arrives with neither value nor description — certain exotic objects or a transient inspector hiccup mid-session.

Common situations: Long-running REPL sessions; evaluating constructs that return unusual remote-object shapes; intermittent occurrences after heavy session activity.

Related errors

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/4247298af5a1aa64. Report an issue: GitHub.