windmill-labs/windmill · error

QuickJS evaluation error: {}

Error message

QuickJS evaluation error: {}

What it means

Generic wrapper: any QuickJS runtime error during expression evaluation that is not a memory-limit abort is converted to 'QuickJS evaluation error: <detail>'. The original QuickJS error text is embedded, e.g. syntax errors, TypeError, undefined variable.

Source

Thrown at backend/windmill-jseval/src/lib.rs:419

        }
        rquickjs::CaughtError::Value(v) => v.is_null() || v.is_undefined(),
        rquickjs::CaughtError::Error(_) => false,
    };
    if is_oom {
        let remediation = match env_override {
            Some(var) => format!(
                "Reduce the amount of data handled in the expression, or raise the \
                 cap via the {var} environment variable."
            ),
            None => "Reduce the amount of data handled in the expression.".to_string(),
        };
        anyhow::anyhow!(
            "The expression evaluation exceeded the memory limit of {} MB. {}",
            memory_limit / (1024 * 1024),
            remediation
        )
    } else {
        anyhow::anyhow!("QuickJS evaluation error: {}", err)
    }
}

#[cfg(feature = "quickjs")]
async fn eval_quickjs_inner(
    expr: &str,
    transform_context: HashMap<String, Arc<Box<RawValue>>>,
    flow_input: Option<mappable_rc::Marc<HashMap<String, Box<RawValue>>>>,
    flow_env: Option<HashMap<String, Box<RawValue>>>,
    authed_client: Option<AuthedClient>,
    by_id: Option<IdContext>,
    extra_ctx: Option<Vec<(String, String)>>,
    context_keys: Vec<String>,
    memory_limit: usize,
) -> anyhow::Result<Box<RawValue>> {
    let runtime = AsyncRuntime::new()?;
    runtime.set_memory_limit(memory_limit).await;
    let context = AsyncContext::full(&runtime).await?;

View on GitHub (pinned to e474e8803c)

Solutions

1. Read the embedded QuickJS message to identify the failing expression part
2. Test the expression against the actual step data shape (guard optional fields with ?. and ??)
3. Fix syntax/runtime errors, then redeploy the flow

Example fix

// before
results.user.profile.email  // TypeError if profile undefined
// after
results.user?.profile?.email ?? null

// guard references before eval
if (!results?.user?.profile) throw new Error('results.user.profile missing');

try {
  return await evalExpression(expr, inputs);
} catch (e) {
  if (String(e).startsWith('QuickJS evaluation error')) {
    console.error('expression failed:', e); // inspect embedded QuickJS message
  }
  throw e;
}

When it happens

Trigger: A transform expression with a JavaScript syntax error, a reference to an undefined variable/property, calling a non-function, or any runtime exception raised inside the QuickJS sandbox.

Common situations: Typos in property names of previous-step results; assuming a field exists on all items of an array; paste-in JS that relies on browser/node APIs unavailable in the sandbox.

Related errors

• The expression evaluation `{expr}` took too long to execute
• The expression evaluation exceeded the memory limit of {} MB
• Job ${jobId} was not successful: ${JSON.stringify(error)}
• Draft flow "${path}" has no value.
• Invalid JSON after replacement: ${message}

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/1997f5b24651f71a. Report an issue: GitHub.