Pug != Interpolation Operator Handling of Promise Values

Short answer

There is no compiler option, flag, or language feature in Pug that automatically awaits a Promise when you use the unescaped interpolation operator `!=`. The template engine is compiled to a synchronous function, and `!=` simply calls toString() on whatever value the expression evaluates to.

Why this happens

• Pug compiles templates into plain JavaScript functions that run synchronously.
• The runtime unescape helper used by `!=` does String(value) (or value.toString()) without any async handling.
• Because of that, a Promise is rendered as [object Promise] regardless of the operator used.

What you can do instead

1. Resolve data before calling pug.render:

   In your route/controller layer, await all asynchronous operations and pass fully resolved values in the locals object.

2. Wrap pug.render with a helper that awaits promises:

   const pug = require('pug');
   
   async function renderAsync(template, locals) {
     const resolved = await Promise.all(
       Object.entries(locals).map(async ([k, v]) => [k, await v])
     );
     const resolvedLocals = Object.fromEntries(resolved);
     return pug.render(template, resolvedLocals);
   }
   

   Use renderAsync instead of the standard pug.render when you need to pass unresolved Promises.

3. Use a third‑party middleware (e.g., express-pug-async or pug-async) that automatically awaits all Promise‑valued locals before rendering. Verify that the package is compatible with the Pug version you use.

What you cannot do

• There is no filter that can unwrap a Promise at render time because filters run during compilation and receive only the template source string.
• There is no syntax to mark an expression as async inside a Pug template; adding such a feature would require a fundamental change to how the compiler and runtime work.

Next step for you

Check which version of Pug you are using. If you are on a recent 3.x release, the above patterns work as documented. If you need a more integrated solution, consider wrapping pug.render with a small async helper as shown, or switch to a middleware that performs this step automatically.