Block-helper options.fn() can return promise in some cases.

[nknapp]

In the following example, options.fn() returns a promise instead of a string.

var promisedHandlebars = require('../')
var Q = require('q')
var Handlebars = promisedHandlebars(require('handlebars'))

// Register a helper that returns a promise for a boolean
Handlebars.registerHelper('eventually-true', function () {
  return Q.delay(100).then(function () {
    return true
  })
})

// Trim whitespaces from block-content result.
Handlebars.registerHelper('trim', function(options) {
  return options.fn().trim();
});

var template = Handlebars.compile('{{#trim}}{{#if (eventually-true)}}   abc{{/if}}{{/trim}}')

// The whole compiled function returns a promise as well
template({}).done(console.log)

The if-helper returns a promise because it has a promise as argument. This promise has a toString()-method that creates a placeholder, but it does not have a .trim() method.

[nknapp]

The solution would be to check if the helper is used as a block helper and then convert the return value to a string in the helper wrapper.
This implies the assumption that block-helpers can never be used in other helpers arguments (which is probably true).

However, simple helpers must be able to return promises, which can be seen in the same example.

[nknapp]

After thinking about it some more, this is probably unfixable. It will be kept open to be visible to users who have similar problems.