Handlebars/Mustache - Is there a built in way to loop through the properties of an object?

Built-in support since Handlebars 1.0rc1

Support for this functionality has been added to Handlebars.js, so there is no more need for external helpers.

How to use it

For arrays:

{{#each myArray}}
    Index: {{@index}} Value = {{this}}
{{/each}}

For objects:

{{#each myObject}}
    Key: {{@key}} Value = {{this}}
{{/each}}

Note that only properties passing the hasOwnProperty test will be enumerated.

It's actually quite easy to implement as a helper:

Handlebars.registerHelper('eachProperty', function(context, options) {
    var ret = "";
    for(var prop in context)
    {
        ret = ret + options.fn({property:prop,value:context[prop]});
    }
    return ret;
});

Then using it like so:

{{#eachProperty object}}
    {{property}}: {{value}}<br/>
{{/eachProperty }}

EDIT: Handlebars now has a built-in way of accomplishing this; see the selected answer above. When working with plain Mustache, the below still applies.

Mustache can iterate over items in an array. So I'd suggest creating a separate data object formatted in a way Mustache can work with:

var o = {
  bob : 'For sure',
  roger: 'Unknown',
  donkey: 'What an ass'
},
mustacheFormattedData = { 'people' : [] };

for (var prop in o){
  if (o.hasOwnProperty(prop)){
    mustacheFormattedData['people'].push({
      'key' : prop,
      'value' : o[prop]
     });
  }
}

Now, your Mustache template would be something like:

{{#people}}
  {{key}} : {{value}}
{{/people}}

Check out the "Non-Empty Lists" section here: https://github.com/janl/mustache.js