handlebar js custom helper for else if

Since Handlebars 3.0 the syntax described by OP does work out of the box! See: https://github.com/wycats/handlebars.js/pull/892 for more information.

Note: For Ember, since version 1.10 and up this does work too. See also: http://emberjs.com/blog/2015/02/07/ember-1-10-0-released.html#toc_chained-else-blocks

You can't do this with a custom helper as Handlebars if-ish helpers only understand two parts: the "if" part and the "else" part. You can nest things though:

{{#if type.one}}
    do something ... IF
{{else}}
    {{#if type.two}}
        do something ... ELSE IF
    {{else}}
        {{#if type.three}}
            ...
        {{else}}
            ...
        {{/if}}
    {{/if}}
{{/if}}

That sort of thing will get nasty fast so you probably don't want to do that. A better approach would (as usual with Handlebars) be to push the logic into your JavaScript so that at most one of type.one, type.two, type.three, ... would be true; then you could:

{{#if type.one}}
    ...
{{/if}}
{{#if type.two}}
    ...
{{/if}}
{{#if type.three}}
    ...
{{/if}}

If you have a lot of options for type or if the bodies in your {{#if}}s are complicated, you could switch to partials. You'd have to add a custom helper to build a partial name based on a template variable though; something like this:

Handlebars.registerHelper('show_type', function(type) {
    var types = ['one', 'two', 'three'];
    var partial;
    for(var i = 0; i < types.length; ++i) {
        if(!type[types[i]])
            continue;
        partial = '_partial_' + types[i];
        break;
    }
    if(partial)
        return Handlebars.partials[name](this);
    else
        return '';
});

and then, assuming your partials are all registered and consistently named, you could say:

{{show_type type}}