The {{#unless}} block helper comes built in with Handlebars. {{#unless}} is essentially the opposite of {{#if}}. If you want to test a negative conditional only, i.e. if you only need the {{else}} part of an {{#if}} statement, then {{#unless}} is what you need. It works exactly the same as {{#if}} and supports both {{else}} and ^ negation if you want to get really confusing! Unless also uses the exact same conditional evaluation rules as {{#if}}.

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Example code

Basic unless example, will execute the template between its start and end tags only if featured evaluates to false. 
{{#unless featured}}
  ...do something...
{{/unless}}
If you want, you can also include an else block, although in the majority of cases, if you need an else, then using {{#if}} is more readable: 
<!-- This is identical to if, but with the blocks reversed -->
{{#unless featured}}
  ...do thing 1...
{{else}}
  ...do thing 2...
{{/unless}}