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Sometimes you may only want to display part of your template if a property exists.

We can use the {{#if}} helper to conditionally render a block:

{{#if person}}
  Welcome back, <b>{{person.firstName}} {{person.lastName}}</b>!

Handlebars will not render the block if the argument passed evaluates to false, undefined, null or [] (i.e., any "falsy" value).

If the expression evaluates to falsy, we can also display an alternate template using {{else}}:

{{#if person}}
  Welcome back, <b>{{person.firstName}} {{person.lastName}}</b>!
  Please log in.

To only render a block if a value is falsy, use {{#unless}}:

{{#unless hasPaid}}
  You owe: ${{total}}

{{#if}} and {{#unless}} are examples of block expressions. These allow you to invoke a helper with a portion of your template. Block expressions look like normal expressions except that they contain a hash (#) before the helper name, and require a closing expression.