I have a statement like this:

{{#if IsValid}}

I want to know how I can use a negative if statement that would look like that:

{{#if not IsValid}}

Simple answers for simple questions:

{{#unless isValid}}
{{/unless}}

Also keep in mind that you can insert an {{else}} in between an {{#if}} or {{#unless}} and the closing tag.

You have many ways of doing that.

1. Use `{{unless}}`:

{{#unless isValid}}
  ...
{{else}}
  ...
{{/unless}}

2. Use inline-if helper:

{{#if (if isValid false true)}}
  ...
{{else}}
  ...
{{/if}}

{{#if (not isValid)}}
  ...
{{else}}
  ...
{{/if}}

unless block helper (built-in helper)

unless helper is the inverse of the if helper.

Its block will be rendered if the expression returns a falsy value.

  {{#unless valid}}
  <h3 class="warning">WARNING</h3>
  {{/unless}}

Venkat Thotakura

{{#if items.length}}
    //Render
{{/if}}

Here items.length .. if it returns some value except null, then only it will enters into the if loop.

NOTE : You can check Boolean values also. In If block

{{#if booleanFloag}}

it can be done in multiple ways.

1 use unless

{{#unless IsValid}}
<Your Code>
{{/unless}}

2.use if else

{{#if IsValid}}
{{else}}
<Your Code>
{{/if}}

3.use not helper

{{#if (not IsValid)}}
<Your Code>
{{/if}}

Below Statements Will help full if you want to use if and else :

{{#if author}}
    <h1>{{firstName}} {{lastName}}</h1>
{{else}}
    <h1>Unknown Author</h1>
{{/if}}

NOTE : Dont close the if Block until logic finished ...

来源：https://stackoverflow.com/questions/10540176/how-to-implement-not-with-if-statement-in-ember-handlebars

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