{{#key}}BLOCK{{/key}}

Render blocks of text one or more times, depending on the value of the key in the current context.

Parameters

key {key}

A key that references a value within the current or parent context. If the value is a function or can.compute, the function's return value is used.

Returns

`{String}`

Depending on the value's type, the following actions happen:

Array or can.List - the block is rendered for each item in the array. The context is set to the item within each block rendering.
A truthy value - the block is rendered with the context set to the value.
A falsey value - the block is not rendered.

The rendered result of the blocks, block or an empty string is returned.

Sections contain text blocks and evaluate whether to render it or not. If the object evaluates to an array it will iterate over it and render the block for each item in the array. There are four different types of sections.

Falseys or Empty Arrays

If the value returns a false, undefined, null, "" or [] we consider that a falsey value.

If the value is falsey, the section will NOT render the block.

{
  friends: false
}

{{#friends}}
  Never shown!
{{/friends}}

Arrays

If the value is a non-empty array, sections will iterate over the array of items, rendering the items in the block.

For example, a list of friends will iterate over each of those items within a section.

{
    friends: [
        { name: "Austin" },
        { name: "Justin" }
    ]
}

<ul>
    {{#friends}}
        <li>{{name}}</li>
    {{/friends}}
</ul>

would render:

<ul>
    <li>Austin</li>
    <li>Justin</li>
</ul>

Reminder: Sections will reset the current context to the value for which it is iterating. See the basics of contexts for more information.

Truthys

When the value is a non-falsey object but not a list, it is considered truthy and will be used as the context for a single rendering of the block.

{
    friends: { name: "Jon" }
}

{{#friends}}
    Hi {{name}}
{{/friends}}

would render:

Hi Jon!

Understanding when to use Sections with lists

Section iteration will re-render the entire section for any change in the list. It is the prefered method to use when a list is replaced or changing significantly. Whereas {{#each key}} iteration will do basic diffing and aim to only update the DOM where the change occured. When doing single list item changes frequently, {{#each key}} iteration is the faster choice.

For example, assuming "list" is a can.List instance:

{{#if list}} will check for the truthy value of list. This is akin to checking for the truthy value of any JS object and will result to true, regardless of list contents or length.

{{#if list.length}} will check for the truthy value of the length attribute. If you have an empty list, the length will be 0, so the #if will result to false and no contents will be rendered. If there is a length >= 1, this will result to true and the contents of the #if will be rendered.

{{#each list}} and {{#list}} both iterate through an instance of can.List, however we setup the bindings differently.

{{#each list}} will setup bindings on every individual item being iterated through, while {{#list}} will not. This makes a difference in two scenarios:

You have a large list, with minimal updates planned after initial render. In this case, {{#list}} might be more advantageous as there will be a faster initial render. However, if any part of the list changes, the entire {{#list}} area will be re-processed.
You have a large list, with many updates planned after initial render. A grid with many columns of editable fields, for instance. In this case, you many want to use {{#each list}}, even though it will be slower on initial render(we're setting up more bindings), you'll have faster updates as there are now many sections.

{{#key}}