Block params

A block head may end with as |name1 name2| to bind local names to the values the block yields. The names are scoped to the block body (and any blocks nested inside it) and shadow context keys of the same name.

{{#each users as |user index|}}
  {{index}}.
  {{user.name}}
{{/each}}

Context and output

Context

{ "users": [{ "name": "Alice" }, { "name": "Bob" }] }

Generated output

0. Alice
1. Bob

This is the Handlebars block-params syntax. It is most useful for naming the current item and index so you can reach them by name from inside nested blocks, instead of relying on $parent / $root or capturing variables.

What the names bind to

The values come from what the block does with its argument, so block params work on every block form:

Block	as |a b| binds
Array loop ({{#each xs}}, {{#xs}})	a = current element, b = index (number)
Object context ({{#with obj}}, {{#obj}})	a = the object, b = undefined
Truthy scalar section	a = the value, b = undefined
Multi-argument {{#with x y}}	a = x, b = y (one name per pushed frame)

Names with no corresponding value resolve to undefined. Because the values are derived from how the block renders its argument, declaring as |...| works on custom-helper blocks too — the params bind to the helper's return value as the block renders it.

{{#with account as |acct|}}
  {{acct.owner}}
  —
  {{acct.balance}}
{{/with}}

Shadowing and precedence

Inside the body, a bare name (or the first segment of a dotted path) resolves to a block param before the surrounding context:

{{#each items as |item|}}
  {{item}}
{{/each}}

…renders the array elements even if the context also has a key named item.

Block params do not shadow the prefixed namespaces — @index, @key, @first, @last, $parent, $root, $this, and $-variables keep their meaning. Inner declarations shadow outer ones:

{{#each rows as |row r|}}
  {{#each row as |cell c|}}
    {{r}},{{c}}:
    {{cell}}
  {{/each}}
{{/each}}

Both r (outer index) and c (inner index) are reachable by name from the inner body — something the @index variable alone cannot do, since it always refers to the innermost loop (see Iteration variables).

Notes

• The clause goes only on a block opener. A closing tag ({{/each}}) or a bare {{else}} may not declare block params.
• Names follow the usual identifier rules (letters, digits, underscores; not starting with a digit).
• as is only treated as a block-params keyword when it is immediately followed by |. A helper argument that happens to be the path as still parses normally.