> For the complete documentation index, see [llms.txt](https://hybrids.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hybrids.gitbook.io/docs/template-engine/iteration.md).

# Iteration

For iteration, an expression should return an `array` with a list of expressions. Items can be primitive values, nested templates as well as nested arrays.

```javascript
html`
  <todo-list>
    ${names.map((name) => `Name: ${name}`)}

    ${items.map(({ name }) => html`<todo-item>${name}</todo-item>`)}
  </todo-list>
`;
```

> Click and play with todo list example:
>
> [![Edit \<app-todos> web components built with hybrids library](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/app-todos-web-components-built-with-hybrids-library-behpb?file=/src/index.js)

## Keys

For default, array `index` identifies expressions for re-render. However, you can use `key` method provided by the result of `html` call for efficient re-order (it sets a key and returns update function). When the list changes and a key is found, the existing template is updated rather than a new one is created.

```javascript
html`
  <todo-list>
    ${items.map(({ id, name }) => 
      html`<todo-item>${name}</todo-item>`.key(id),
    )}
  </todo-list>
`
```


---

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