---
title: Attributes.setIfUnset()
---
The `Attributes.setIfUnset()` method sets the attribute identified by `key` to
value `value` but only if it's currently unset (`undefined`).
## Signature
```js
Attributes attributes.setIfUnset(string key, string value)
```
## Example
```js
const attr = new Attributes()
.setIfUnset('class', 'classA')
.setIfUnset('class', 'classB')
const class = attr.get('class')
// class now holds: "classA"
```
## Notes
This will never overwrite any value and thus is a safe way to set attributes
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