<script> let current = 'foo'; </script> <style> button { display: block; } .active { background-color: #ff3e00; color: white; } </style> <button class="{current === 'foo' ? 'active' : ''}" on:click="{() => current = 'foo'}" >foo</button> <button class="{current === 'bar' ? 'active' : ''}" on:click="{() => current = 'bar'}" >bar</button> <button class="{current === 'baz' ? 'active' : ''}" on:click="{() => current = 'baz'}" >baz</button>