, {key: u.id}, u.name)
})

// PREFER
users.map(function(u) {
    return m.fragment({key: u.id}, m(&quot;button&quot;, u.name))
})</code></pre>
<h4 id="avoid-variable-types"><a href="#avoid-variable-types">Avoid variable types</a></h4>
<p>Keys must be strings if present or they will be cast to strings if they are not. Therefore, <code>&quot;1&quot;</code> (string) and <code>1</code> (number) are considered the same key.</p>
<p>You should use either strings or numbers as keys in one array, but not mix both.</