<blockquote>
The static <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect/ownKeys">Reflect.ownKeys()</a> method returns an array of the target object's own property keys.
</blockquote>
Arrays under the hood in JavaScript are Objects. Their keys are numbers, and their values are the elements. By default, all arrays also have a property <code>length</code> that reflects the number of elements in that array. In the case of a sparse array, holes do not add the corresponding key to the array.
Also, the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax">spread operator</a> converts these holes to <code>undefined</code>
<code>1.</code> Its an empty array so answer only contains <code>length</code>
<code>2.</code> This array [,] is also an empty sparse array hence holes are ignored
<code>3.</code> [1,,2] has values defined at index 0 and 2, hence it prints ["0","2","length"]
<code>4.</code> Using Spread Operator changes the input array to [1,undefined,2] thus all the keys give following output ["0","1","2","length"]
<pre class="shiki github-dark shaku"><code>console.log(Reflect.ownKeys([])) // ["length"]console.log(Reflect.ownKeys([,])) // ["length"]console.log(Reflect.ownKeys([1,,2])) // ["0","2","length"]console.log(Reflect.ownKeys([...[1,,2]])) // ["0","1","2","length"]</code></pre>

Reflect.ownKeys

What does the code snippet to the right output by console.log?…