<pre class="shiki github-dark shaku"><code>/** * @param {any} arg * @returns any */function undefinedToNull(arg) { function processArr(arr) { return arr.map(item =&gt; Array.isArray(item) ? processArritem : !!item &amp;&amp; typeof item === 'object' ? processObj(item) : item === undefined ? null : item ); } function processObj(obj) { return Object.keys(obj).reduce((acc, curr) =&gt; { if (obj[curr] === undefined) { acc[curr] = null; } else { const value = obj[curr]; if (Array.isArray(value)) { acc[curr] = processArr(value); } else if (typeof value === 'object') { acc[curr] = processObj(value) } else { acc[curr] = value; } } return acc; }, {}) } return Array.isArray(arg) ? processArr(arg) : processObj(arg);}</code></pre>

One of the differences between null and undefined is how they are treated differently in JSON.stringify(). JSON.stringify({a: null}) // '{"a":null}'JS…