<pre class="shiki github-dark shaku"><code>/** * @param {any} arg * @returns any */function undefinedToNull(arg) { let ans = arg; if(Array.isArray(ans)){ for (let i=0;i&lt;ans.length;i++){ let value = ans[i]; if(value === undefined){ ans[i] = null; }else if(value &amp;&amp; (typeof value === 'object' || Array.isArray(value))){ ans[i] = undefinedToNull(value) } } }else if(typeof arg === 'object'){ ans = { ...arg }; for (const property in ans) { if(ans[property] === undefined){ ans[property] = null; }else if(Array.isArray(ans[property])){ let subArray = ans[property]; for (let i=0;i&lt;subArray.length;i++){ let value = subArray[i]; if(subArray[i] === undefined){ subArray[i] = null; }else if(value &amp;&amp; (typeof value === 'object' || Array.isArray(value))){ subArray[i] = undefinedToNull(subArray[i]) } } }else if(typeof ans[property] === 'object'){ ans[property] = undefinedToNull(ans[property]) } } } ans = JSON.stringify(ans); ans = ans.replaceAll('undefined', null); ans = JSON.parse(ans); return ans;} console.log(undefinedToNull(['BFE.dev', undefined, null, {a: ['BFE.dev', undefined]}] ))</code></pre>

Time complexity: O(n log k),  Space complexity: O(n + d)

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