<pre class="shiki github-dark shaku"><code>/** * @param {any} arg * @returns {any} */function undefinedToNull(arg) { // your code here if(Array.isArray(arg)){ return arrToUndef(arg) } else{ let _arg = arg; const keys = Object.keys(_arg); keys.map(item=&gt;{ if(_arg[item] === undefined) return _arg[item] = null; if(Array.isArray(_arg[item])){ return _arg[item] = arrToUndef(_arg[item]) } else if(typeof _arg[item] === 'object'){ return undefinedToNull(_arg[item]); } return _arg[item] = _arg[item]; }) return _arg }}function arrToUndef (args){ return args.map(item=&gt; { if(item === null) return item; if(item === undefined) return null; if(Array.isArray(item)){ return arrToUndef(item) }else if(typeof item === 'object'){ return undefinedToNull(item); } return item })}</code></pre>

time complexity: O(n), space complexity: O(d)+O(n)

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