<pre class="shiki github-dark shaku"><code>/** * @param {any} arg * @returns any */function undefinedToNull(arg,isArray = false) { // your code here isArray = Array.isArray(arg); const res = {}; for(const key in arg) { if(arg[key ] !== null &amp;&amp; typeof arg[key] === "object") { res[key] = undefinedToNull(arg[key]) continue; } if(arg[key] == undefined) { res[key] = null }else{ res[key] = arg[key] } } return isArray ? Object.values(res) : res;}console.log(undefinedToNull({a: ['BFE.dev', undefined, 'bigfrontend.dev']}));</code></pre>

recursion,

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