<pre class="shiki github-dark shaku"><code>/** * @param {any} arg * @returns any */function undefinedToNull(arg) { if (typeof arg == 'undefined') return null; if (Array.isArray(arg)) { return arg.map(i =&gt; undefinedToNull(i)); } if (typeof arg == 'object' &amp;&amp; arg !== null) { const res = {} Object.keys(arg).forEach(i =&gt; { res[i] = undefinedToNull(arg[i]); }) return res; } return arg;}console.log(undefinedToNull({ a: undefined, b: 'BFE.dev' }));console.log(undefinedToNull({ a: ['BFE.dev', undefined, 'bigfrontend.dev'] }));</code></pre>

o(n) recursive (danger for the memory)

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