<pre class="shiki github-dark shaku"><code>/** * @param {Thunk} thunk * @return {Thunk} */function flattenThunk(thunk) { return function(finalCallback) { const unwrap = (error, result) =&gt; { if(error) { finalCallback(error); } else if(typeof result === 'function') { result(unwrap); } else { finalCallback(undefined, result); } } thunk(unwrap); }}// flattenThunk(func3) -&gt; return a function// new function(finalCallback)// finalCallback = (error, data) =&gt; {// //// }</code></pre>

ac solution

Suppose we have a Callback type type Callback = (error: Error, result: any | Thunk) => void A Thunk is a function that take a Callback as parameter ty…