I'll break down this JavaScript Promise chain line by line and explain what happens at each step:
<pre class="shiki github-dark shaku"><code>Promise.resolve(1) // Step 1: Creates a Promise that resolves with value 1 .then((val) =&gt; { // Step 2: First .then handler console.log(val) // Logs: 1 return val + 1 // Returns 2 to next .then }) .then((val) =&gt; { // Step 3: Second .then handler console.log(val) // Logs: 2 }) // No explicit return, so returns undefined .then((val) =&gt; { // Step 4: Third .then handler console.log(val) // Logs: undefined return Promise.resolve(3) // Creates new Promise that resolves with 3 .then((val) =&gt; { // Inner .then console.log(val) // Logs: 3 }) // No explicit return, so returns undefined }) .then((val) =&gt; { // Step 5: Fourth .then handler console.log(val) // Logs: undefined return Promise.reject(4) // Rejects the Promise with value 4 }) .catch((val) =&gt; { // Step 6: Catches the rejection console.log(val) // Logs: 4 }) .finally((val) =&gt; { // Step 7: Finally block executes console.log(val) // Logs: undefined (finally doesn't receive value) return 10 // Return value is ignored in finally }) .then((val) =&gt; { // Step 8: Last .then handler console.log(val) // Logs: undefined })</code></pre>
The output will be:
<pre><code>1
2
undefined
3
undefined
4
undefined
undefined
</code></pre>
Let's understand why each value is logged:
<ol>
<li>The first log prints 1 from the initial Promise.resolve(1)</li>
<li>The second log prints 2 because we returned val + 1 (1 + 1)</li>
<li>The third log prints undefined because the previous .then didn't return anything</li>
<li>The fourth log prints 3 from the inner Promise.resolve(3)</li>
<li>The fifth log prints undefined because the inner .then didn't return anything</li>
<li>The sixth log prints 4 from the catch block catching the rejected Promise</li>
<li>The seventh log prints undefined because finally doesn't receive any value</li>
<li>The eighth log prints undefined because finally's return value is ignored</li>
</ol>
Key concepts demonstrated:
<ol>
<li>Promise chaining</li>
<li>Implicit undefined returns when no value is explicitly returned</li>
<li>Promise rejection handling with .catch()</li>
<li>finally() block behavior (doesn't receive values and its returns are ignored)</li>
<li>Nested Promise handling</li>
</ol>

Detail Explanation of Code

What does the code snippet to the right output by console.log?…