Node.js async patterns: promises, async/await, and streams

<p>I need to understand best practices for async JavaScript/TypeScript: when to use Promise.all vs Promise.allSettled, how to handle errors in async patterns, and how to avoid common pitfalls like unhandled rejections.</p>

<p>Modern async/await patterns in Node.js:</p> <div class="highlight"><pre><span></span><code><span class="c1">// Promise.all vs Promise.allSettled:</span> <span class="k">async</span><span class="w"> </span><span class="kd">function</span><span class="w"> </span><span class="nx">fetchTraceData</span><span class="p">(</span><span class="nx">traceId</span><span class="o">:</span><span class="w"> </span><span class="kt">string</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// Promise.all: fails fast if ANY promise rejects</span> <span class="w"> </span><span class="kd">const</span><span class="w"> </span><span class="p">[</span><span class="nx">trace</span><span class="p">,</span><span class="w"> </span><span class="nx">tags</span><span class="p">,</span><span class="w"> </span><span class="nx">votes</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">await</span><span class="w"> </span><span class="nb">Promise</span><span class="p">.</span><span class="nx">all</span><span class="p">([</span> <span class="w"> </span><span class="nx">getTrace</span><span class="p">(</span><span class="nx">traceId</span><span class="p">),</span> <span class="w"> </span><span class="nx">getTags</span><span class="p">(</span><span class="nx">traceId</span><span class="p">),</span> <span class="w"> </span><span class="nx">getVotes</span><span class="p">(</span><span class="nx">traceId</span><span class="p">),</span> <span class="w"> </span><span class="p">]);</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">trace</span><span class="p">,</span><span class="w"> </span><span class="nx">tags</span><span class="p">,</span><span class="w"> </span><span class="nx">votes</span><span class="w"> </span><span class="p">};</span> <span class="p">}</span> <span class="c1">// Promise.allSettled: get all results even if some fail</span> <span class="k">async</span><span class="w"> </span><span class="kd">function</span><span class="w"> </span><span class="nx">bulkFetch</span><span class="p">(</span><span class="nx">ids</span><span class="o">:</span><span class="w"> </span><span class="kt">string</span><span class="p">[])</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kd">const</span><span class="w"> </span><span class="nx">results</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">await</span><span class="w"> </span><span class="nb">Promise</span><span class="p">.</span><span class="nx">allSettled</span><span class="p">(</span><span class="nx">ids</span><span class="p">.</span><span class="nx">map</span><span class="p">(</span><span class="nx">id</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="nx">getTrace</span><span class="p">(</span><span class="nx">id</span><span class="p">)));</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">results</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">r</span><span class="p">,</span><span class="w"> </span><span class="nx">i</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">({</span> <span class="w"> </span><span class="nx">id</span><span class="o">:</span><span class="w"> </span><span class="kt">ids</span><span class="p">[</span><span class="nx">i</span><span class="p">],</span> <span class="w"> </span><span class="nx">trace</span><span class="o">:</span><span class="w"> </span><span class="kt">r.status</span><span class="w"> </span><span class="o">===</span><span class="w"> </span><span class="s1">'fulfilled'</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="nx">r.value</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="kt">null</span><span class="p">,</span> <span class="w"> </span><span class="nx">error</span><span class="o">:</span><span class="w"> </span><span class="kt">r.status</span><span class="w"> </span><span class="o">===</span><span class="w"> </span><span class="s1">'rejected'</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="nx">r.reason.message</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="kt">null</span><span class="p">,</span> <span class="w"> </span><span class="p">}));</span> <span class="p">}</span> <span class="c1">// Sequential with await in loop (only when order matters):</span> <span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kd">const</span><span class="w"> </span><span class="nx">trace</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="nx">traces</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">await</span><span class="w"> </span><span class="nx">processTrace</span><span class="p">(</span><span class="nx">trace</span><span class="p">);</span><span class="w"> </span><span class="c1">// Sequential -- waits for each</span> <span class="p">}</span> <span class="c1">// Concurrent with limit using p-limit:</span> <span class="k">import</span><span class="w"> </span><span class="nx">pLimit</span><span class="w"> </span><span class="kr">from</span><span class="w"> </span><span class="s1">'p-limit'</span><span class="p">;</span> <span class="kd">const</span><span class="w"> </span><span class="nx">limit</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">pLimit</span><span class="p">(</span><span class="mf">5</span><span class="p">);</span> <span class="kd">const</span><span class="w"> </span><span class="nx">results</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">await</span><span class="w"> </span><span class="nb">Promise</span><span class="p">.</span><span class="nx">all</span><span class="p">(</span> <span class="w"> </span><span class="nx">traces</span><span class="p">.</span><span class="nx">map</span><span class="p">(</span><span class="nx">t</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="nx">limit</span><span class="p">(()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="nx">processTrace</span><span class="p">(</span><span class="nx">t</span><span class="p">)))</span> <span class="p">);</span> <span class="c1">// Unhandled rejection prevention:</span> <span class="nx">process</span><span class="p">.</span><span class="nx">on</span><span class="p">(</span><span class="s1">'unhandledRejection'</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="nx">reason</span><span class="p">,</span><span class="w"> </span><span class="nx">promise</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">error</span><span class="p">(</span><span class="s1">'Unhandled rejection:'</span><span class="p">,</span><span class="w"> </span><span class="nx">reason</span><span class="p">);</span> <span class="w"> </span><span class="nx">process</span><span class="p">.</span><span class="nx">exit</span><span class="p">(</span><span class="mf">1</span><span class="p">);</span> <span class="p">});</span> </code></pre></div> <p>Key points: - Promise.all fails fast -- use when ALL results are needed - Promise.allSettled for independent operations where partial failure is ok - Avoid await inside forEach -- forEach is not async-aware - p-limit for concurrency limiting -- equivalent to asyncio.Semaphore in Python - Always attach .catch() or use try/catch to prevent unhandled rejections</p>