TypeScript conditional types for API response handling

<p>Building a typed API client where response types differ based on request parameters. Need type-safe response handling without runtime checks or type assertions.</p>

<p>Use conditional types with generic constraints:</p> <div class="highlight"><pre><span></span><code><span class="kr">type</span><span class="w"> </span><span class="nx">ApiResponse</span><span class="o">&lt;</span><span class="nx">T</span><span class="w"> </span><span class="k">extends</span><span class="w"> </span><span class="s1">'list'</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="s1">'detail'</span><span class="o">&gt;</span><span class="w"> </span><span class="o">=</span> <span class="w"> </span><span class="nx">T</span><span class="w"> </span><span class="k">extends</span><span class="w"> </span><span class="s1">'list'</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">items</span><span class="o">:</span><span class="w"> </span><span class="kt">Item</span><span class="p">[];</span><span class="w"> </span><span class="nx">total</span><span class="o">:</span><span class="w"> </span><span class="kt">number</span><span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="nx">T</span><span class="w"> </span><span class="k">extends</span><span class="w"> </span><span class="s1">'detail'</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">item</span><span class="o">:</span><span class="w"> </span><span class="kt">Item</span><span class="p">;</span><span class="w"> </span><span class="nx">related</span><span class="o">:</span><span class="w"> </span><span class="kt">Item</span><span class="p">[]</span><span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="nx">never</span><span class="p">;</span> <span class="k">async</span><span class="w"> </span><span class="kd">function</span><span class="w"> </span><span class="nx">fetchApi</span><span class="o">&lt;</span><span class="nx">T</span><span class="w"> </span><span class="k">extends</span><span class="w"> </span><span class="s1">'list'</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="s1">'detail'</span><span class="o">&gt;</span><span class="p">(</span> <span class="w"> </span><span class="nx">endpoint</span><span class="o">:</span><span class="w"> </span><span class="kt">string</span><span class="p">,</span><span class="w"> </span><span class="nx">mode</span><span class="o">:</span><span class="w"> </span><span class="kt">T</span> <span class="p">)</span><span class="o">:</span><span class="w"> </span><span class="nb">Promise</span><span class="o">&lt;</span><span class="nx">ApiResponse</span><span class="o">&lt;</span><span class="nx">T</span><span class="o">&gt;&gt;</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">resp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">await</span><span class="w"> </span><span class="nx">fetch</span><span class="p">(</span><span class="sb">`</span><span class="si">${</span><span class="nx">endpoint</span><span class="si">}</span><span class="sb">?mode=</span><span class="si">${</span><span class="nx">mode</span><span class="si">}</span><span class="sb">`</span><span class="p">);</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">resp</span><span class="p">.</span><span class="nx">json</span><span class="p">();</span> <span class="p">}</span> <span class="kd">const</span><span class="w"> </span><span class="nx">list</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">await</span><span class="w"> </span><span class="nx">fetchApi</span><span class="p">(</span><span class="s1">'/items'</span><span class="p">,</span><span class="w"> </span><span class="s1">'list'</span><span class="p">);</span><span class="w"> </span><span class="c1">// typed as { items, total }</span> </code></pre></div>