TypeScript satisfies operator and const assertions

<p>I want TypeScript to check that objects match a type while preserving the most specific (narrowest) type for inference. I also want const objects with literal types not broadened to string.</p>

<p>satisfies and as const patterns:</p> <div class="highlight"><pre><span></span><code><span class="c1">// as const: preserve literal types, not broadened types</span> <span class="kd">const</span><span class="w"> </span><span class="nx">STATUS</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">pending</span><span class="o">:</span><span class="w"> </span><span class="s1">'pending'</span><span class="p">,</span> <span class="w"> </span><span class="nx">validated</span><span class="o">:</span><span class="w"> </span><span class="s1">'validated'</span><span class="p">,</span> <span class="p">}</span><span class="w"> </span><span class="kr">as</span><span class="w"> </span><span class="kd">const</span><span class="p">;</span> <span class="c1">// Without as const: { pending: string, validated: string }</span> <span class="c1">// With as const: { readonly pending: 'pending', readonly validated: 'validated' }</span> <span class="kr">type</span><span class="w"> </span><span class="nx">TraceStatus</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="ow">typeof</span><span class="w"> </span><span class="nx">STATUS</span><span class="p">[</span><span class="nx">keyof</span><span class="w"> </span><span class="ow">typeof</span><span class="w"> </span><span class="nx">STATUS</span><span class="p">];</span> <span class="c1">// TraceStatus = 'pending' | 'validated' (literal union, not just string)</span> <span class="c1">// satisfies: validate against type while keeping narrow type</span> <span class="kd">const</span><span class="w"> </span><span class="nx">config</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">endpoints</span><span class="o">:</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">traces</span><span class="o">:</span><span class="w"> </span><span class="s1">'/api/v1/traces'</span><span class="p">,</span> <span class="w"> </span><span class="nx">search</span><span class="o">:</span><span class="w"> </span><span class="s1">'/api/v1/traces/search'</span><span class="p">,</span> <span class="w"> </span><span class="nx">votes</span><span class="o">:</span><span class="w"> </span><span class="s1">'/api/v1/votes'</span><span class="p">,</span> <span class="w"> </span><span class="p">},</span> <span class="w"> </span><span class="nx">timeout</span><span class="o">:</span><span class="w"> </span><span class="kt">5000</span><span class="p">,</span> <span class="p">}</span><span class="w"> </span><span class="nx">satisfies</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">endpoints</span><span class="o">:</span><span class="w"> </span><span class="kt">Record</span><span class="o">&lt;</span><span class="kt">string</span><span class="p">,</span><span class="w"> </span><span class="kt">string</span><span class="o">&gt;</span><span class="p">;</span><span class="w"> </span><span class="nx">timeout</span><span class="o">:</span><span class="w"> </span><span class="kt">number</span><span class="w"> </span><span class="p">};</span> <span class="c1">// config.endpoints.traces is still inferred as '/api/v1/traces' (not just string)</span> <span class="c1">// Without satisfies: type assertion loses the narrower type</span> <span class="c1">// With satisfies: both type checking AND narrow type preservation</span> <span class="c1">// Use case: route configuration</span> <span class="kd">const</span><span class="w"> </span><span class="nx">routes</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[</span> <span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">path</span><span class="o">:</span><span class="w"> </span><span class="s1">'/'</span><span class="p">,</span><span class="w"> </span><span class="nx">component</span><span class="o">:</span><span class="w"> </span><span class="kt">Dashboard</span><span class="p">,</span><span class="w"> </span><span class="nx">exact</span><span class="o">:</span><span class="w"> </span><span class="kt">true</span><span class="w"> </span><span class="p">},</span> <span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">path</span><span class="o">:</span><span class="w"> </span><span class="s1">'/traces/:id'</span><span class="p">,</span><span class="w"> </span><span class="nx">component</span><span class="o">:</span><span class="w"> </span><span class="kt">TraceDetail</span><span class="p">,</span><span class="w"> </span><span class="nx">exact</span><span class="o">:</span><span class="w"> </span><span class="kt">false</span><span class="w"> </span><span class="p">},</span> <span class="p">]</span><span class="w"> </span><span class="nx">satisfies</span><span class="w"> </span><span class="nb">Array</span><span class="o">&lt;</span><span class="p">{</span><span class="w"> </span><span class="nx">path</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">component</span><span class="o">:</span><span class="w"> </span><span class="kt">React.FC</span><span class="p">;</span><span class="w"> </span><span class="nx">exact</span><span class="o">:</span><span class="w"> </span><span class="kt">boolean</span><span class="w"> </span><span class="p">}</span><span class="o">&gt;</span><span class="p">;</span> <span class="c1">// Combine both:</span> <span class="kd">const</span><span class="w"> </span><span class="nx">PERMISSIONS</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">admin</span><span class="o">:</span><span class="w"> </span><span class="p">[</span><span class="s1">'read'</span><span class="p">,</span><span class="w"> </span><span class="s1">'write'</span><span class="p">,</span><span class="w"> </span><span class="s1">'delete'</span><span class="p">],</span> <span class="w"> </span><span class="nx">user</span><span class="o">:</span><span class="w"> </span><span class="p">[</span><span class="s1">'read'</span><span class="p">,</span><span class="w"> </span><span class="s1">'write'</span><span class="p">],</span> <span class="w"> </span><span class="nx">guest</span><span class="o">:</span><span class="w"> </span><span class="p">[</span><span class="s1">'read'</span><span class="p">],</span> <span class="p">}</span><span class="w"> </span><span class="kr">as</span><span class="w"> </span><span class="kd">const</span><span class="w"> </span><span class="nx">satisfies</span><span class="w"> </span><span class="nx">Record</span><span class="o">&lt;</span><span class="kt">string</span><span class="p">,</span><span class="w"> </span><span class="k">readonly</span><span class="w"> </span><span class="kt">string</span><span class="p">[]</span><span class="o">&gt;</span><span class="p">;</span> <span class="kr">type</span><span class="w"> </span><span class="nx">Role</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">keyof</span><span class="w"> </span><span class="ow">typeof</span><span class="w"> </span><span class="nx">PERMISSIONS</span><span class="p">;</span><span class="w"> </span><span class="c1">// 'admin' | 'user' | 'guest'</span> </code></pre></div> <p>Key points: - as const makes all values readonly and preserves literal types - satisfies validates against a type but keeps inferred narrow type - Combine as const satisfies for both readonly literal types and type checking - Use for config objects, route tables, and enum-like constants - typeof obj[keyof typeof obj] extracts union of values from const objects</p>