<pre class="shiki github-dark shaku"><code>interface Matcher { toBe(data: any): void}function myExpect(input: any): Matcher &amp; { not: Matcher } { const generate = (isNegated: boolean) =&gt; { return { toBe(data: any) { const result = isNegated ? !Object.is(input, data) : Object.is(input, data); if (!result) { throw new Error("Invalid"); } }, get not() { return generate(!isNegated) } } } return generate(false)}</code></pre>