<span class="hljs-keyword">var</span> <span class="hljs-title function_">f</span> = x => x; <span class="hljs-title function_">f</span>(<span class="hljs-function"><span class="hljs-params">x</span> =></span> x + <span class="hljs-function">(<span class="hljs-params">y=<span class="hljs-number">2</span>, z=<span class="hljs-literal">undefined</span>, ...rest</span>) =></span> y); <span class="hljs-function">() =></span> <span class="hljs-literal">null</span>; <span class="hljs-keyword">const</span> <span class="hljs-title function_">FC</span> = props => <span class="language-xml"><span class="hljs-tag"><<span class="hljs-name">p</span>></span>functional component<span class="hljs-tag"></<span class="hljs-name">p</span>></span></span>; <span class="hljs-keyword">const</span> <span class="hljs-title function_">good</span> = (<span class="hljs-params"></span>) => <span class="hljs-number">0</span>; <span class="hljs-keyword">const</span> <span class="hljs-title function_">good</span> = (<span class="hljs-params">x</span>) => <span class="hljs-number">0</span>; <span class="hljs-keyword">const</span> bad = (<span class="hljs-function"><span class="hljs-params">a</span> =></span> [...a, b]); <span class="hljs-keyword">const</span> bad = (<span class="hljs-function"><span class="hljs-params">_</span> =></span> <span class="hljs-title function_">doSomething</span>()); <span class="hljs-keyword">const</span> bad = (<span class="hljs-function">() =></span> <span class="hljs-number">0</span>); <span class="hljs-keyword">const</span> bad = (<span class="hljs-function">(<span class="hljs-params">a, b</span>) =></span> [...a, b]); <span class="hljs-keyword">const</span> array = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>].<span class="hljs-title function_">reduce</span>(<span class="hljs-function">(<span class="hljs-params">acc, next</span>) =></span> [...acc, next], []); sides.<span class="hljs-title function_">every</span>(<span class="hljs-function">(<span class="hljs-params">length,width=(<span class="hljs-number">3</span>+<span class="hljs-number">2</span>+(<span class="hljs-number">4</span>/<span class="hljs-number">5</span>))</span>) =></span> length > <span class="hljs-number">0</span> );