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<html lang=en>
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  <meta charset=utf-8>
  <title>Prime numbers in range</title>
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  <h1>Prime numbers in the range 1 to 20</h1>
  <pre id=out></pre>
  <script>!function(){function n(n){if(n<=1)return!1;for(let t=2;t*t<=n;t++)if(n%t==0)return!1;return!0}var t,e=[];for(t=1;t<=20;t++)n(t)&&e.push(String(t));var r=[];r.push("Prime numbers in the range 1 to 20:"),r.push(e.join(" ")),document.getElementById("out").textContent=r.join("\n")}()</script>
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JavaScript Prime Number — Range 1 to 20 (Try It)