[Webkit-unassigned] [Bug 237385] New: let vs var performance

Wed Mar 2 11:33:28 PST 2022

https://bugs.webkit.org/show_bug.cgi?id=237385

            Bug ID: 237385
           Summary: let vs var performance
           Product: WebKit
           Version: Safari 15
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jeremy.tellaa at gmail.com

I have built a very basic POC, and I found that looping when the loop is initialized with `let` instead of `var` is about seven times slower:

```
<!doctype html>
<html lang="en">

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <meta name="theme-color" content="#000000">
  <title>POC</title>
</head>

<body>
  <div id="root"></div>
  <script type="text/javascript">
  const nb = 1000 * 1000 * 1000;
  const t0 = performance.now();
  for (var i = 0; i < nb; i++) { };
  const t1 = performance.now();
  for (let j = 0; j < nb; j++) { };
  const t2 = performance.now();
  const resVar = Math.floor(t1 - t0);
  const resLet = Math.floor(t2 - t1);

  res = `Looping ${nb} times with 'var' took ${resVar}ms.<br>Looping ${nb} times with 'let' took ${resLet}ms.`
  document.getElementById('root').innerHTML = res;
  </script>
</body>

</html>
```

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