console.log in array not working as expect

[JiangWeixian]

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Describe the bug
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const a = []
a[2] = 1
console.log(a.length)
for (let i = 0; i < a.length; i++) {
  console.log(i)
}

only console.log(a.length) worked

Expected behavior
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To Reproduce
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1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Version (please complete the following information):

• Version [e.g. 22]
• OS [e.g macos]

Additional context
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• Mini reproduction