refactor(index): avoid Object.assign

[alexander-akait]

ℹ️ Describe the big picture of your changes here to communicate to the maintainers
why we should accept this pull request. If it fixes a bug or resolves a feature
request, be sure to link to that issue.

Type

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ℹ️ What types of changes does your code introduce?

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• Fix
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• Chore
• Feature
• Refactor

SemVer

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ℹ️ What changes to the current semver range does your PR introduce?

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• Bug (:label: Patch)
• Feature (:label: Minor)
• Breaking Change (:label: Major)

Issues

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ℹ️ What issue (if any) are closed by your PR?

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not required

Checklist

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ℹ️ You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. This is a reminder of what we are going to look for before merging your code.

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• Lint and unit tests pass with my changes
• I have added tests that prove my fix is effective/works
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[coveralls]

Coverage decreased (-0.5%) to 88.82% when pulling 6a2f0b0 on evilebottnawi-patch-1 into 2484275 on master.

[michael-ciniawsky]

#326