[DOC]Style Guid : Why is this example used of "indexOf" than "includes" on Prop definitions?

[mymactive]

Hi,
I feel a bit uncomfortable code on Props definitions.
This example is following:

// Even better!
props: {
  status: {
    type: String,
    required: true,
    validator: function (value) {
      return [
        'syncing',
        'synced',
        'version-conflict',
        'error'
      ].indexOf(value) !== -1
    }
  }
}

For this example, "indexOf" is used than "includes".
Semantically, I think "includes" is natural than "indexOf" in this context.
Would you teach the reason why "indexOf" is better than "includes"?
If "includes" is better, I will edit the style guide.

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