narrowing return type of "array.includes"

[YanagiEiichi]

The array.includes is used usually to check data type, such as:

const typeList = [ 'apple', 'banana', 'orange' ] as const;

// Return 'apple' | 'banana' | 'orange' is better than any.
function getValueOrDefault(what: any) {
  if (typeList.includes(what)) {
    return what;
  }
  return 'apple';
}

Fixes partial #36275

interface TextMessage { type: 'text', text: string }

interface ImageMessage { type: 'image', url: string }

type Message = TextMessage | ImageMessage;

const message: Message = JSON.parse('{}') as Message;

// Prepare a const type list
const typeList = ['text'] as const;

// Take the `type` as any
let type: any = message.type;

// Narrowing the `type`
if (typeList.includes(type)) {
    // Narrowing the `message`
    if (message.type === type) {
        // It's work
        message.text = message.text.trim();
    }
}

[typescript-bot]

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