Distributive as const

Search Terms

as const, widening, literal widening

Suggestion

Allow to use as const on ternary operator as well as others code branches that currently we can't.

Use Cases

Let's say that we have this code:

const result = (
  value > 5
    ? { amount: 'many' } as const
    : { amount: 'few' } as const
)

We need to type as const twice. So would be nice if we could type just once:

const result = (
  value > 5
    ? { amount: 'many' }
    : { amount: 'few' }
) as const

Currently we can't do that (playground).
It's just a syntax sugar. It won't change anything on as const!

Analogously, maybe we could allow this syntax sugar on function:

const func = (param: number) => {
  if (param > 5) {
    return 'many' as const
  }

  return 'few' as const
}

// would be just...
const func = (param: number): const => {
  if (param > 5) {
    return 'many'
  }

  return 'few'
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

リンクされている TypeScript Playground の例から始めて、拒否される as const の配置を再現し、受け入れられる分岐レベルの形式と比較してください。ソースファイルやテストは指定されていません。const assertions に関するコンパイラーとテストのエントリーポイントを特定し、提案された三項構文が、生成される JavaScript や既存の動作を変更せずに受け入れられることを確認してください。