Add support for type guarding to if statements

Suggestion

🔍 Search Terms

type guard guarding assertion

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

This proposal introduces a new feature that would allow developers to use type guard statements with if-statements rather than only with functions. This feature would provide a more concise and expressive syntax for type guards, making code easier to read and maintain.

📃 Motivating Example

With the proposed feature, developers could use the is keyword directly within an if-statement to perform a type guard. For example…

Before — requires separate function for type guarding

function isIterable<T>(obj: Iterable<T> | ArrayLike<T>): obj is Iterable<T> {
  return typeof (obj as Iterable<T>)[Symbol.iterator] === 'function';
}

function iterateOver<T>(obj: Iterable<T> | ArrayLike<T>, expose: Function) {
  if (isIterable(obj)) {
    // some logic here
  } else {
    // some logic here
  }
}

After — can handle type guarding inline via if statement

function iterateOver<T>(obj: Iterable<T> | ArrayLike<T>, expose: Function) {
  const isIterable = typeof (obj as Iterable<T>)[Symbol.iterator] === 'function';
  if (isIterable): obj is Iterable<T> {
    // some logic here
  } else {
    // some logic here
  }
}

** this could just as easily be rewritten to get rid of the isIterable placeholder altogether, like this:

function iterateOver<T>(obj: Iterable<T> | ArrayLike<T>, expose: Function) {
  if (typeof (obj as Iterable<T>)[Symbol.iterator] === 'function'): obj is Iterable<T> {
    // some logic here
  } else {
    // some logic here
  }
}

💻 Use Cases

Currently, if developers want to use a type guard to narrow down the type of a value within an if statement, they need to define a separate function (AFAIK) that returns a boolean value indicating whether the value is of a certain type. This approach can be cumbersome and can lead to code that is harder to read and understand.

从 issue 中描述的动机示例和现有类型守卫行为开始。没有指定源文件、测试或编译器入口点。要视为完成，需要确定内联类型守卫的语法和语义，并完成相应的实现和验证。