typescript - 在 Typescript 中，我创建了一个带有字符串字段的函数接口(interface)，为什么实现必须是 const？

Question

interface DescribableFunction  {
  description: string;
  (f: number,s:number): number;
};

function doSomething(fn: DescribableFunction) {
  console.log( " returned " + fn(3,4));
}

//here,"const" is right,"let" will be error
const p:DescribableFunction=(first:number,second:number)=>first+second;
p.description="hello";

doSomething(p);

如果 DescribableFunction 没有“描述”字段，const 或 let 都可以我在 https://www.typescriptlang.org/play 测试它

Answer 1

Here you have a documentation :

TypeScript 3.1 brings the ability to define properties on function declarations and const-declared functions, simply by assigning to properties on these functions in the same scope. This allows us to write canonical JavaScript code without resorting to namespace hacks. For example:

function readImage(path: string, callback: (err: any, image: Image) => void) {
  // ...
}
readImage.sync = (path: string) => {
  const contents = fs.readFileSync(path);
  return decodeImageSync(contents);
};

所以，这是设计决策。