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);
if DescribableFunction has no "description" field,const or let will be ok I test it at https://www.typescriptlang.org/play
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);
};
So, this is design decision.
const mutable = (a:string)=>console.log(a); mutable.b = 2; is right,but const mutable ={a:1}; mutable.b = 2; has error for this reason.