I have 2 errors with a function that I have attempted to convert to TypeScript. The problem is with the parameters which are of type Set

import type {Set} from 'typescript'

function union<T>(setA: Set<T>, setB: Set<T>) {
  const _union = new Set(setA);
  for (const elem of setB) {
    _union.add(elem)
  }
  return _union
}

The first error is on line 4: new Set(setA)

TS2769: No overload matches this call.

The second error is on line 5: for (const elem of setB)

TS2495: Type 'Set ' is not an array type or a string type.

Otherwise, this function works as expected.

tsconfig.json

{
    "compilerOptions": {
        "sourceMap": true,
        "outDir": "dist",
        "module": "commonjs",
        "target": "es5",
        "lib": ["esnext"],
        "esModuleInterop": true
    },
    "exclude": [
        "node_modules"
    ],
    "include": [
        "src/**/*"
    ]
}

CodePudding user response：

The Problem

When you import { Set } from 'typescript', you actually import the type definition for the Set object. Then, I believe, the type definition overlaps the Set object given by javascript(without the need to import it). If you look for the Set type definition in typescript, you'll find out that Set represents a Typescript interface:

    interface Set<T> extends ReadonlySet<T>, Collection<T> {
        add(value: T): this;
        delete(value: T): boolean;
    }

But when you call new Set(), the interpreter looks for a constructor to initiate the object, but it doesn't find one, as the imported Set is an interface. And that is why you get the errors:

• No overload matches this call.: There is no constructor for the imported Set, so Typescript doesn't find any relevant function that could initiate the Set.
• TS2495: Type 'Set ' is not an array type or a string type.: Set is indeed not an array type, nor a string as it is an interface.

The Solution

As javascript gives us the Set object, without any need to import it, you don't have to import it at all. In addition, you also don't have to import the Set type definition, as it is built in.