How can type annotations be properly incorporated into this TypeScript code using an inline function class?

How can I provide type annotations to inform TypeScript that this code is correct?

Indeed, I do require that inline function class. The code presented here is a simplified version of my actual problem.

let x = 10;
const obj = new (function() {
  if(--x) return this.constructor();
  return {};
});
'this' implicitly has type 'any' because it does not have a type annotation.ts(2683)
'new' expression, whose target lacks a construct signature, implicitly has an 'any' type.ts(7009)

I attempted fully annotating everything but it did not resolve the issue. Here is what it looks like with annotations:

type myType = { constructor(): myType };
let x: number = 10;
const obj: myType = new (function(): myType {
  if(--x) return (this as myType).constructor();
  return {} as myType;
});
'new' expression, whose target lacks a construct signature, implicitly has an 'any' type.ts(7009)

Is there something in the TypeScript documentation that I might be overlooking? It appears that there are no generic Constructor types and the available Constructor types are not specifically for constructors.

Answer №1

When defining a type as being "newable", meaning it can be used with the new operator, you should utilize a construct signature. This construct signature can take the form of { new (⋯): ⋯} similar to a call signature, or new (⋯) => ⋯ akin to an arrow function expression.

To ensure your code compiles correctly, consider following this example:

let x = 10;
type MyType = { new(): {} };
const obj = new ((function (this: {}) {
  console.log(x);
  if (--x) return this.constructor();
  return {};
}) as any as MyType);

In this scenario, MyType is defined as { new(): {} }, specifying a construct signature that does not require constructor arguments and produces a value of type {}.

The assertion that the anonymous function expression matches type MyType is done using (⋯) as any as MyType. It's worth noting that direct assertions between callable types and newable types are not allowed in TypeScript due to their different behaviors in JavaScript. An intermediate type like any or Function is needed to bridge the gap.

In addition, the anonymous function is given a this parameter of type {} for compiler satisfaction when dealing with this inside the function body.

With these adjustments, there should be no errors, and the inferred type of obj will be {}. As you work on your own projects, make sure to tailor your typings accordingly, ensuring at least one construct signature is involved.

For further exploration and experimentation, visit the TypeScript Playground link.

Answer №2

Convert a generic function from (...a: A) => B to new (...a: A) => B

function createNewable<F extends (...a: any) => any>(
  fn: F,
): (new (...a: Parameters<F>) => ReturnType<F>) /* ` & F` if additional props are needed */ {
  return fn as any
}
function bar() {
  return Math.random() > 0.5 ? 'bar' : { bar: 'baz' };
}
let x = new (fakeNewable(bar))
//  ^?
// let x: string | { bar: string; }

(note that returning undefined inside the new call will result in getting the constructed object)

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