TypeScript asserts that the Function is not callable

Question

TypeScript asserts that the Function is not callable

There seems to be an issue with TypeScript not recognizing that a function of type Function is not callable.

type Constructable = { new(...args: any[]): any }

function isClass(func: any) {
  return (
    typeof func === 'function' &&
    /^class\s/.test(Function.prototype.toString.call(func))
  )
}

function coerceOne(data: any, fn: Function | Constructable) {
  if (isClass(fn)) {
    const constructor = fn as Constructable
    return new constructor(data)
  } else {
    return fn(data) // <-- ERROR
  }
}

Error:

This expression is not callable.
  No constituent of type 'Function | Constructable' is callable.(2349)

https://www.typescriptlang.org/play?ssl=14&ssc=11&pln=14&pc=6#code/C4TwDgpgBAwg9gOwM7AE4FcDGwCGAjAG2gF4oBvKBCAdwAoA6RnVAcyQC4ocEQBtAXQCUnbiCgBfAFCSAZugTYAloiiKkMAjiRJachSJ6DykqFFQRg6VAii0TpqKEhwZUPZijEvUAOTvgygg+UABkIfamAPQAepia2gA6SJH0wBAotABi8kqI9GCocMBF4BCpcADKaIoILPSYOAQEujmCgvbtUrI5ASqYcBComBAA8lS0ACY4uAYgADRuCJzZCr02AD6wiCgY2PhERmT2iq60ahpaOjIIbcYOUP3IwA-baFjFqJ6LXEhbT7u4QgQCJmCxWGxUagvf7vOCoSbTHDtUziKAQAhIaBHe7mSzWRYI3DIiSSKRAA

Any suggestions on how to troubleshoot this problem?

typescript

Answer 1

Answer №1

Your custom type guard isClass requires a return type predicate: : func is Constructable

type Constructable = { new(...args: any[]): any }

function isClass(func: any): func is Constructable {
  return (
    typeof func === 'function' &&
    /^class\s/.test(Function.prototype.toString.call(func))
  )
}

function coerceOne(data: any, fn: Function | Constructable) {
  if (isClass(fn)) {
    const constructor = fn as Constructable
    return new constructor(data)
  } else {
    return fn(data)
  }
}

console.log(coerceOne('works', console.log))

Click here for more details

Answer 2

Your custom type guard isClass requires a return type predicate: : func is Constructable

type Constructable = { new(...args: any[]): any }

function isClass(func: any): func is Constructable {
  return (
    typeof func === 'function' &&
    /^class\s/.test(Function.prototype.toString.call(func))
  )
}

function coerceOne(data: any, fn: Function | Constructable) {
  if (isClass(fn)) {
    const constructor = fn as Constructable
    return new constructor(data)
  } else {
    return fn(data)
  }
}

console.log(coerceOne('works', console.log))

Click here for more details

TypeScript asserts that the Function is not callable

Answer №1

Similar questions

React JS displayed the string of /static/media/~ instead of rendering its markdown content

Effortlessly collapsing cards using Angular 2 and Bootstrap

Exploration of mapping in Angular using the HttpClient's post

Sending an event from a child component to another using parent component in Angular

What causes the oninput event to act differently in Angular as opposed to regular JavaScript?

Is there a way to utilize req.query, req.params, or req.* beyond its original scope without the need to store it in a database?

Ionic app: refresher functionality works in browser but not on iOS home screen app

Discovering an entry that lacks a specific value within an array in the records

Can you use TypeScript to define generic React functional components?

Error TS2322: Type 'boolean' cannot be assigned to type 'undefined'. What is the best approach for dynamically assigning optional properties?

How can I determine the existence of an S3 bucket and, if it doesn't exist, create it using TypeScript CDK?

Custom fields provided by Express are not accessible within routes

TypeScript seems to be failing to detect the necessary checks before they are used

Experimenting with a file system library function using Jest and Typescript alongside a placeholder function

Ways to extend the default timeout duration in Angular

I can't figure out why I'm receiving undefined even though all the variables are populated with the necessary

What's causing Angular to not display my CSS properly?

Guide on invoking child components' functions from the parent component in Angular 6

The specified module '...' is identified as a non-module entity and therefore cannot be imported using this specific construct

Accessing and manipulating a intricate JSON structure within an Ionic 3 framework to seamlessly connect it to the user