Error Cannot invoke an expression whose type lacks a call signature

Question

I am brand new to typescript  and I have two classes  In the parent class I have   abstract class Component     public deps  any         public props  any          public setProp prop  string   any       return  lt T gt  val  T   T   gt          this props prop    val        return val                 In the child class I have   class Post extends Component     public toggleBody  string     constructor         this toggleBody   this setProp  showFullBody           public showMore    boolean       return this toggleBody true          public showLess    boolean       return this toggleBody false           Both showMore and ShowLess give me the error   Cannot invoke an expression whose type lacks a call signature    But the function that setProp returns DOES have a call signature  I think  I think I m misunderstanding something important about typings of functions  but I don t know what it is   Thanks

User · Accepted Answer

The function that it returns has a call signature  but you told Typescript to completely ignore that by adding   any in its signature

User · Answer

I think what you want is   abstract class Component     public deps  any         public props  any          public makePropSetter lt T gt  prop  string    val  T    gt  T       return function val          this props prop    val       return val              class Post extends Component     public toggleBody   val  boolean    gt  boolean     constructor          super       this toggleBody   this makePropSetter lt boolean gt   showFullBody          showMore     boolean       return this toggleBody true         showLess     boolean       return this toggleBody false          The important change is in setProp  i e   makePropSetter in the new code    What you re really doing there is to say  this is a function  which provided with a property name  will return a function which allows you to change that property   The  lt T gt  on makePropSetter allows you to lock that function in to a specific type   The  lt boolean gt  in the subclass s constructor is actually optional   Since you re assigning to toggleBody  and that already has the type fully specified  the TS compiler will be able to work it out on its own   Then  in your subclass  you call that function  and the return type is now properly understood to be a function with a specific signature   Naturally  you ll need to have toggleBody respect that same signature

User · Answer

Let s break this down    The error says     Cannot invoke an expression whose type lacks a call signature   The code    The problem is in this line public toggleBody  string   amp   it s relation to these lines       return this toggleBody true       return this toggleBody false      The result    Your saying toggleBody is a string but then your treating it like something that has a call signature  i e  the structure of something that can be called  lambdas  proc  functions  methods  etc  In JS just function tho    You need to change the declaration to be public toggleBody   arg  boolean    gt  boolean    Extra Details    invoke  means your calling or applying a function    an expression  in Javascript is basically something that produces a value  so this toggleBody   counts as an expression    type  is declared on this line public toggleBody  string   lacks a call signature  this is because your trying to call something this toggleBody   that doesn t have signature i e  the structure of something that can be called  lambdas  proc  functions  methods  etc   that can be called  You said this toggleBody is something that acts like a string   In other words the error is saying     Cannot call an expression  this toggleBody  because it s type   string  lacks a call signature  bc it has a string signature

User · Answer

It means you re trying to call something that isn t a function  const foo    string  foo      error

User · Answer

This error can be caused when you are requesting a value from something and you put parenthesis at the end  as if it is a function call  yet the value is correctly retrieved without ending parenthesis   For example  if what you are accessing is a Property  get  in Typescript   private IMadeAMistakeHere    void       let mynumber   this SuperCoolNumber       private IDidItCorrectly    void       let mynumber   this SuperCoolNumber     private get SuperCoolNumber    number       let response   42      return response

User · Answer

Cannot invoke an expression whose type lacks a call signature     In your code     class Post extends Component     public toggleBody  string     constructor         this toggleBody   this setProp  showFullBody           public showMore    boolean       return this toggleBody true          public showLess    boolean       return this toggleBody false           You have public toggleBody  string   You cannot call a string as a function  Hence errors on   this toggleBody true   and this toggleBody false

User · Answer

Add a type to your variable and then return   Eg   const myVariable   string        hello    there     const result   myVaraible map x  gt      return         x id              Important part is adding the string   type etc

User · Answer

I had the same error message  In my case I had inadvertently mixed the ES6 export default function myFunc syntax with const myFunc   require    myFunc      Using module exports   myFunc  instead solved the issue

[javascript] Error: Cannot invoke an expression whose type lacks a call signature

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