Defining TypeScript callback type

Question

I ve got the following class in TypeScript   class CallbackTest       public myCallback       public doWork    void                 doing some work            this myCallback      calling callback           I am using the class like this   var test   new CallbackTest    test myCallback        gt  alert  done    test doWork      The code works  so it displays a messagebox as expected   My question is  Is there any type I can provide for my class field myCallback  Right now  the public field myCallback is of type any as shown above  How can I define the method signature of the callback  Or can I just set the type to some kind of callback-type  Or can I do nether of these  Do I have to use any  implicit explicit    I tried something like this  but it did not work  compile-time error    public myCallback         or  public myCallback  function    I couldn t find any explanation to this online  so I hope you can help me

User · Answer

I m a little late  but  since some time ago in TypeScript you can define the type of callback with   type MyCallback    KeyboardEvent    gt  void    Example of use   this addEvent document   keydown    e    gt        if  e keyCode     1          e preventDefault               addEvent element  eventName  callback  MyCallback        element addEventListener eventName  callback  false

User · Answer

Here is an example - accepting no parameters and returning nothing    class CallbackTest       public myCallback       void         public doWork    void                 doing some work            this myCallback      calling callback          var test   new CallbackTest    test myCallback        gt  alert  done    test doWork      If you want to accept a parameter  you can add that too   public myCallback    msg  string   void      And if you want to return a value  you can add that also   public myCallback    msg  string   number

User · Answer

I came across the same error when trying to add the callback to an event listener  Strangely  setting the callback type to EventListener solved it  It looks more elegant than defining a whole function signature as a type  but I m not sure if this is the correct way to do this   class driving          the answer from this post - this works        private callback       gt  void           this also works      private callback EventListener       constructor            this callback        gt  this startJump            window addEventListener  keydown   this callback              startJump   void           console log  jump             window removeEventListener  keydown   this callback

User · Answer

To go one step further  you could declare a type pointer to a function signature like   interface myCallbackType    myArgument  string   void     and use it like this   public myCallback   myCallbackType

User · Answer

If you want a generic function you can use the following  Although it doesn t seem to be documented anywhere   class CallbackTest     myCallback  Function

User · Answer

You can use the following    Type Alias  using type keyword  aliasing a function literal  Interface Function Literal     Here is an example of how to use them   type myCallbackType    arg1  string  arg2  boolean    gt  number   interface myCallbackInterface    arg1  string  arg2  boolean   number     class CallbackTest                   public myCallback2  myCallbackType      public myCallback3  myCallbackInterface      public myCallback1   arg1  string  arg2  boolean    gt  number

User · Answer

You can declare a new type   declare type MyHandler    myArgument  string    gt  void   var handler  MyHandler    Update   The declare keyword is not necessary  It should be used in the  d ts files or in similar cases

User · Answer

I just found something in the TypeScript language specification  it s fairly easy  I was pretty close   the syntax is the following   public myCallback   name  type    gt  returntype    In my example  it would be  class CallbackTest       public myCallback       gt  void       public doWork    void                 doing some work            this myCallback      calling callback

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