Are strongly-typed functions as parameters possible in TypeScript

Question

In TypeScript  I can declare a parameter of a function as a type Function  Is there a  type-safe  way of doing this that I am missing  For example  consider this   class Foo       save callback  Function    void             Do the save         var result   number   42    We get a number from the save operation           Can I at compile-time ensure the callback accepts a single parameter of type number somehow          callback result            var foo   new Foo    var callback    result  string    void   gt        alert result     foo save callback     The save callback is not type safe  I am giving it a callback function where the function s parameter is a string but I am passing it a number  and compiles with no errors  Can I make the result parameter in save a type-safe function   TL DR version  is there an equivalent of a  NET delegate in TypeScript

User · Answer

type FunctionName    n  inputType    gt  any   class ClassName       save callback  FunctionName    void           callback data             This surely aligns with the functional programming paradigm

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Here are TypeScript equivalents of some common  NET delegates   interface Action lt T gt         item  T   void     interface Func lt T TResult gt         item  T   TResult

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Sure  A function s type consists of the types of its argument and its return type  Here we specify that the callback parameter s type must be  function that accepts a number and returns type any    class Foo       save callback   n  number    gt  any    void           callback 42           var foo   new Foo     var strCallback    result  string    void   gt        alert result     var numCallback    result  number    void   gt        alert result toString        foo save strCallback      not OK foo save numCallback      OK   If you want  you can define a type alias to encapsulate this   type NumberCallback    n  number    gt  any   class Foo          Equivalent     save callback  NumberCallback    void           callback 42

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I realize this post is old  but there s a more compact approach that is slightly different than what was asked  but may be a very helpful alternative  You can essentially declare the function in-line when calling the method  Foo s save   in this case   It would look something like this   class Foo       save callback   n  number    gt  any    void           callback 42             multipleCallbacks firstCallback   s  string    gt  void  secondCallback   b  boolean    gt  boolean   void           firstCallback  hello world            let result  boolean   secondCallback true          console log  Resulting boolean      result           var foo   new Foo       Single callback example     Just like with  RyanCavanaugh s approach  ensure the parameter s  and return    types match the declared types above in the  save    method definition  foo save  newNumber  number    gt        console log  Some number      newNumber          This is optional  since  any  is the declared return type      return newNumber        Multiple callbacks example     Each call is on a separate line for clarity     Note that  firstCallback    has a void return type  while the second is boolean  foo multipleCallbacks       s  string    gt             console log  Some string      s              b  boolean    gt            console log  Some boolean      b          let result   b  amp  amp  false          return result           The multipleCallback   approach is very useful for things like network calls that may succeed or fail  Again assuming a network call example  when multipleCallbacks   is called  behavior for both a success and failure can be defined in one spot  which lends itself to greater clarity for future code readers   Generally  in my experience  this approach lends itself to being more concise  less clutter  and greater clarity overall   Good luck all

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In TS we can type functions in the in the following manners   Functions types signatures  This is used for real implementations of functions methods it has the following syntax    arg1  Arg1type  arg2  Arg2type    ReturnType   Example   function add x  number  y  number   number       return x   y     class Date     setTime time  number   number                      Function Type Literals  Function type literals are another way to declare the type of a function  They re usually applied in the function signature of a higher-order function  A higher-order function is a function which accepts functions as parameters or which returns a function  It has the following syntax    arg1  Arg1type  arg2  Arg2type    gt  ReturnType   Example   type FunctionType1    x  string  y  number    gt  number   class Foo       save callback   str  string    gt  void                             doStuff callback  FunctionType1

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Because you can t easily union a function definition and another data type  I find having these types around useful to strongly type them  Based on Drew s answer  type Func lt TArgs extends any    TResult gt        args  TArgs    gt  TResult     Syntax sugar type Action lt TArgs extends any   gt    Func lt TArgs  undefined gt     Now you can strongly type every parameter and the return type  Here s an example with more parameters than what is above  save callback  Func lt  string  Object  boolean   number gt    number       let str    quot  quot       let obj           let bool   true      let result  number   callback str  obj  bool       return result     Now you can write a union type  like an object or a function returning an object  without creating a brand new type that may need to be exported or consumed    THIS DOESN T WORK let myVar1  boolean    parameters  object    gt  boolean     This works  but requires a type be defined each time type myBoolFunc    parameters  object    gt  boolean  let myVar1  boolean   myBoolFunc     This works  with a generic type that can be used anywhere let myVar2  boolean   Func lt  object   boolean gt

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If you define function type first then it would be looked like  type Callback    n  number    gt  void   class Foo       save callback  Callback    void                   callback 42            var foo   new Foo    var stringCallback    result  string    void   gt        console log result      var numberCallback    result  number    void   gt        console log result      foo save stringCallback     --will be showing error foo save numberCallback     Without function type by using plain property syntax it would be   class Foo       save callback   n  number    gt  void    void                   callback 42            var foo   new Foo    var stringCallback    result  string    void   gt        console log result      var numberCallback    result  number    void   gt        console log result      foo save stringCallback     --will be showing error foo save numberCallback     If you want by using an interface function like c  generic delegates it would be   interface CallBackFunc lt T  U gt         input T   U      class Foo       save callback  CallBackFunc lt number void gt     void                   callback 42            var foo   new Foo    var stringCallback    result  string    void   gt        console log result      var numberCallback    result  number    void   gt        console log result      let strCBObj CallBackFunc lt string void gt    stringCallback  let numberCBObj CallBackFunc lt number void gt    numberCallback   foo save strCBObj     --will be showing error foo save numberCBObj

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Besides what other said  a common problem is to declare the types of the same function that is overloaded  Typical case is EventEmitter on   method which will accept multiple kind of listeners  Similar could happen When working with redux actions - and there you use the action type as literal to mark the overloading  In case of EventEmitters  you use the event name   literal type   interface MyEmitter extends EventEmitter     on name  click   l  ClickListener  void   on name  move   l  MoveListener  void   on name  die   l  DieListener  void     and a generic one   on name string  l     a any     gt any  void    type ClickListener    e ClickEvent   gt void type MoveListener    e MoveEvent   gt void     etc     will type check the correct listener when writing something like  myEmitter on  click   e  gt     lt --- autocompletion

[typescript] Are strongly-typed functions as parameters possible in TypeScript?

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