TypeScript function overloading

Question

Section 6 3 of the TypeScript language spec talks about function overloading and gives concrete examples on how to implement this   However if I try something like this   export class LayerFactory         constructor  public styleFactory  Symbology StyleFactory           createFeatureLayer  userContext   Model UserContext  mapWrapperObj   MapWrapperBase    any                       throw  not implemented                               createFeatureLayer layerName   string  style   any    any           throw  not implemented                       I get a compiler error indicating duplicate identifier even though function parameters are of different types   Even if I add an additional parameter to the second createFeatureLayer function  I still get a compiler error   Ideas  please

User · Accepted Answer

This may be because  when both functions are compiled to JavaScript  their signature is totally identical  As JavaScript doesn t have types  we end up creating two functions taking same number of arguments  So  TypeScript restricts us from creating such functions   TypeScript supports overloading based on number of parameters  but the steps to be followed are a bit different if we compare to OO languages  In answer to another SO question  someone explained it with a nice example  Method overloading    Basically  what we are doing is  we are creating just one function and a number of declarations so that TypeScript doesn t give compile errors  When this code is compiled to JavaScript  the concrete function alone will be visible  As a JavaScript function can be called by passing multiple arguments  it just works

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As a heads up to others  I ve oberserved that at least as manifested by TypeScript compiled by WebPack for Angular 2  you quietly get overWRITTEN instead of overLOADED methods   myComponent     method      console info  no args         method arg     console info  with arg          Calling   myComponent method     seems to execute the method with arguments  silently ignoring the no-arg version  with output   with arg

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You can declare an overloaded function by declaring the function as having a type which has multiple invocation signatures   interface IFoo       bar             s  string   number           n  number   string            Then the following   var foo1  IFoo         var n  number   foo1 bar  baz           OK var s  string   foo1 bar 123            OK var a  number     foo1 bar  1 2 3       ERROR   The actual definition of the function must be singular and perform the appropriate dispatching internally on its arguments   For example  using a class  which could implement IFoo  but doesn t have to    class Foo       public bar s  string   number      public bar n  number   string      public bar arg  any   any                if  typeof arg       number               return arg toString            if  typeof arg       string               return arg length            What s interesting here is that the any form is hidden by the more specifically typed overrides   var foo2  new Foo     var n  number   foo2 bar  baz           OK var s  string   foo2 bar 123            OK var a  number     foo2 bar  1 2 3       ERROR

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When you overload in TypeScript  you only have one implementation with multiple signatures    class Foo       myMethod a  string       myMethod a  number       myMethod a  number  b  string       myMethod a  any  b   string            alert a toString               Only the three overloads are recognized by TypeScript as possible signatures for a method call  not the actual implementation    In your case  I would personally use two methods with different names as there isn t enough commonality in the parameters  which makes it likely the method body will need to have lots of  ifs  to decide what to do    TypeScript 1 4  As of TypeScript 1 4  you can typically remove the need for an overload using a union type  The above example can be better expressed using   myMethod a  string   number  b   string        alert a toString         The type of a is  either string or number

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What is function overloading in general      Function overloading or method overloading is the ability to create multiple functions of the same name with different implementations  Wikipedia      What is function overloading in JS   This feature is not possible in JS - the last defined function is taken in case of multiple declarations   function foo a1  a2    return    a1     a2     function foo a1    return    a1        replaces above  foo  declaration foo 42   foo       42        and in TS   Overloads are a compile-time construct with no impact on the JS runtime   function foo s  string   string    overload  1 of foo function foo s  string  n  number   number    overload  2 of foo function foo s  string  n   number   string   number                foo implementation   A duplicate implementation error is triggered  if you use above code  safer than JS   TS chooses the first fitting overload in top-down order  so overloads are sorted from most specific to most broad     Method overloading in TS  a more complex example  Overloaded class method types can be used in a similar way to function overloading   class LayerFactory       createFeatureLayer a1  string  a2  number   string     createFeatureLayer a1  number  a2  boolean  a3  string   number     createFeatureLayer a1  string   number  a2  number   boolean  a3   string            number   string         your implementation        const fact   new LayerFactory   fact createFeatureLayer  foo   42     string fact createFeatureLayer 3  true   bar      number   The vastly different overloads are possible  as the function implementation is compatible to all overload signatures - enforced by the compiler   More infos    Playground sample Do s and Don ts for overloads

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Function overloading in typescript   According to Wikipedia   and many programming books  the definition of method function overloading is the following      In some programming languages  function overloading or method   overloading is the ability to create multiple functions of the same   name with different implementations  Calls to an overloaded function   will run a specific implementation of that function appropriate to the   context of the call  allowing one function call to perform different   tasks depending on context    In typescript we cannot have different implementations of the same function that are called according to the number and type of arguments  This is because when TS is compiled to JS  the functions in JS have the following characteristics    JavaScript function definitions do not specify data types for their parameters JavaScript functions do not check the number of arguments when called   Therefore  in a strict sense  one could argue that TS function overloading doesn t exists  However  there are things you can do within your TS code that can perfectly mimick function overloading   Here is an example   function add a  number  b  number  c  number   number  function add a  number  b  number   any  function add a  string  b  string   any   function add a  any  b  any  c   any   any     if  c        return a   c        if  typeof a      string         return  a is   a   b is   b        else       return a   b          The TS docs call this method overloading  and what we basically did is supplying multiple method signatures  descriptions of possible parameters and types  to the TS compiler  Now TS can figure out if we called our function correctly during compile time and give us an error if we called the function incorrectly

[typescript] TypeScript function overloading

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