TypeScript typed array usage

Question

I have a TypeScript class definition that starts like this   module Entities                  export class Person           private  name  string          private  possessions  Thing            private  mostPrecious  Thing           constructor  name  string                this  name   name              this  possessions   new Thing 100               Looks like an array of type Thing does not get translated correctly to the corresponding Javascript array type   This is a snippet from the generated JavaScript   function Person name        this  name   name      this  possessions   new Entities Thing 100         Executing code containing a Person object  throw an exception when attempting to initialize the  possession field      Error is  0x800a138f - Microsoft JScript runtime error  Unable to get value of the property  100   object is null or undefined     If I change the type of  possession to any    and initialize  possession with new Array   exception is not thrown   Did I miss something

User · Answer

The translation is correct  the typing of the expression isn t  TypeScript is incorrectly typing the expression new Thing 100  as an array  It should be an error to index Thing  a constructor function  using the index operator  In C  this would allocate an array of 100 elements  In JavaScript this calls the value at index 100 of Thing as if was a constructor  Since that values is undefined it raises the error you mentioned  In JavaScript and TypeScript you want new Array 100  instead   You should report this as a bug on CodePlex

User · Answer

You have an error in your syntax here   this  possessions   new Thing 100       This doesn t create an  array of things   To create an array of things  you can simply use the array literal expression   this  possessions         Of the array constructor if you want to set the length   this  possessions   new Array 100     I have created a brief working example you can try in the playground   module Entities          class Thing                       export class Person           private  name  string          private  possessions  Thing            private  mostPrecious  Thing           constructor  name  string                this  name   name              this  possessions                   this  possessions push new Thing                this  possessions 100    new Thing

User · Answer

You could try either of these  They are not giving me errors    It is also the suggested method from typescript for array declaration    By using the Array lt Thing gt  it is making use of the generics in typescript  It is similar to asking for a List lt T gt  in c  code       Declare with default value private  possessions  Array lt Thing gt    new Array lt Thing gt        or private  possessions  Array lt Thing gt           or - gt  prefered by ts-lint private  possessions  Thing           or     declare private  possessions  Array lt Thing gt      or - gt  preferd by ts-lint private  possessions  Thing     constructor          assign     this  possessions   new Array lt Thing gt           or     this  possessions

[arrays] TypeScript typed array usage

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