TypeScript typed array usage-Collection of common programming errors

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?

1. 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();
           }
       }
   }
   

2. 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.

Originally posted 2013-11-09 18:41:17.