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?

Answer 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();
        }
    }
}