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?
This question is related to
arrays
typescript
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();
}
}
}
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.
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<Thing> it is making use of the generics in typescript. It is similar to asking for a List<T> in c# code.
// Declare with default value
private _possessions: Array<Thing> = new Array<Thing>();
// or
private _possessions: Array<Thing> = [];
// or -> prefered by ts-lint
private _possessions: Thing[] = [];
or
// declare
private _possessions: Array<Thing>;
// or -> preferd by ts-lint
private _possessions: Thing[];
constructor(){
//assign
this._possessions = new Array<Thing>();
//or
this._possessions = [];
}
Source: Stackoverflow.com