无法设置未定义的属性“ someProperty”-Angular Typescript

Question

I have a class Article which has property of type Image and looks like this:

export class Article {
   public image:Image;
   public images: Image[]; 
}

If I comment this.article.image = new Image(); like this:

constructor()
{
    this.article = new Article();
    //this.article.image = new Image();
}

And If I try to use an this.image later in code, like:

 this.article.image.fileName = file.name;

I will get following error:

ERROR TypeError: Cannot set property '..' of undefined

But I realized in case when I try to add something to an array for example:

this.article.images.push(something);

I am not getting error! And I'm wondering why I don't get error when I use array (even if I did not said this.article.images = new Images[].. , and why I get error when I'm using simple object/not array..

Answer 1

I guess the .. in your error code means fileName .

You actually explained the error yourself.

If I comment this.article.image = new Image();

You removed the creation of your image, thus, this.article.image is undefined.

So by using this.article.image.fileName , you are trying to access the property fileName of undefined .

However, for the property images , the method .push() already exists because it's a method defined by Array , which is known thanks to the declaration :

public images: Image[];