How to initialize an empty array of objects in Typescript?

Question

I have fetched a set of data documents, and am looping through them to create an object out of each called 'Item'; each Item object has an 'amount' key and an 'id' key.

I need to append each created Item object to an array called 'Items'. However, when I create this array at the top (currently like this: var itemObjects: [Item]) and then push each item to the array like this:

snapshot.forEach((doc: any) => {
let docData = doc.data()
let items = docData.items
items.forEach((item: any) => {
  let itemObject = new Item(item.amount, item.transactionType)
  console.log("converted item to Item Object:", itemObject)
  itemObjects.push(itemObject)
})

It gives me this error: Unhandled error TypeError: Cannot read properties of undefined (reading 'push')\n

I believe I am incorrectly initializing the variable array up top. Any help is appreciated thanks.

EDIT- the other piece of code (for the Item class) is:

interface IItem {
amount: number
id: string
}

export class Item implements IItem {
 amount: number
 id: string

 constructor(amount: number, id: string) {
  this.amount = amount
  this.id = id
  }
 }

Thomas · Accepted Answer · 2022-08-12 14:40:09Z

1

You are not initializing the variable, merely declaring it. After TypeScript removes the type annotations, all that is left in the resulting JavaScript is:

var itemObjects

So just give it a value:

var itemObjects: Item[] = []
                        ^^^^

The other issue (also fixed above) is that [Item] is a tuple of a single Item. At runtime it's just an array, but you can't create one with more or fewer than one Item. Instead, use Item[] to denote an array of items.

edited Aug 12, 2022 at 14:40

answered Aug 12, 2022 at 14:21

Thomas

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9 Comments

ohu Over a year ago

Type '[]' is not assignable to type '[Item]'. Source has 0 element(s) but target requires 1.

ohu Over a year ago

that is the error I get when I try initializing it in that way.

Thomas Over a year ago

Right, see edit.

ohu Over a year ago

I understand your edit, but it is giving me a similar error when written that way:

ohu Over a year ago

Argument of type 'Item[]' is not assignable to parameter of type '[Item]'. Target requires 1 element(s) but source may have fewer.

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dpimonov · Accepted Answer · 2022-08-12 15:00:35Z

1

You can declare a typed array like the following:

var items = new Array<Item>();

or

var items: Item[] = [];

Both ways will give you exactly same behavior.

Btw why not just use map function instead of forEach?

var items = docs.map((item: any) => new Item(item.amount, item.id));

edited Aug 12, 2022 at 15:00

answered Aug 12, 2022 at 14:52

dpimonov

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1 Comment

Ruan Mendes Over a year ago

I agree that in this case, the type is obvious and inferred by TS and I would not write it in manually.

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