How to create an array of custom types typescript

Question

I have an interface with the following:

interface orderItem {
  symbol: string,
  side: string,
  price: number
  quantity: number
}

I want to create an array "orders" consisting of "orderItems" in my constructor.

However, orders : [orderItem] doesn't seem to be doing the trick. Does anyone know how I can proceed?

Here is what I am trying to do in my constructor:

   this.state = {
      orders: orderItem []

  } 

Answer 1

Since you're doing an object literal, the colon is used to divide the key from the value (ie, it has its normal javascript meaning, not its typescript meaning). As a result, trying to set the type the way you're doing is causing an error.

There are a few ways you could do what you want. You could set the type using the as keyword:

this.state = {
    orders: undefined as orderItem[]
};

or with an empty array:

this.state = {
    orders: [] as orderItem[]
};

Or you could create something ahead of time, and then insert that into the object:

let myOrders: orderItem[] = [];
this.state = {
     orders: myOrders
};

Or you could have a separate interface for the state and use that:

interface myState {
    orders: orderItem[]
}

class myClass {
    private state: myState;
    constructor () {
        this.state = {
            orders: undefined
        }
    }
}