Typescript Initialize object member of class defined in the class

Question

In a situation like this (simplified):

    class Order{
        items:{prices:number[],lastUpdate:Date};
    }

I want to have orders not undefined.

    class Order{
        items:{prices:number[],lastUpdate:Date} = {prices:[],lastUpdate:new Date};
    }

does it but I am not sure is the cleanest (lazy) way. What I don't want in this case is create an OrderItem class. Suggestions?

eeehm... what is the question? if you want an OrderItem without having orders undefined, could you instantiate it with constructor and creating it anywhere with new Order(items: {[], new Date}) — Luca Taccagni
– Luca Taccagni, Commented Feb 2, 2018 at 14:14

jcalz · Accepted Answer · 2018-02-02 14:43:43Z

1

TypeScript will happily infer types of variables and properties for you, and you only have to correct it if it infers something you don't like.

So you could do this:

class Order {
  items = { prices: [], lastUpdate: new Date };
}

which infers items to be of type { prices: never[]; lastUpdate: Date; }. That's close to what you want but it has no idea that an empty array will hold numbers. So you can then do this:

class Order {
  items = { prices: [] as number[], lastUpdate: new Date };
}

Now you've told it (via type assertion) that the empty array is for numbers, and items is inferred as { prices: number[]; lastUpdate: Date; } as desired.

Hope that helps.

answered Feb 2, 2018 at 14:43

jcalz

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1 Comment

tru7 Over a year ago

Yes! (I think I'm getting a crush on typescript :-)

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