How define types of objects in Typescript?

Question

I have following code:

type Machine = {
    id: string,
    name: string,
    tonnage?: number
}

const machines = {
    "machine1": {id: "S01", name: "AnyMachine"} as Machine,
}

is possible to indicate that key is string and value is typeof Machine? Casting as Machine seems to work, but I would like to have less verbose alternative.

Aron · Accepted Answer · 2017-07-13 14:45:43Z

2

You can do something like this:

interface MachineHolder {
  [key: string]: Machine;
}

type Machine = {
  id: string,
  name: string,
  tonnage?: number
}

const machines: MachineHolder = {
  "machine1": {id: "S01", name: "AnyMachine"},
}

answered Jul 13, 2017 at 14:45

Aron

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Comments

Nitzan Tomer · Accepted Answer · 2017-07-13 14:46:51Z

2

You can do this:

type Machines = {
    [key: string]: Machine;
}

const machines: Machines = {
    "machine1": { id: "S01", name: "AnyMachine" }
}

But, depending on what you're trying to do, it might not be necessary.
The Typescript type system is based on the structure of the types, so you can just do this and it will work:

const machines = {
    "machine1": { id: "S01", name: "AnyMachine" }
}

function fn(machine: Machine) {
    // ...
}

fn(machines.machine1);

Here the compiler infers that you're passing the right type even though machines isn't defined as a map of Machines.

answered Jul 13, 2017 at 14:46

Nitzan Tomer

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1 Comment

Pavel Hanpari Over a year ago

Thanks, my intentions are quite strange. I just want to use this as custom typed records and to use typescript tooling to check my types without compiling. I know I should use better tools like database but this is less painless for small datas :)

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