TypeScript: How to use a variable inferred type to define another variable with the same type, outside of a class

Question

Let's say I have this variable:

const x = {
  name: 'shachar',
} as const;

This variable has the inferred type of

{
  name: 'shachar';
}

I want to use that type to create another type alias like this:

const y: {[key in keyof *TYPE_OF_X*]: boolean} = {
  name: true;
}

Without having to define this type explicitly.

This can be done if I was inside a class:

class MyClass {
  x = {
   name: 'shachar',
  } as const;

  y: {[key in keyof this['x']]: boolean} = {
    name: true;
  }
}

But can this be done if I'm outside a class?

I've also found the following workaround:

class MyClass {
    x = {
      name: 'shachar',
    } as const;
}

const x = (new MyClass()).x;

const y: { [key in keyof MyClass['x']]: boolean} = {
    name: true,
}

But it doesn't look really good, it requires me to add a redundant class, and might make the code look messy when I repeat this method over and over...

Etheryte · Accepted Answer · 2021-01-21 16:32:10Z

1

Your pseudocode is already very close, using key in keyof typeof x will work as expected:

const x = {
  name: 'shachar',
} as const;

const y: {[key in keyof typeof x]: boolean} = {
  name: true,
};

answered Jan 21, 2021 at 16:32

Etheryte

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