How to assign defined string constant to type [Typescript]

Question

I have a predefined object:

const obj = {
  ONE: 'ONE',
  TWO: 'TWO'
};

I want to define assign the constant obj.ONE to a new type like this:

type TOne = obj.ONE

I get the following warning: Cannot find namespace 'obj'. or 'ONE' refers to a value, but is being used as a type here. if I don't put the strings inside an object. Is there a fix around this to allow me to assign a predefined string to a type?

@CertainPerformance that would assign the type string to TOne, but I want type "ONE" — CaptainObvious
– CaptainObvious, Commented May 10, 2020 at 13:13
If you can't control how the object is defined, then you can't get to 'ONE' due to automatic type widening — CertainPerformance
– CertainPerformance, Commented May 10, 2020 at 13:14
If you can control where it's defined, declare the object as const, or as 'ONE' in the property definition. Otherwise, you're out of luck — CertainPerformance
– CertainPerformance, Commented May 10, 2020 at 13:20

CertainPerformance · Accepted Answer · 2020-05-10 13:59:04Z

2

Usually, to extract the value of a property from an object, you can use typeof, eg:

const TOne = typeof obj.ONE;

But, in object literals, Typescript automatically widens the types of the values by default, unfortunately, so

const obj = {
  ONE: 'ONE',
  TWO: 'TWO'
};

results in an obj of type

{
  ONE: string;
  TWO: string;
}

The 'ONE' string type is lost. You can avoid the type widening by declaring the object as const, or by declaring the property as const or as 'ONE':

const obj = {
  ONE: 'ONE',
  TWO: 'TWO'
} as const;

const obj = {
  ONE: 'ONE' as 'ONE',
  TWO: 'TWO'
};

answered May 10, 2020 at 13:59

CertainPerformance

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