What causes TypeScript to be unable to locate declared constants?

I am facing an issue with the following simple code snippet:

const getMethod = 'get';
const postMethod = 'post';

export type RequestMethod = getMethod | postMethod;

When I try this code in TypeScript Playground, it shows an error stating that it cannot find the constants getMethod and postMethod, even though a compiled version is displayed on the right side.

I used to use this code successfully before, so what could be wrong now? How can I resolve this error?

For reference, here is a screenshot of the error message:

https://i.sstatic.net/AWrME.png

typescript

Answer №1

According to the information provided in the spec document, a union type consists of two types combined together. It is important to note that constants themselves are not considered as types in TypeScript. String literals, on the other hand, are treated as types as mentioned in the spec. This explains why expressions like 'get' | 'post' are valid.

To utilize the string literal type, you can use the type of the constant itself:

const getMethod = 'get';
const postMethod = 'post';

export type RequestMethod = typeof getMethod | typeof postMethod;

Answer №2

The reason for this issue is that you are attempting to assign constants as data types.

If you wish for RequestMethod to function as a union type, it is necessary to define getMethod and postMethod as individual types.

type getMethod = 'get';
type postMethod = 'post';

export type RequestMethod = getMethod | postMethod;

Answer №3

getMethod should be defined as a specific type, interface, or enum, not as a variable.

You could consider:

type getMethod = 'get';
type postMethod = 'post';

export type RequestMethod = getMethod | postMethod;

or 

enum RequestMethod { 
    GET = 'get',
    POST = 'post' 
}

const getMethod = RequestMethod.GET;

function makeRequest(method: RequestMethod) {

}

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