How to declare and import typescript interfaces in a separate file

I want to define several interfaces in their own file in my typescript-based project, from which I'll implement classes for production as well as mocks for testing. However, I can't figure out what the correct syntax is. I've found plenty of tutorials on declaring interfaces and implementing them, but they all have a trivial implementation of both the interface and derived classes in the same file, which isn't very real-world. What's the right way to export and import the interfaces?

You need to export the interface from the file in which is defined and import it wherever you want to use it.

in IfcSampleInterface.ts:

export interface IfcSampleInterface {
   key: string;
   value: string;
}

In SampleInterface.ts

import { IfcSampleInterface } from './IfcSampleInterface';
let sampleVar: IfcSampleInterface;

Use definition (d.ts) files and namespaces, no need to import/export modules this way. DefinitelyTyped project has guidance and huge number of examples how to do it.

Export only a few interfaces

Without spreading multiple exports, you can group them in one single export {} block (in this case, no file default type should be declared):

// interfaces.ts
interface IWords {
  [key: string]: string; 
}

interface INumbers {
  [key: string]: number; 
}

interface IBooleans {
  [key: string]: boolean; 
}

interface IValues {
  [key: string]: string | number; 
}

interface IStructures {
  [key: string]: INumbers | IBooleans | IValues;
}

export {
  // not exporting IWords | INumbers
  IBooleans,
  IValues,
  IStructures,
}

Import example

import { IBooleans, IValues, IStructures } from 'interfaces';

const flags: IBooleans = { read: true, write: false, delete: false };

const userFile: IValues = { user: 1, username: 'One', file: 'types.txt' };

const userContext: IStructure = {
  file: userFile,
  permissions: flags,
  counts: { views: 3, writes: 1 } // => INumbers (lint: try to remove IValues from IStructures)
};

You need to export the interfaces in the file the are defined in and import them in the files they are used in. See this link for examples.

x.ts

interface X{
    ...
}
export default X

y.ts

import X from "./x.ts"
// You can use X now

For more information see https://www.typescriptlang.org/docs/handbook/modules.html

You can use the following syntax in relatively new projects

`import type { xxx } from './xxx'`

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html