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Proper explanation for NodeJS/Typescript Export / Import?

Posted by: admin November 30, 2017 Leave a comment

Questions:

Could someone please explain exactly how exports and imports work in NodeJS using Typescript?

My setup is:

  • NodeJS
  • Everything in Typescript
  • TSLint
  • Typings

I am messing about with exports/imports instead of doing some proper coding, its driving me nuts, and cannot find any proper explanation of how it works.

Import

Can you please explain following:

var module = require ("module");

import module = require("module");

import module from "module";

import {something} from "module";

import * as module from "module";

Export

Can you please explain following

export = something;

export default something;

export interface|class something;

Questions

  • How do properly export/import a class without initiating it
  • How do properly export/import a class with it being initiated (construct did run)
  • How do I properly export/import a class and interface
  • How do I properly export/import class and multiple interfaces
  • When to use modules declarations, what are they good for and why is tslint throwing errors at me if I try to use them.

I cannot seem to find proper way of doing exports vs. imports so my IDE is not covered in red and throwing hundreds of errors at me.

General Questions

  • Is there even a way to not get errors using nodejs/typescript/tslint?
  • TSLint is screaming at me for almost anything I type… it is extremely frustrating since there is 0 explanation to most of the errors. Is it even worth using it?

List of questions goes on and on, but im sure once some of the above is answered, I can pick up on the rest.

Thank you and sorry for such a general question, but my frustration level just reached the top…

Answers:

Agreed, import/export syntax is confusing for at least two reasons:

  • the commonjs syntax: var module = require ("module"); works but that is commonjs -> no typings
  • it changed: the syntax import x = require('y') is now deprecated in TS

TL;DR;: Use the ‘es6 style’ syntax introduced in TS 1.5

The ‘best’ resource on import/export in TS I know is this

Overall I recommend reading this excellent handbook which will provide answers to most of your questions

To import

From a default export

Something was exported as a default (single) export ie export default Something in ts/es6

Use

import Something from "module"

You can actually import a default export with a different name. import SomethingElse from 'module' will also work

From named exports

Something was exported as a named export in “module” using export {Something} or export class|interface Something{} in ts/es6

You want to import only that, use

import {Something} from "module"    

You want to import everything that is exported from “module” under the namespace mod

import * as mod from "module

Then use const c:mod.Something = whatever

To export

See imports above

The form export = something is deprecated in favor of the new ES6 style syntax. It is mostly found in definition files to express the fact that a js library exports a single function/object e.g. module.exports=something.

What you should try using

Use ES6 style syntax and avoid default exports: they have the advantage that they can be imported using a different name but

  • a default import cannot be re-exported, which may be problematic if you are writing a library
  • they will confuse a lot of IDEs (not to mention yourself) when refactoring
  • a named export can actually be locally renamed when imported i.e. import {Something as SomethingElse} from "module"

Concretely, export whatever needs to be exported and import it specifically

In api.ts

export interface MyInterface {
}

export class MyClass {
}

In main.ts

 import {MyInterface, MyClass} from './api'

Linting

There are a lot of good IDEs out there that provide excellent linting: VSCode, Atom Typescript and Webstorm to name a popular few, the first two being free and the third one even manages the imports for you.