TypeScript - difference between import … and impor

问题:

Coming from Java to TS, I've omitted the {...} around the imported type.

import DiscriminatorMappingData from './DiscriminatorMappingData';

instead of

import {DiscriminatorMappingData} from './DiscriminatorMappingData';

See TypeScript - storing a class as a map value?.

I've read the documentation and didn't understand much. I only took from it that when I only need one type from a file, I can omit the {}.
However, that caused weird errors, like "Unknown name", or unexpected type incompatibilites.

So, what's the difference, put simply?

回答1:

The difference between your two import declarations is covered in the TypeScript specification. From §11.3.2, Import Declarations:

An import declaration of the form

import d from "mod";

is exactly equivalent to the import declaration

import { default as d } from "mod";

Thus, you would omit the braces only when you are importing something that was exported as the default entity of the module (with an export default declaration, of which there can only be one per module). The name you provide in the import declaration becomes an alias for that imported entity.

When importing anything else, even if it's just one entity, you need to provide the braces.

The Default exports section of the TypeScript handbook has a few examples.

回答2:

This is imports in destructruring format. Essentially grouping all the entities that we want to be imported from a definition file.

If you are familiar with other programming languages you might be familiar with the destructuring notion. Actually destructuring was added in the form of assignments as part of es6.

Import restructuring support was first suggested in TypeScript in this Github ticket but then got grouped under this ticket that was tracking all the es6 modules.