Typescript: Import type information only

[blakeembrey]

There's a few other issues that touch on something similar (https://github.com/Microsoft/TypeScript/issues/5098, https://github.com/Microsoft/TypeScript/issues/278), but is there a way to explicitly import only type information from an external module? I imagine something like below, where there's a reserved word used to modify an existing import:

import type * as ts from 'typescript'
import type ts = require('typescript')

ts.parseJsonConfigFileContent(...) // Error, type information only.

var TS: ts = require('typescript')

ts.parseJsonConfigFileContent(...) // Works great.

This construct would import type information only, and any runtime usage of the namespace in the current module could be considered an error. This is useful in a few place (E.g. ts-node and tsconfig) where I need the type information from TypeScript, but I also need to guarantee that I don't accidentally import it as a dependency in the compiled output. Not sure if others have a similar use-case, but the reverse at least seems to be an issue (importing for side-effects only, which is optimized out by the compiler currently).

[DanielRosenwasser]

So you're not going for eliding modules, right? You're specifically trying to make it so that users don't accidentally use values withing a module?

[basarat]

but I also need to guarantee that I don't accidentally import it as a dependency in the compiled output

I tend to use _ prefix for _type import only_ scenarios. e.g.

import _atomUtils = require('../main/atom/atomUtils');
var atomUtils: typeof _atomUtils;

Use case lazy loading Or I might have misunderstood the question :rose:

[blakeembrey]

@basarat Absolutely correct, that's the same use-case I have with ts-node too. I wanted the TypeScript compiler type information, but the TypeScript compiler is lazy loaded depending on the compiler string the user specifies.

@DanielRosenwasser Yes, eliding modules works. But I need to be explicitly sure that I haven't accidentally used the value before it's been defined (E.g. guarantee that require('typescript') does not get emitted).

Edit: Also, yes, _ works and is a reasonable workaround with the current behaviour. But making this explicit also fixes the other side of the coin - where I need an import but aren't using it anywhere (E.g. a polyfill or something else relying solely on side-effects).

[mhegazy]

see #2812 for more details.

[blakeembrey]

Just realised Flow actually handles this and the syntax is identical: http://flowtype.org/blog/2015/02/18/Import-Types.html.

[philkunz]

https://www.npmjs.com/package/smartsystem