import type should not need runtime modules support

[antoinep92]

Suggestion

🔍 Search Terms

import type
module
--outFile

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Intuitively I expect import type to be a compile-time only construct. This is currently not really the case, because using import type triggers the compiler into thinking we are using ES modules.

📃 Motivating Example

Consider this test.ts:

import type { mkdir as mkdir_proto } from 'fs';
const { mkdir } = require('fs') as { mkdir: typeof mkdir_proto };

This to compile to a ŧest.js which is more or less:

"use strict";
const { mkdir } = require('fs');

Which does not use ES modules. Nevertheless, the compiler refuses to compile this if modules are disabled, e.g.
tsc test.ts --outFile test.js --target es2020 fails with:

error TS6131: Cannot compile modules using option 'outFile' unless the '--module' flag is 'amd' or 'system'.

1 import type { mkdir as mkdir_proto } from 'fs';

Which makes me think the compilers treats import and import type the same way.

💻 Use Cases

This is clearly not a showstopper, more of an optimization. One use-case is trying to build simple Node.JS scripts, written in typescript as a single file (with the --outFile flag), in order to deploy each script as a single javascript file, that I can run natively on a Node.JS server with no typescript support (no tsc or ts-node installed).
Since the script only has dependency to node's standard library, I was not expecting typescript to refuse to compile as a single file.

We can work arround by defining a tsconfig.json and building in a directory, then extracting the js file generated in that build/ dir. And I realize there are a lot of issues around here about packaging as a single file. But in this case, there is no concatenation required at all. This is just about import type triggering typescript into thinking we are using modules, when we really aren't.

As I said, this is clearly not critical, and I have no idea if this would be easy to implement. So it's really just a suggestion :)

[MartinJohns]

Essentially a duplicate of #41513 and #41562, with a possible solution discussed at #41567.

As soon as you're using the import or export statements you're dealing with modules. This is by design.

[antoinep92]

I see, thanks, and sorry for the duplicate.
For future reference, this explains the rationale: #35200 (comment)

Although it leaves unanswered the question of how to reference modules, and in particular type modules from script :(

[MartinJohns]

Although it leaves unanswered the question of how to reference modules, and in particular type modules from script :(

That has been answered in the linked issues. In short: You can't. At least currently not.

I stand corrected by @Jamesernator.

[Jamesernator]

Although it leaves unanswered the question of how to reference modules, and in particular type modules from script :(

That has been answered in the linked issues. In short: You can't. At least currently not.

This isn't entirely true, although it's kinda awkward without proper import. Basically you can use:

type MkdirProto = typeof import("fs")["mkdir"];

This isn't too bad to be honest, although becomes tedious if you want a lot of types as there's no destructuring or such to speak of in the type system. So you'll need a type ImportedType = typeof import("someModule")["ImportedType"] for each and every imported type.

[antoinep92]

Ah excellent thanks. I saw that from some other comments but wasn't aware this syntax wasn't considered a module import as opposed to import type. Is this documented somewhere ?