denoland-deno/docs/typescript/faqs.md

FAQs about TypeScript in Deno

Can I use TypeScript not written for Deno?

Maybe. That is the best answer, we are afraid. For lots of reasons, Deno has chosen to have fully qualified module specifiers. In part this is because it treats TypeScript as a first class language. Also, Deno uses explicit module resolution, with no magic. This is effectively the same way browsers themselves work, thought they don't obviously support TypeScript directly. If the TypeScript modules use imports that don't have these design decisions in mind, they may not work under Deno.

Also, in recent versions of Deno (starting with 1.5), we have started to use a Rust library to do transformations of TypeScript to JavaScript in certain scenarios. Because of this, there are certain situations in TypeScript where type information is required, and therefore those are not supported under Deno. If you are using tsc as stand-alone, the setting to use is "isolatedModules" and setting it to true to help ensure that your code can be properly handled by Deno.

One of the ways to deal with the extension and the lack of magical resolution is to use import maps which would allow you to specify "packages" of bare specifiers which then Deno could resolve and load.

What version(s) of TypeScript does Deno support?

Deno is built with a specific version of TypeScript. To find out what this is, type the following on the command line:

> deno --version

The TypeScript version (along with the version of Deno and v8) will be printed. Deno tries to keep up to date with general releases of TypeScript, providing them in the next patch or minor release of Deno.

There was a breaking change in the version of TypeScript that Deno uses, why did you break my program?

We do not consider changes in behavior or breaking changes in TypeScript releases as breaking changes for Deno. TypeScript is a generally mature language and breaking changes in TypeScript are almost always "good things" making code more sound, and it is best that we all keep our code sound. If there is a blocking change in the version of TypeScript and it isn't suitable to use an older release of Deno until the problem can be resolved, then you should be able to use --no-check to skip type checking all together.

In addition you can utilize @ts-ignore to ignore a specific error in code that you control. You can also replace whole dependencies, using import maps, for situations where a dependency of a dependency isn't being maintained or has some sort of breaking change you want to bypass while waiting for it to be updated.

Why are you forcing me to use isolated modules, why can't I use const enums with Deno, why do I need to do export type?

As of Deno 1.5 we defaulted to isolatedModules to true and in Deno 1.6 we removed the options to set it back to false via a configuration file. The isolatedModules option forces the TypeScript compiler to check and emit TypeScript as if each module would stand on its own. TypeScript has a few type directed emits in the language at the moment. While not allowing type directed emits into the language was a design goal for TypeScript, it has happened anyways. This means that the TypeScript compiler needs to understand the erasable types in the code to determine what to emit, which when you are trying to make a fully erasable type system on top of JavaScript, that becomes a problem.

When people started transpiling TypeScript without tsc, these type directed emits became a problem, since the likes of Babel simply try to erase the types without needing to understand the types to direct the emit. In the internals of Deno we have started to use a Rust based emitter which allows us to optionally skip type checking and generates the bundles for things like deno bundle. Like all transpilers, it doesn't care about the types, it just tries to erase them. This means in certain situations we cannot support those type directed emits.

So instead of trying to get every user to understand when and how we could support the type directed emits, we made the decision to disable the use of them by forcing the isolatedModules option to true. This means that even when we are using the TypeScript compiler to emit the code, it will follow the same "rules" that the Rust based emitter follows.

This means that certain language features are not supportable. Those features are:

• Re-exporting of types is ambigious and requires to know if the source module is exporting runtime code or just type information. Therefore, it is recommended that you use import type and export type for type only imports and exports. This will help ensure that when the code is emitted, that all the types are erased.
• const enum is not supported. const enums require type information to direct the emit, as const enums get written out as hard coded values. Especially when const enums get exported, they are a type system only construct.
• export = and import = are legacy TypeScript syntax which we do not support.
• Only declare namespace is support. Runtime namespace is legacy TypeScript syntax that is not supported.

Why don't you support language service plugins or transformer plugins?

While tsc supports language service plugins, Deno does not. Deno does not always use the built in TypeScript compiler to do what it does, and the complexity of adding support for a language service plugin is not feasible. TypeScript does not support emitter plugins, but there are a few community projects which hack emitter plugins into TypeScript. First, we wouldn't want to support something that TypeScript doesn't support, plus we do not always use the TypeScript compiler for the emit, which would mean we would need to ensure we supported it in all modes, and the other emitter is written in Rust, meaning that any emitter plugin for TypeScript wouldn't be available for the Rust emitter.

The TypeScript in Deno isn't intended to be a fully flexible TypeScript compiler. Its main purpose is to ensure that TypeScript and JavaScript can run under Deno. The secondary ability to do TypeScript and JavaScript emitting via the runtime API Deno.emit() is intended to be simple and straight forward and support a certain set of use cases.