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 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 Node.js non-standard resolution logic is to use 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:

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.

In addition you can utilize @ts-ignore to ignore a specific error in code that you control. You can also replace whole dependencies, using , 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.

How do I write code that works in Deno and a browser, but still type checks?

You can do this by using a configuration file with the --config option on the command line and adjusting the "lib" option in the "compilerOptions" in the file. For more information see .

As of Deno 1.5 we defaulted to isolatedModules to 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.

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

• const enum is not supported. s 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 is intended to be simple and straight forward and support a certain set of use cases.

The Deno language server supports the ability to have a “per-resource” configuration of enabling Deno or not. This also requires a client IDE to support this ability. For Visual Studio Code the official supports the vscode concept of multi-root workspace. This means you just need to add folders to the workspace and set the deno.enable setting as required on each folder.