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Configuring TypeScript in Deno

TypeScript comes with a load of different options that can be configured, but Deno strives to make it easy to use TypeScript with Deno. Lots of different options frustrates that goal. To make things easier, Deno configures TypeScript to "just work" and shouldn't require additional configuration.

That being said, Deno does support using a TypeScript configuration file, though like the rest of Deno, the detection and use of use of a configuration file is not automatic. To use a TypeScript configuration file with Deno, you have to provide a path on the command line. For example:

> deno run --config ./tsconfig.json main.ts

⚠️ Do consider though that if you are creating libraries that require a configuration file, all of the consumers of your modules will require that configuration file too if you distribute your modules as TypeScript. In addition, there could be settings you do in the configuration file that make other TypeScript modules incompatible. Honestly it is best to use the Deno defaults and to think long and hard about using a configuration file.

How Deno uses a configuration file

Deno does not process a TypeScript configuration file like tsc does, as there are lots of parts of a TypeScript configuration file that are meaningless in a Deno context or would cause Deno to not function properly if they were applied.

Deno only looks at the compilerOptions section of a configuration file, and even then it only considers certain compiler options, with the rest being ignored.

Here is a table of compiler options that can be changed, their default in Deno and any other notes about that option:

Option Default Notes
allowJs true This almost never needs to be changed
allowUnreachableCode false
allowUnusedLabels false
checkJs false If true causes TypeScript to type check JavaScript
experimentalDecorators true We enable these by default as they are already opt-in in the code and when we skip type checking, the Rust based emitter has them on by default. We strongly discourage the use of legacy decorators, as they are incompatible with the future decorators standard in JavaScript
jsx "react"
jsxFactory "React.createElement"
jsxFragmentFactory "React.Fragment"
keysofStringsOnly false
lib [ "deno.window" ] The default for this varies based on other settings in Deno. If it is supplied, it overrides the default. See below for more information.
noFallthroughCasesInSwitch false
noImplicitAny true
noImplicitReturns false
noImplicitThis true
noImplicitUseStrict true
noStrictGenericChecks false
noUnusedLocals false
noUnusedParameters false
reactNamespace React
strict true
strictBindApply true
strictFunctionTypes true
strictPropertyInitialization true
strictNullChecks true
suppressExcessPropertyErrors false
suppressImplicitAnyIndexErrors false

For a full list of compiler options and how they affect TypeScript, please refer to the TypeScript Handbook

What an implied tsconfig.json looks like

It is impossible to get tsc to behave like Deno. It is also difficult to get the TypeScript language service to behave like Deno. This is why we have built a language service directly into Deno. That being said, it can be useful to understand what is implied.

If you were to write a tsconfig.json for Deno, it would look something like this:

{
  "compilerOptions": {
    "allowJs": true,
    "esModuleInterop": true,
    "experimentalDecorators": true,
    "inlineSourceMap": true,
    "isolatedModules": true,
    "jsx": "react",
    "lib": ["deno.window"],
    "module": "esnext",
    "strict": true,
    "target": "esnext",
    "useDefineForClassFields": true
  }
}

You can't copy paste this into a tsconfig.json and get it to work, specifically because of the built in type libraries that are custom to Deno which are provided to the TypeScript compiler. This can somewhat be mocked by running deno types on the command line and piping the output to a file and including that in the files as part of the program, removing the "lib" option, and setting the "noLib" option to true.

If you use the --unstable flag, Deno will change the "lib" option to [ "deno.window", "deno.unstable" ]. If you are trying to load a worker, that is type checked with "deno.worker" instead of "deno.window".

Using the "lib" property

[TBC]