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Using Visual Studio Code

In this section we are going to go into depth about developing Deno applications using Visual Studio Code and the official vscode_deno extension.

Installing

The vscode extension integrates directly to the Deno CLI using the language server protocol. This helps ensure that the information you get about your code aligns to how that code will work when you try to run it under the Deno CLI.

The Deno extension is installed like other extensions in vscode, by browsing the extensions in vscode and choosing to install the Deno extension. Or if you have vscode installed, you can view the extension via this link and install it if you haven’t already done so.

Once you install the extension for the first time, you should receive a splash page that welcomes you to the extension. (If you missed it, or want to see it again, just use the Deno: Welcome command from the command palette.)

Configuring the extension

The following sections will detail out how to configure the extension to work best for you and will cover most of the settings available.

Deno enabling a workspace

We realize that not every project you might edit with vscode is a Deno project. By default, vscode comes with a built-in TypeScript/JavaScript language service which is used when editing TypeScript or JavaScript files.

In order to have support for Deno APIs as well as the ability to resolve modules as the Deno CLI does, you need to enable Deno for the workspace. The most direct way to do this is to use the Deno: Initialize Workspace Configuration from the vscode command palette. This will activate a helper which will ask if you want to also enable linting and the Deno unstable APIs for the project. This command will instruct vscode to store these settings in the workspace configuration (your workspace root .vscode/settings.json). Once the helper is finished, you will get a notification that Deno is setup for the project.

These settings (and other settings) are available via the vscode settings panel. In the panel the setting is Deno: Enable and when manually editing the JSON, the setting is deno.enable.

⚠️ vscode has user and workspace settings. You probably don’t want to enable Deno in the user settings, as then by default, every workspace will be Deno enabled.

When a project is enabled, the extension will get information directly from the installed Deno CLI. The extension will also mute the built-in TypeScript/JavaScript extension.

Partially Deno enabling a workspace

While vscode supports Workspace Folders, they can be challenging to configure and use. Because of this, the option Deno: Enable Paths has been introduced (or "deno.enablePaths" if manually editing). In a given workspace (or workspace folder), sub-paths can be enabled for Deno, while code outside those paths will be not be enabled and the vscode built-in JavaScript/TypeScript language server will be used.

For example if you have a project like this:

project
├── worker
└── front_end

Where you only want to enabled the worker path (and its subpaths) to be Deno enabled, you will want to add ./worker to the list of Deno: Enable Paths in the configuration.

Using linting

The same engine that provides the diagnostics when using deno lint can also be used via the extension. By enabling the Deno: Lint setting in the settings panel (or deno.lint if editing settings in JSON) the editor should start to display lint “warnings” in your code. See the Linter section for more information on how to use the Deno linter.

Using import maps

It is possible to use import maps in the editor. The option Deno: Import Map (or deno.importMap if manually editing) should be set to the value of the import map file. If the path is a relative path, it will be resolved relative to the root of the workspace.

Using a configuration file

Typically a configuration file is not required for a Deno project. There are a few scenarios though where it might be useful, and if you want to have the same settings applied as when specifying the --config option on the command line, the Deno: Config option can be used (or deno.config if manually editing).

The Deno extension will also auto-identify and apply a deno.jsonc or deno.json by looking in the workspace root for the configuration file and applying it. Manually specifying a Deno: Config option will override this automatic behavior.

Using formatting

The Deno CLI comes with a built-in formatter which can be accessed using deno fmt but can also be configured to be used by vscode. Deno should be on the drop down list for the Editor: Default formatter setting (or if you are editing settings manually, it would be "editor.defaultFormatter": "denoland.vscode-deno").

See the Code formatter for more information on how to use the formatter.

Setting a path to the Deno CLI

The extension looks for the Deno CLI executable in the host’s PATH, but sometimes that isn’t desirable and the Deno: Path can be set (or deno.path if manually editing) to point to the Deno executable. If the path provided is relative, it will be resolved relative to the root of the workspace.

Import suggestions

When attempting to import a module, the extension will offer suggestions to complete the import. Local relative files will be included in the suggestions, plus also any cached remote files.

The extension supports registry auto-completions, where a remote registry/website of modules can optionally provide metadata that allows a client to discover modules. By default, the extension will check hosts/origins to see if they support suggestions, and if it does, the extension will prompt you to see if you want to enable it. This behavior can be changed by unsetting Deno > Suggest > Imports: Auto Discover (or deno.suggest.imports.autoDiscover if manually editing).

Individual hosts/origins can be enabled or disabled by editing the Deno > Suggest > Imports: Hosts/deno.suggest.imports.hosts setting in the appropriate settings.json.

Caching remote modules

Deno supports remote modules and will fetch remote modules and store them locally in a cache. When you do something like deno run, deno test, deno info or deno cache on the command line, the Deno CLI will go and try to fetch any remote modules and their dependencies and populate the cache.

While developing code in the editor, if the module is not in the cache, you will get a diagnostic like Uncached or missing remote URL: “https://deno.land/example/mod.ts” for any missing remote modules. Deno will not automatically try to cache the module, unless it is a completion from a registry import suggestion (see above).

In addition to running a command on a command line, the extension provides ways to cache dependencies within the editor. A missing dependency will have a quick fix which is to have Deno try to cache the dependency. Fixes can be accessed by pressing CTRL . or ⌘ . when the editor is positioned in the import specifier, or hovering over the specifier and selecting Quick Fix….

There is also the Deno: Cache Dependencies command in the command palette which will attempt to cache any dependencies of the module currently active in the editor.

Code lenses

The language server currently supports several code lenses (actionable contextual information interspersed in the code) that allow you to get greater insight into the code. Most are disabled by default, but can easily be enabled:

  • Deno > Code Lens: Implementations/deno.codeLens.implementations - Provides a lens that will list out any implementations of an item elsewhere in the code.
  • Deno > Code Lens: References/deno.codeLens.references - Provides a lens that will list out any references to an item elsewhere in the code.
  • Deno > Code Lens: References All Functions/deno.codeLens.referencesAllFunctions - Provides a lens that will list out all references to all functions in the code. All functions are excluded from just References mention above.

Testing code lens

The Deno CLI includes a built-in testing API available under Deno.test. The extension and language server have a code lens enabled by default which provides the ability to run a test from within the editor.

When you have a block of code that provides a test, like:

import { assert } from "https://deno.land/std@0.185.0/testing/asserts.ts";

Deno.test({
  name: "a test case",
  fn() {
    let someCondition = true;
    assert(someCondition);
  },
});

You will see a code lens like the following just above the test:

▶ Run Test

This is a link that if you click it, the extension will start up the Deno CLI to run the test for you and display the output. Based on your other settings, the extension will try to run your test with the same settings. If you need to adjust the arguments provided when doing deno test, you can do so by setting the deno.codeLens.testArgs setting.

The extension will also try to track if in the same module you destructure the Deno.test function or assign it to a variable. So you can do something like this and still have the code lens work:

const { test: denoTest } = Deno;

denoTest({
  name: "example test",
  fn() {},
});

If you want to disable this feature, you can do so by unsetting the Deno > CodeLens: Test/deno.codeLens.test setting.

Using the debugger

The extension provides integration with the built-in VSCode debugger. You can generate a configuration by: going to Run and Debug panel, clicking create a launch.json file and selecting Deno option from the available debugger options.

By default, the configuration will use --inspect-wait flag if the configured Deno version is greater than 1.29, or --inspect-brk for versions older than 1.29. This ensures that the debugger has a chance to connect to your program and register all the breakpoints specified in the code.

Tasks

The extension communicates directly to the language server, but for some development tasks, you might want to execute the CLI directly. The extension provides a task definition for allowing you to create tasks that execute the deno CLI from within the editor.

Deno CLI tasks

The template for the Deno CLI tasks has the following interface, which can be configured in a tasks.json within your workspace:

interface DenoTaskDefinition {
  type: "deno";
  // This is the `deno` command to run (e.g. `run`, `test`, `cache`, etc.)
  command: string;
  // Additional arguments pass on the command line
  args?: string[];
  // The current working directory to execute the command
  cwd?: string;
  // Any environment variables that should be set when executing
  env?: Record<string, string>;
}

Several of the commands that are useful in the editor are configured as templates and can be added to your workspace by select Tasks: Configure Task in the command palette and searching for deno tasks.

And example of what a deno run mod.ts would look like in a tasks.json:

{
  "version": "2.0.0",
  "tasks": [
    {
      "type": "deno",
      "command": "run",
      "args": [
        "mod.ts"
      ],
      "problemMatcher": [
        "$deno"
      ],
      "label": "deno: run"
    }
  ]
}

Workspace folders

The Deno language server and this extension supports multi-root workspaces configuration, where certain settings can be applied to workspace folders within a workspace.

When you add folders to your workspace and open the settings, you will have access to the per folder settings. If you look at the .vscode/settings.json in a folder, you will see a visual indication of what settings apply to folder, versus those that come from the workspace configuration:

screenshot of the .vscode/setting.json configured as a workspace folder

Workspace folder settings

These are the settings that can be set on a workspace folder. The rest of the settings currently only apply to the workspace:

  • deno.enable - Controls if the Deno Language Server is enabled. When enabled, the extension will disable the built-in vscode JavaScript and TypeScript language services, and will use the Deno language server instead. boolean, default false
  • deno.enablePaths - Controls if the Deno Language Server is enabled for only specific paths of the workspace folder. Defaults to an empty list.
  • deno.codeLens.test - Controls if the test code lens is enabled. boolean, default true
  • deno.codeLens.testArgs - The list of arguments that are passed to deno test when activating a test code lens. string array, default ["--allow-all"]

Mixed-Deno projects

While you can use this feature to enable mixed-Deno projects, you might want to consider partially Deno enabling a workspace. But with this feature, you can have a mixed Deno project, where some of the workspace folders are Deno enabled and some are not. This is useful when creating a project that might have a front-end component, where you want a different configuration for that front end code.

In order to support this, you would create a new workspace (or add a folder to an existing workspace) and in the settings configure one of the folders to have deno.enable set to true and one set to false. Once you save the workspace configuration, you notice that the Deno language server only applies diagnostics to the enabled folders, while the other folder will use the built-in TypeScript compiler of vscode to supply diagnostics for TypeScript and JavaScript files.

Using a development container

Using a development container with vscode is a great way to have an isolated development environment without having to worry about having to install the Deno CLI on your local system.

To use development containers, you need to have a few prerequisites installed:

The way a development container is configured is by having a .devcontainer folder as part of the workspace with configuration information in the folder. If you are opening a project that already contains a dev container with Deno, you will be prompted to build the dev container and access the project under that. Everything should “just work”.

If you have an existing Deno project that you would like to add dev container support to, you will want to execute the command Remote-Containers: Add Development Container Configuration Files… in the command palette and then choose Show All Definitions… and then search for the Deno definition. This will setup a baseline .devcontainer configuration for you that will install the latest version of the Deno CLI in the container.

Once added, vscode will prompt if you want to open the project in a dev container. If you choose to, vscode will build the development container and re-open the workspace using the development container, which will have the Deno CLI and the vscode_deno extension installed in it.

Troubleshooting

The following sections cover challenges you might face when using the extension and try to give likely causes.

Errors/diagnostics like An import path cannot end with a '.ts' extension. or Cannot find name 'Deno'.

This is normally a situation where Deno is not enabled on a Deno project. If you look at the source of the diagnostic you are probably going to see a ts(2691). The ts indicates that it is coming from the built-in TypeScript/JavaScript engine in vscode. You will want to check that your configuration is set properly and the Deno: Enable/deno.enable is true.

You can also check what the Deno language server thinks is your current active configuration by using Deno: Language Server Status from the command palette. This will display a document from the language server with a section named Workspace Configuration. This will provide you with what vscode is reporting the configuration is to the language server.

Also check if the VSCode configuration called enableProjectDiagnostics, located in TypeScript › Tsserver › Experimental: Enable Project Diagnostics is disabled. This setting allows TypeScript language server to execute in the background to check the entire project at once and Deno cannot disable its behaviour so the errors keep showing even when all other settings are correctly set.

If "enable" is set to true in there, and the error message still persists, you might want to try restarting vscode, as the part of the extension that “mutes” the built-in TypeScript diagnostics for files is not working as designed. If the issue still persists after a restart, you may have encountered a bug that we didn’t expect and searching the issues and reporting a bug at https://github.com/denoland/vscode_deno is the next step.

References
Testing API