Visual Studio Code Extension Development

I don’t like long introductions in posts because you know what brought you here and don’t need the fluff. Let’s get right into it…


Format of this Post

The format of this post is as follows: you’ll be presented with a question that is assumed to be on your mind and provided some answers and supporting details for that question.

What kind of extensions can I build?

You can build whatever your heart desires to build. There are a few main “types” of extensions you can build though which give you a better idea of where to start. Let’s break them down by what you might want to do.

1. Customize VS Code’s Look and Feel

What does this mean? Color themes and Icon Themes. A Color theme is actually a great place to start for your first extension. Let’s first break them down a bit more for better understanding.

2. Extend Existing Functionality

What does this mean? It means there is some existing capability/feature in VS Code but you wish it had something more to it. A simple example of this is creating new keymaps that make it easier for someone who’s used to certain keyboard shortcuts found in another editor. A similar example is an extension that provides code snippets to generate common code that is reused in Node.js applications.

Here are some simple examples:

More complex examples:

3. Add New Functionality

What does this mean? It means VS Code doesn’t provide some sort of functionality that you’re looking to use. In that case we build out an extension that adds that functionality.


4. Aggregate Other Extensions into One Place

What does this mean? You bring many extensions together under one pack that help fulfill a specific focus/category of actions. Essentially there are many extensions out there that alone are great for the type of development you may be doing but you may not be aware of them all. Someone can gather them together to make them easier to find and install at once.

An example of this is the Angular Essentials extension pack that packs a bunch of Angular related extensions into one place. Check out the source code on GitHub to see what’s involved in creating one.

These types of extensions are driven by their package.json file to define the other extensions to be included in the pack. See the example from Angular Essentials’ package.json

How do I build an extension?

One of the simplest extensions you can start with is a Color Theme extension. Let’s step through building one of those now.

Building a Color Theme Extension

Let’s build a color theme extension from scratch just to get a feel for creating our first extension. We’re going to use the VS Code Yeoman Generator so make sure you executed the following command in your terminal/command prompt:

npm i -g generator-code

This installs the Yeoman CLI tool and the VS Code templating/scaffolding generator as an option within it

Once you’ve installed that go to your main folder/directory that you’d like to use for development (example: cd /my/dev/folder) and run yo code to get started:

function test(a, b) {
  const sum = a + b;
  return sum;

You’ll be prompted to choose the type of extension you want to create. Use your keyboard arrow keys to move to the “New Color Theme” option and press your Enter key.

VS Code Yeoman Generator - results from running yo code in the terminal/command prompt

Next, you’ll be prompted with an option to import or convert an existing color theme. Choose the option “No, start fresh”

VS Code Yeoman Generator - results from running yo code in the terminal/command prompt

From here, the prompt will ask you to provide details about the extension.

VS Code Yeoman Generator - results from running yo code in the terminal/command prompt

When you’ve finished entering all the options for your color theme press the Enter key for the last prompt to start generatoring everything. You’ll see some progress in the terminal/command prompt which shows the files and folders it is generating for you.

VS Code Yeoman Generator - results from running yo code in the terminal/command prompt

When it’s done you should see a message that says the following: “To start editing with Visual Studio Code, use the following commands:” Run the commands shown below it and you’ll see VS Code open the folder for your color theme extension.

VS Code with the current color theme extension folder open

Open up the debugger view in VS Code using the keyboard shortcut CTRL/CMD + SHIFT + D (or click on the icon with a bug in it). Click on the green “play” button (as seen in the screenshot below) to start debugging/running your color theme extension.

VS Code with the current color theme .json file open

Once running you’ll see a new instance of VS Code open up with a title of “Extension Development Host”. This is a separate instance of VS Code running with your extension loaded into it. Back in a previous step when generating this color theme extensions in the terminal/command prompt I chose the “Dark” option for my base theme so I see this as the default color in the Extension Development Host instance of VS Code. You may see different colors based on the base theme you chose during that step. Let’s test out changing a color to the theme.

VS Code with the current color theme .json file open

Go back to your other instance of VS Code that has the extension folder opened. Expand the “themes” folder and double click on the .json file to open/view the contents.

VS Code with the current color theme .json file open

Now find the editor.background key in the JSON file under the colors key. Hover your mouse over the string value to bring up the color picker view in VS Code. Carefully move your mouse into the color picker pop up view and change the color to something different. You’ll see the string value change.

Editing the background color for the color theme in VS

Once you save your changes you’ll see the “Extension Development Host” of VS Code will automatically update and display the color change.

Editing the background color for the color theme in VS

You’ve done it! Congratulations on creating your first extension for Visual Studio Code! Feel free to continue tweaking color options in the .json file to further customize your color theme extension.

Building Something More

Creating a color theme extension is great and all but it doesn’t dive deep into enhancing or creating features within VS Code. Let’s walk through creating an extension that results in creating a file that lists all the extensions we’re using in VS Code.

Go to your main folder/directory that you’d like to use for development (example: cd /my/dev/folder) and run the following command to get started:

yo code

You’ll be prompted to choose the type of extension you want to create. Use your keyboard arrow keys to move to the “New Extension (TypeScript)” option and press your Enter key.

Terminal/command line prompt using the Yeoman generator for VS Code and selecting 'New Extension (TypeScript)'

From here, the prompt will ask you to provide details about the extension.

Filling in options for creating a new VS Code extension with the Yeoman generator

Change to the directory for the newly generated extension and open it up in VS Code

Changing directories and opening the folder in VS Code via the terminal/command line

VS Code extensions leverage package.json as their manifest and extend it with some further options beyond the conventional options. We won’t go into detail for all of them but we’ll need to change a few for the purposes of this extension.

  1. Open up the package.json file and find the activationEvents key. This defines what action will activate your extension. At the moment it’s defined to activate your extension when the command “helloWorld” is triggered. You’ll see this as string value "onCommand:extension.helloWorld". To make sure we keep things consistent with the intent of this new extension replace helloWorld with createList as that will be the name of the command we’ll create.

  2. Shortly below the activationEvents key you’ll find another key contributes. This defines the many ways your extension can “contribute” to VS Code. At the moment it has one option defined and that is via the commands option. This is an array defining various commands the extension will make available to VS Code via the command palette. You should see one command defined as extension.helloWorld. The “extension” part of that command acts as a namespace when wiring up the command within VS Code and the “helloWorld” part creates a unique name for the command within your extension. We need to make sure this gets changed to match what we updated in the activationEvents so replace helloWorld here with createList.

  3. The title key within the command definition provides the text that will be shown to users within the command palette. Change the value of this key to be Create List. The last thing we’ll change in this file is add another key below title so put a comma at the end of the title value and press your “Enter” key for a new line. Add a key "category" (intellisense should pop up to help here too) and give it a value "Extension Lister". This category helps group commands visually such that they show up as the following in the command palette <Extension Category>: <Command Title> (Example: “Extension Listener: Create List”)

Modifying the package.json file for our new extension

Let’s get into building out the functionality for this extension. Within the src folder you’ll see a file extension.ts. This is the main file that’s used to load up your extension and corresponding functionality. Take a moment to read the comments within the code that helps describe some of the parts of code that were generated for you… Seriously go read it and come back.

Now that you’re a bit more familiar with the code let’s get our first command set up. We first need to update our command code to match the changes we made in the package.json. Find the code registerCommand('extension.helloWorld' and replace helloWorld with createList.

Changing the name of the registered command in code

Ok the command is wired up correctly and we’re ready to start gathering the list of extensions. The VS Code API provides a namespace extensions which contains a list of all extensions available in the instance under the all array. We access the API via the import * as vscode from 'vscode'; statement. Since that was already done within the registerCommand (on line 17) let’s grabbed the array and assign it to a constant. Replace the line of code vscode.window.showInformationMessage('Hello World!'); (and the comment above it) with the following code:

const activeExtensions = vscode.extensions.all;

If you run and debug the extension with a breakpoint on this line you’ll notice the array contains literally all extensions installed and available for the instance of VS Code the extension is running within. Feel free to try this yourself now but it is not required. This is a good start but if you inspect collection of extensions you’ll notice it includes extensions that are built-in for VS Code, ones that are not currently active and extensions that are themes.

Being that we’re getting extensions outside the scope of what we want to include in the list (active, non-theme type, extensions) we need to filter some of these results first. Add the following array method, filter, to capture the extensions within scope.

const activeExtensions = vscode.extensions.all.filter(
  (extension: vscode.Extension<any>) => {
    return (
      extension.isActive && // make sure it is active
      !extension.packageJSON.isBuiltin && // don't include built in
        // don't include themes
        (category: string) => category.toLocaleLowerCase() === 'themes'

Next we want to format the content of what will be written to the file to be as follows: <extension name>: <extension marketplace URL>. This will make it easy to share and allow others to quickly install the extensions. To do this let’s iterate over the array of extensions to create a string of the content to write to the file. Use the following code to accomplish that and add it after the activeExtensions array is defined and assigned:

let extensionListData = ''; // variable to hold the file contents as a string
activeExtensions.forEach((extension: vscode.Extension<any>) => {
  // thanks to TypeScript and the exposed Extension type from the VS Code API we get intellisense to see the properties of each extension.
  // In particular we want to read the display name property found in the `packageJSON` object and generate the URL using the extension ID property
  extensionListData += `${extension.packageJSON.displayName}:${}\n`;

At this point we have the data we need and in a human readable format so we’re ready to ask the user where they want to save this file. For visual prompts and queues VS Code provides options via its window namespace. In particular we want the showSaveDialog function. There are some options/arguments we can provide to it such that it helps create a better user experience in this dialog. We know this will just be a plaintext file so we’ll provide a filter option that says to limit the file type to *.txt.

When the user has completed their actions in the save dialog the resulting URI (universal resource indicator) for the file will be returned to us in the promise .then() resolve function. We’ll need to make sure a URI was indeed provided first. If it wasn’t we can show an error dialog with vscode.window.showErrorMessage(). Once we know we at least have a value we can write to the file but let’s hold on that just for a moment. See the following code which accomplishes what we’ve discussed in this section:

vscode.window.showSaveDialog({ filters: { '*': ['txt'] } }).then((uri) => {
  if (!uri) {
    // This pops up an error notification dialog
      'You must select a file location to create the extension list'
    return; // Don't proceed if we don't have a file URI to write to
  // We'll add the code to write to a file here next...

We want to implement writing to a file on the Operating System’s file system, however, we shouldn’t pollute our main extension code with that code. Let’s create a separate file to handle writing a file to the file system. Open VS Code’s file explorer view by clicking on the icon or using the keyboard shortcut CTRL/CMD + SHIFT + E. Right-click on the src folder and choose “New File”

Creating a new file in the 'src' folder

Give it the name file-manager and press enter to complete the file creation. VS Code should automatically open up that file but if not double-click on it. We won’t go through the following code as it’s not really pertinent to the creation of VS Code extensions but know that it’s handling writing the contents to a file. Feel free to review the code if you’d like. Add this to the file-manager.ts file and save:

import { writeFile } from 'fs'; // leverage the Node.js file system module's writeFile function

export async function writeExtensionListFile(uri: string, data: any) {
  await writeFile(uri, data, (err: any) => {
    if (err) {
      throw new Error(err);
    return true;

We can now import the writeExtensionListFile() function from the file-manager.ts file we created to handle the last step. Open up the extension.ts file again and add an import statement to the top of it after the vscode import (found on line 3):

import { writeExtensionListFile } from './file-manager';

Sweet, let’s use it. Go down to where we have the showSaveDialog and after the part that’s checking for a uri value add the following code:

// Provide the full path on the file system for the file to write to and the contents we want to write to that file
writeExtensionListFile(uri.fsPath, extensionListData)
  .then(() => {
    // if the file was created successfully show an alert notification
      'Extension list was successfully created'
  .catch((error: any) => {
    // if the file failed to be created show an error notification
      'There was an issue creating the extension list'

What does this code do? It’s calling the function writeExtensionListFile and passing in the full file system path value (a property on the uri object) and the contents we want to be written to it. If that goes through successfully we let the user know through the vscode.window.showInformationMessage() function otherwise we show an error message with the vscode.window.showErrorMessage() function.

In case something isn’t working correctly for you use the following GitHub repository as a reference for the completed code: VS Code Extension Lister

How about we give it a run and test it out? Press F5 to start debugging or open the debug view in VS Code and press the green “Play” button.

Running/debugging the Extension Lister extension

You’ll then see a new VS Code instance load up with the title “Extension Development Host” that has your extension running within it. Open the command palette using the keyboard shortcut CTRL/CMD + SHIFT + P and type lister to see the available command for our extension.

Testing out the Extension Lister extension

Press the “Enter” key on your keyboard and you should be prompted to choose a location and name for your file. Pick a location and name for your file and click the “Save” button. You should see a success notification pop up in the bottom right-hand corner of VS Code.

A success notification for completing the file creation in our extension

Just to make sure go to where you saved the file and open it to see the contents. You should see the full list of active extensions you’re using with their marketplace URLs.

The resulting file contents from the VS Code Extension Lister extension

Congrats you’ve created another VS Code extension and one that adds new functionality!

How do I publish the extension?

Rather than write that same information again here take a look the Publishing Extension documentation. The documentation provided on the VS Code website is pretty straightforward and easy to follow so check that out for further details.

Where do I go to find out more?

The best place to read up more on VS Code extension development is at the official VS Code API Docs site. There you’ll find a table of contents on the left-hand side that is expandable and breaks out important topics around extension development.


If there are more questions you have that haven’t been answered here let me know in the comments. I’ll check in regularly and follow up with a response or a post if it’s more appropriate in that format.

Thanks for reading this post and I hope you found it helpful.

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