Creating a VS Code extension

January 30, 2019
7 minute read
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javascript, tooling, testing
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Where I work we have become quite particular about making our code line up vertically in the import sections and when declaring some objects, so that it looks nice and neat! My colleague commented that it would be nice if there was an extension for VS Code that would do the alignment for us, so I made one, and I suspect that this was his plan all along!.

The full code can be found on my Github, I will explain the steps I took to create it below.

Gif of extension in action

Getting started

The first step is to download Yeoman and the VS Code Extension Generator with NPM:

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npm install -g yo generator-code

Running the generator scaffolds a new project ready for developing. Just type the following and answer the questions:

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$ yo code
_-----_ ╭──────────────────────────╮
| | │ Welcome to the Visual │
|--(o)--| │ Studio Code Extension │
`---------´ │ generator! │
( _´U`_ ) ╰──────────────────────────╯
/___A___\ /
| ~ |
´ ` |° ´ Y `
? What type of extension do you want to create? New Extension (TypeScript)
? What's the name of your extension? my-new-extension
? What's the identifier of your extension? my-new-extension
? What's the description of your extension? It's an extension which is mine and new.
? Enable stricter TypeScript checking in 'tsconfig.json'? No
? Setup linting using 'tslint'? No
? Initialize a git repository? (Y/n) n

Once the project has been created and the dependencies have been installed you can open the folder in VS Code to check out the project structure. Pressing F5 starts the debugger and runs your code in a new "Extension Development Host window". What the team at VS Code have done here is great because you can place breakpoints in the main editor whilst trying out the extension you are writing in a development version of VS Code.

That's all I'm going to say about getting setup, I haven't gone into great detail; if you would like a more in-depth article about getting started then I recommend taking a look at this tutorial on VS Code's website.

Adding a menu item

The first thing I knew that I wanted to do was to add an item to the context menu so that users could right-click on some highlighted text to run the extension. This can be achieved through the package.json, just add the following to the contributes section. Whilst you have this file open you should also change any references to 'helloWorld' to the name of your extension:

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"contributes": {
"commands": [
"command": "extension.alignVertically",
"title": "Align Vertically"
"menus": {
"editor/context": [
"command": "extension.alignVertically",
"group": "YourGroup@1"

Coding the extension

If you chose to write your extension in JavaScript then you will have a file called extension.js which looks like this when you first open it:

Hello world sample

If you are writing your file in TypeScript then it will look different but the logic is going to be the same for both. We need to change the activate function to register our extension, notice that "extension.alignVertically" matches up with the entries in the package.json.

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function activate(context) {
let disposable = vscode.commands.registerCommand(

The second argument passed to vscode.commands.registerCommand is our function which we will call alignVertically. This function will call the functions which handle the formatting of the text and replace it in the editor:

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1 async function alignVertically() {
2 const editor = vscode.window.activeTextEditor
3 const text = editor.document.getText(editor.selection)
4 if (text) {
5 const keyword = await getKeywordFromUser()

Let's take a closer look at this function. On line 2 we get the activeTextEditor from the vscode object (which is required in at the top of the file). Then, on line 3 we get the text which has been highlighted by the user. The if statement on line 4 means that the rest of the program will only run if some text has acutally been selected.

Next (line 5) we prompt the user for the keyword which is going to be used to split the text. You hopefully noticed on line 1 that this function is async which means we can make it wait for some data by using the await command. We call getKeywordFromUser and the execution is paused until the result is returned. Here is the function:

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function getKeywordFromUser() {
return vscode.window.showInputBox({
placeHolder: "Align by which word?"

Using a method on the vscode API's window object, all we need to do is set the placeholder text which we want to be displayed by the input box. The result is returned by the API as a promise which is resolved when the user hits enter.

Back to the alignVertically function:

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4 if (text) {
5 const keyword = await getKeywordFromUser()
6 const lines = getLines(text, keyword)
7 const mask = getMask(lines)
8 const transformedText = transform(lines, mask, getSpaces)
9 const result = joinWithKeyword(transformedText, keyword)
10 editor.edit(builder => builder.replace(editor.selection, result))
11 }

Lines 6 - 9 call the functions which I have written to transform the highlighted text. These are in another file and I will cover them in the next section. By line 10 we have our transformed text held in a variable called result and all that is left to do is to replace the highlighted text in the editor with value of this result variable.

Our editor variable from line 2 has a method called edit which calls a function with an object (named here builder) which has the method replace and it, as you can probably guess, replaces the selected text (editor.selection - previously used on line 3) with result. Phew, that felt like a lot when typing it out, it's not that bad though, hopefully it made sense to read!

The next couple of sections are about the functions which format the text. If you are only interested in how to create an extension then you should probably skip these and jump down to the last section which is about how to publish your extension to the marketplace. If, however, you're interested in how I formatted the text itself then read on!

Test Driven Development

Lines 6 - 9 use five functions which are imported from a separate file. These functions process the text by:

  1. Splitting the text into separate blocks which I could easily work with.
  2. Calculating the current positions of the supplied keyword in each line.
  3. Creating a new block of the correct amount of empty space for each line.
  4. Adding the new blocks of empty space to each line.
  5. Rejoining the blocks of text back together.

One of the advantages of separating your logic like this is that you can write simple functions which just do one thing each. These are pure functions which do not have any side effects - based on what you put in you know exactly what you are going to get out. Because of this, the best and easiest way to develop our functions is to set up test cases where we can control the input to each function and test that the output is as expected.

The VS Code generator creates your project with Mocha already installed, however I prefer to work with Jest. You can install Jest with NPM:

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npm install Jest -D

This will add Jest to the node_modules binaries (.bin) folder, the easiest way to run Jest then is to add this line to the scripts section in your _package.json:

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"scripts": {
"test": "jest"

And run the tests with the command:

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npm test

Jest will search through your codebase for files with .test in the name and run the tests in those files (our functions are in a file which I, rather originally, titled functions.js and the tests are in the, equally originally named, functions.test.js file). You can also start the tests in watch mode so that the tests will re-run each time a change is made; the watch menu includes some useful options that are worth exploring:

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$ npm test -- --watch
No tests found related to files changed since last commit.
Press `a` to run all tests, or run Jest with `--watchAll`.
Watch Usage
› Press a to run all tests.
› Press f to run only failed tests.
› Press p to filter by a filename regex pattern.
› Press t to filter by a test name regex pattern.
› Press q to quit watch mode.
› Press Enter to trigger a test run.

At the top of the file I require in the functions from functions.js. The reason I put these functions in a separate file was so that I would not need to mock anything when running the tests; if I left them in extension.js then Jest would attempt to import the vscode object.

To create a test suite in Jest you use a describe block. The first argument is just a string which will be used at the start of every test and the second argument is the function within which you run each test.

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1 const {
2 getMask,
3 getLines,
4 transform,
5 getSpaces,
6 joinWithKeyword
7 } = require("../functions.js");
9 describe("The function ", () => {
11 const keyword = "SPLIT";
13 const text = [
14 "This is some SPLIT text",
15 "I want SPLIT to test",
16 "Nothing to report here",
17 "It SPLIT should be getting",
18 "split where there SPLIT is"
19 ].join("\n");
21 const expectedLines = [
22 ["This is some ", " text"],
23 ["I want ", " to test"],
24 ["Nothing to report here"],
25 ["It ", " should be getting"],
26 ["split where there ", " is"]
27 ];
29 const expectedMask = [13, 7, 0, 3, 18];
31 const expectedTransformed = [
32 ["This is some ", " text"],
33 ["I want ", " to test"],
34 ["Nothing to report here"],
35 ["It ", " should be getting"],
36 ["split where there ", " is"]
37 ];
39 const expectedResult = [
40 "This is some SPLIT text",
41 "I want SPLIT to test",
42 "Nothing to report here",
43 "It SPLIT should be getting",
44 "split where there SPLIT is"
45 ].join("\n");

First I set up the variables which will be used in each test. At line 11 is the imaginary keyword supplied by our user, next, at line 13 is the block of text which our user has highlighted (declared as an array joined with \n which is a newline). Each following expected output is then used both to test the function has produced the correct output and also as the input for the following function.

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48 test("getLines returns expected", () => {
49 expect(getLines(text, keyword)).toEqual(expectedLines);
50 });
52 test("getMask returns expected", () => {
53 expect(getMask(expectedLines)).toEqual(expectedMask);
54 });
56 test("getSpaces returns expected", () => {
57 expectedMask.forEach(index => {
58 const diff = 18 - index;
59 expect(getSpaces(18, index).length).toBe(diff);
60 });
61 });
63 test("transform returns expected", () => {
64 expect(transform(expectedLines, expectedMask, getSpaces)).toEqual(
65 expectedTransformed
66 );
67 });
69 test("joinWithKeyword returns expected", () => {
70 expect(joinWithKeyword(expectedTransformed, keyword)).toEqual(
71 expectedResult
72 );
73 });

If a test fails then Jest will give you a handy diff to show you why it failed:

Failed tests output

It is possible to set up the VS Code debugger so that you can breakpoint your code as the tests run to help with debugging. I won't go into how to do that here though, maybe in another post!

This is what we're aiming to achieve, all tests passed:

Passed tests output

Formatting the text

It's (finally!) time to take a look at the functions themselves. Here's a reminder of lines 6 - 9 from extensions.js:

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6 const lines = getLines(text, keyword)
7 const mask = getMask(lines)
8 const transformedText = transform(lines, mask, getSpaces)
9 const result = joinWithKeyword(transformedText, keyword)

getLines(text, keyword)

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getLines(text, keyword) {
return text.split("\n").map(line => line.split(keyword));

The getLines function takes the highlighted text and keyword as arguments and uses Array's split method to split the text on the newline character (remember this is how we tested it above?), we then split each line at the keyword which result in an array where each element is a line of text which is itself an array with each element containing either the text before or after the keyword. For example, if the keyword provided was 'SPLIT', the following text:

This is some SPLIT text
It will SPLIT words by newline
Also by keyword

would output as:

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[ ['this is some ', 'text'], ['It will ', 'words by newline'], ['Also by keyword'] ]


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getMask(lines) {
return => {
if (line.length > 1) {
return line[0].length;
return 0;

This function creates an array with one element per line. Each element contains a number which represents the length of the block of text which precedes the keyword. If the keyword is not on a line it will put a zero in its place. The mask for the example above would be [ 13, 8, 0].

getSpaces(max, mask[i])

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getSpaces(max, index) {
const diff = max - index;
return new Array(diff).fill(" ");

This function is used by the transform function (below). It takes the max variable (explained next) and uses it to calculate how much space needs to be added to the block of text. It returns an array of the required length and fills it with empty spaces.

transform(lines, mask, getSpaces)

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1 transform(lines, mask, getSpaces) {
2 const max = Math.max(...mask);
3 return, i) => {
4 if (mask[i]) {
5 const extended = [
6 line[0], ...getSpaces(max, mask[i])
7 ].join("");
8 line.splice(0, 1, extended);
9 }
10 return line;
11 });
12 }

The first thing this function does is get the max number from the mask. This is the position of the rightmost keyword in our lines of text and is where we want to move our other lines to. Next, it maps through the lines array and if the line has a corresponding number in the mask (ie. is greater than zero) then it creates an extended version of the block of text which has the extra spaces added into it.

This happens on line 5. You can see that we create a new array from our first element (line[0]) and spread the output from the above getSpaces function into it. This array would look like:

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['this is some ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ']

Then we join the array, making a string with the block of spaces at the end. On line 6 we use Array's splice method to replace the first element of the line with our new extended version.

joinWithKeyword(transformedText, keyword)

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joinWithKeyword(transformed, keyword) {
return => l.join(keyword)).join("\n");

Finally we just need to put our blocks of text back together. We map through the array output by the transform function and join each line with the keyword. Lastly we join the lines back together using the newline (\n) character again.

So that's everything that we need to do to create the extension. All that remains to do is to make it available for people to use.

Publishing the extension

I'm not going to go into a great deal of detail on this, partly because the documentation on VS Code's site tells you everything you need to know and partly because I feel like I've been prattling on for far too long already! Therefore, for a detailed guide take a look here. The short version is that you need to run:

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npm install -g vsce

Then you need to get a Personal Access Token from the Azure DevOps site. You use this token to set yourself up as a publisher by running:

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vsce create-publisher < publisher name >

Once vsce (which stands for Visual Studio Code Extension) is setup you can run the below to publish your extension to the marketplace:

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vsce publish

If you don't want to publish your extension to the marketplace for public use then you can instead run:

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vcse package

This will create a .vsix file which can be shared with whomever you choose and be easily installed with:

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code --install-extension < path/to/file.vsix >

When I was figuring out how to make this basic extension I struggled a little bit to find the information I needed. I hope that if you have stumbled across this post that it has helped to fill in one or two of the blanks. If you have any comments or questions, or have spotted something that is incorrect or could have been done in a better way then please let me know on Twitter. Cheers

If you've found this helpful then let me know with a clap or two!