Chapter 2

IDE Superpowers

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TypeScript works the same no matter what IDE you use, but for this book we'll assume you're using Visual Studio Code (VS Code).

When you open VS Code, the TypeScript server starts in the background. It will be active as long as you have a TypeScript file open.

Let's take a look at some of the awesome VS Code features that are powered by the TypeScript server.


If I were to name the single TypeScript feature I couldn't live without, it would be autocomplete.

TypeScript knows what type everything is inside your app. Because of that, it can give you suggestions when you're typing - speeding you up enormously.

In the example below, just typing 'p' after audioElement brings up all the properties which start with 'p'.

const audioElement = document.createElement("audio");
audioElement.p; // play, pause, part etc

This is really powerful for exploring APIs you might not be familiar with, like the HTMLAudioElement API in this case.

Manually Triggering Autocomplete

You'll often want to manually trigger autocomplete. In VS Code, the Ctrl + Space keyboard shortcut will show you a list of suggestions for what you're typing.

For example, if you were adding an event listener to an element, you would see a list of available events:

"", // autocomplete here

Hitting the Ctrl + Space shortcut with the cursor inside the quotes will show you a list of events that can be listened for:


If you wanted to narrow down the list to the events you were interested in, you could type "drag" before hitting Ctrl + Space and only the appropriate events would display:


Autocomplete is an essential tool for writing TypeScript code, and it's available right out of the box in VS Code.


Exercise 1: Autocomplete

Here's an example of some code where autocomplete can be triggered:

const acceptsObj = (obj: { foo: string; bar: number; baz: boolean }) => {};
// Autocomplete in here!

Practice using the autocomplete shortcut to fill in the object when calling acceptsObj.

Solution 1: Autocomplete

When you hit Ctrl + Space inside the object, you'll see a list of the possible properties based on the MyObj type:


As you select each property, the autocomplete list will update to show you the remaining properties.

TypeScript Error Checking

The thing TypeScript is most famous for is its errors. These are a set of rules which TypeScript uses to make sure your code is doing what you think it's doing.

For every change you make to a file, the TypeScript server will check your code.

If the TypeScript server finds any errors, it will tell VS Code to draw a red squiggly line under the part of the code that has a problem. Hovering over the underlined code will show you the error message. Once you make a change, the TypeScript server will check again and remove the red squiggly line if the error is fixed.

I like to think of it like a teacher hovering over your shoulder, underlining your work in red pen while you type.

Let's look at these errors a bit more deeply.

Catching Runtime Errors

Sometimes TypeScript will warn you about things that will definitely fail at runtime.

For example, consider the following code:

const a = null;
a.toString(); // red squiggly line under `a`

TypeScript tells us that there is a problem with a. Hovering over it shows the following error message:

'a' is possibly 'null'.

This tells us where the problem is, but it doesn't necessarily tell us what the problem is. In this case, we need to stop and think about why we can't call toString() on null. If we do, it will throw an error at runtime.

Uncaught TypeError: Cannot read properties of null (reading 'toString').

Here, TypeScript is telling us that an error might happen even without us needing to check it. Very handy.

Warnings About Non-Runtime Errors

Not everything TypeScript warns us about will actually fail at runtime.

Take a look at this example where we're assigning a property to an empty object:

const obj = {};
const result =; // red squiggly line under `foo`

TypeScript draws a red squiggly line below foo. But if we think about it, this code won't actually cause an error at runtime. We're trying to assign a property that doesn't exist in this object: foo. This won't error, it will just mean that result is undefined.

It may seem strange that TypeScript would warn us about something that won't cause an error, but it's actually a good thing. If TypeScript didn't warn us about this, it would be saying that we can access any property on any object at any time. Over the course of an entire application, this could result in quite a few bugs.

It's best to think of TypeScript's rules as opinionated. They are a collection of helpful tips that will make your application safer as a whole.

Warnings Close to the Source of the Problem

{/* TODO Consider moving this and the next section to later, perhaps the weird parts */}

TypeScript will try to give you warnings as close to the source of the problem as possible.

Let's take a look at an example.

type Album = {
artist: string;
title: string;
year: number;
const album: Album = {
artsist: "Television", // red squiggly line under `artsist`
title: "Marquee Moon",
year: 1977,

We define an 'Album' type - an object with three properties. Then, we say that const album needs to be of that type via const album: Album. Don't worry if you don't understand all the syntax yet - we'll cover it all later.

Can you see the problem? There's a typo of the artist property when creating an album. Hovering over artsist shows the following error message:

Type '{ artsist: string; title: string; year: number; }' is not assignable to type 'Album'.
Object literal may only specify known properties, but 'artsist' does not exist in type 'Album'. Did you mean to write 'artist'?

That's because we've said that the album variable needs to be of type Album, but we've misspelled artist as artsist. TypeScript is telling us that we've made a mistake, and even suggests the correct spelling.

Dealing With Multi-Line Errors

However, sometimes TypeScript will give you an error in a more unhelpful spot.

In this example, we have a function called logUserJobTitle that logs the job title of a user:

const logUserJobTitle = (user: {
job: {
title: string;
}) => {

Don't worry about the syntax for now - we'll cover it later. The important thing is that logUserJobTitle takes a user object with a job property that has a title property.

Now, let's call logUserJobTitle with a user object where the job.title is a number, not a string.

const exampleUser = {
job: {
title: 123,
logUserJobTitle(exampleUser); // red squiggly line under `exampleUser`

It might seem like TypeScript should give us an error on title in the exampleUser object. But instead, it gives us an error on the exampleUser variable itself.

Hovering over exampleUser shows the following error message:

Argument of type '{ job: { title: number; }; }' is not assignable to parameter of type '{ job: { title: string; }; }'.
The types of 'job.title' are incompatible between these types.
Type 'number' is not assignable to type 'string'.

It's multiple lines long, which can feel pretty scary. A good rule of thumb with multi-line errors is to start at the bottom:

Type 'number' is not assignable to type 'string'.

This tells us that a number is being passed into a slot where a string is expected. This is the root of the problem.

Let's go to the next line:

The types of 'job.title' are incompatible between these types.

This tells us that the title property in the job object is the problem.

Already, we understand the issue without needing to see the top line, which is usually a long summary of the problem.

Reading errors bottom-to-top can be a helpful strategy when dealing with multi-line TypeScript errors.

Introspecting Variables and Declarations

You can hover over more than just error messages. Any time you hover over a variable or declaration, VS Code will show you information about it.

In this example, we could hover over thing and see that it's of type number:

let thing = 123;
// hovering over `thing` shows:
let thing: number;

Hovering works for more involved examples as well. Here otherObject spreads in the properties of otherThing as well as adding thing:

let otherThing = {
name: "Alice",
const otherObject = {

Hovering over otherObject will give us a computed readout of all of its properties:

// hovering over `otherObject` shows:
const otherObject: {
thing: number;
name: string;

Depending on what you hover over, VS Code will show you different information. For example, hovering over .thing in otherObject.thing will show you the type of thing:

(property) thing: number

Get used to the ability to float around your codebase introspecting variables and declarations, because it's a great way to understand what the code is doing.


Exercise 1: Hovering a Function Call

In this code snippet we're trying to grab an element using document.getElementById with an ID of 12. However, TypeScript is complaining.

let element = document.getElementById(12); // red squiggly line under 12

How can hovering help to determine what argument document.getElementById actually requires? And for a bonus point, what type is element?

Solution 1: Hovering a Function Call

First of all, we can hover over the red squiggly error itself. In this case, hovering over 12, we get the following error message:

Argument of type 'number' is not assignable to parameter of type 'string'.

We'll also get a readout of the getElementById function:

(method) Document.getElementById(elementId: string): HTMLElement | null

In the case of getElementById, we can see that it requires a string as an argument, and it returns an HTMLElement | null. We'll look at this syntax later, but it basically means either a HTMLElement or null.

This tells us that we can fix the error by changing the argument to a string:

let element = document.getElementById("12");

We also know that element's type will be what document.getElementById returns, which we can confirm by hovering over element:

// hovering over element shows:
const element: HTMLElement | null;

So, hovering in different places reveals different information. When I'm working in TypeScript, I hover constantly to get a better sense of what my code is doing.

JSDoc Comments

JSDoc is a syntax for adding documentation to the types and functions in your code. It allows VS Code to show additional information in the popup that shows when hovering.

This is extremely useful when working with a team

Here's an example of how a logValues function could be documented:

* Logs the values of an object to the console.
* @param obj - The object to log.
* @example
* ```ts
* logValues({ a: 1, b: 2 });
* // Output:
* // a: 1
* // b: 2
* ```
const logValues = (obj: any) => {
for (const key in obj) {
console.log(`${key}: ${obj[key]}`);

The @param tag is used to describe the parameters of the function. The @example tag is used to provide an example of how the function can be used, using markdown syntax.

There are many, many more tags available for use in JSDoc comments. You can find a full list of them in the JSDoc documentation.

Adding JSDoc comments is a useful way to communicate the purpose and usage of your code, whether you're working on a library, a team, or your own personal projects.


Exercise 1: Adding Documentation for Hovers

Here's a simple function that adds two numbers together:

const myFunction = (a: number, b: number) => {
return a + b;

In order to understand what this function does, you'd have to read the code.

Add some documentation to the function so that when you hover over it, you can read a description of what it does.

Solution 1: Adding Documentation for Hovers

In this case, we'll specify that the function "Adds two numbers together". We can also use an @example tag to show an example of how the function is used:

* Adds two numbers together.
* @example
* myFunction(1, 2);
* // Will return 3
const myFunction = (a: number, b: number) => {
return a + b;

Now whenever you hover over this function, the signature of the function along with the comment and full syntax highlighting for anything below the @example tag:

// hovering over myFunction shows:
const myFunction: (a: number, b: number) => number
Adds two numbers together.
myFunction(1, 2);
// Will return 3

While this example is trivial (we could, of course, just name the function better), this can be an extremely important tool for documenting your code.

The TypeScript server also provides the ability to navigate to the definition of a variable or declaration.

In VS Code, this "Go to Definition" shortcut is used with Command + Click on a Mac or F12 on Windows for the current cursor position. You can also right click and select "Go to Definition" from the context menu on either platform. For the sake of brevity, we'll use the Mac shortcut.

Once you've arrived at the definition location, repeating the Command + Click shortcut will show you everywhere that the variable or declaration is used. This is called "Go To References". This is especially useful for navigating around large codebases.

The shortcut can also be used for finding the type definitions for built-in types and libraries. For example, if you Command + Click on document when using the getElementById method, you'll be taken to the type definitions for document itself.

This is a great feature for understanding how built-in types and libraries work.

Rename Symbol

In some situations, you might want to rename a variable across your entire codebase. Let's imagine that a database column changes from 'id' to 'entityId'. A simple find and replace won't work, because 'id' is used in many places for different purposes.

A TypeScript-enabled feature called 'Rename Symbol' allows you to do that with a single action.

Let's take a look at an example.

{/* We could convert this to an exercise? */}

const filterUsersById = (id: string) => {
return users.filter((user) => === id);

Right click on the id parameter of the findUsersById function and select "Rename Symbol".

A panel will be displayed that prompts for the new name. Type in userIdToFilterBy and hit enter. VS Code is smart enough to realize that we only want to rename the id parameter for the function, and not the property:

const filterUsersById = (userIdToFilterBy: string) => {
return users.filter((user) => === userIdToFilterBy);

The rename symbol feature is a great tool for refactoring code, and it works across multiple files as well.

Automatic Imports

Large JavaScript applications can be composed of many, many modules. Manually importing from other files can be tedious. Fortunately, TypeScript supports automatic imports.

When you start typing the name of a variable you want to import, TypeScript will show a list of suggestions when you use the Ctrl + Space shortcut. Select a variable from the list, and TypeScript will automatically add an import statement to the top of the file.

You do need to be a little bit careful when using autocompletion in the middle of a name since the rest of the line could be unintentionally altered. To avoid this issue, make sure your cursor is at the end of the name before hitting Ctrl + Space.

Quick Fixes

VS Code also offers a "Quick Fix" feature that can be used to run quick refactor scripts. For now, let's use it to import multiple missing imports at the same time.

To open the Quick Fix menu, hit Command + .. If you do this on a line of code which references a value that hasn't been imported yet, a popup will show.

const triangle = new Triangle(); // red squiggly line under `Triangle`

One of the options in the Quick Fix menu will be 'Add All Missing Imports'. Selecting this option will add all the missing imports to the top of the file.

import { Triangle } from "./shapes";
const triangle = new Triangle();

We'll look at the Quick Fixes menu again in the exercises. It provides a lot of options for refactoring your code, and it's a great way to learn about TypeScript's capabilities.

Restarting the VS Code Server

We've looked at several examples of the cool things that TypeScript can do for you in VS Code. However, running a language server is not a simple task. The TypeScript server can sometimes get into a bad state and stop working properly. This might happen if configuration files are changed or when working with a particularly large codebase.

If you're experiencing strange behavior, it's a good idea to restart the TypeScript server. To do this, open the VS Code Command Palette with Shift + Command + P, then search for "Restart TS Server".

After a couple of seconds, the server should kick back into gear and ensure that errors are being reported properly.

Working in JavaScript

If you're a JavaScript user, you might have noticed that lots of these features are already available without using TypeScript. Autocomplete, organizing imports, auto imports and hovering all work in JavaScript. Why is that?

It's because of TypeScript. TypeScript's IDE server is not just running on TypeScript files, but on JavaScript files too. That means that some of TypeScript's amazing IDE experience is also available in JavaScript.

Some features aren't available in JavaScript out of the box. The most prominent is in-IDE errors. Without type annotations, TypeScript isn't confident enough about the shape of your code to give you accurate warnings.

TIP: There is a system for adding types to .js files using JSDoc comments which TypeScript supports, but it isn't enabled by default. We'll learn how to configure it later.

The reason TypeScript does this is, first of all, to support a better experience working in JavaScript for VS Code users. A subset of TypeScript's features is better than nothing at all.

But the upshot is that moving to TypeScript should feel extremely familiar for JavaScript users. It's the same IDE experience, just better.


Exercise 1: Quick Fix Refactoring

Let's revisit VS Code's Quick Fixes menu we looked at earlier.

In this example, we have a function that contains a randomPercentage variable, which is created by calling Math.random() and converting the result to a fixed number:

const func = () => {
// Refactor this to be its own function
const randomPercentage = `${(Math.random() * 100).toFixed(2)}%`;

The goal here is to refactor the logic that generates the random percentage into its own separate function.

Highlight a variable, line, or entire code block then hit Command + . to open the Quick Fix menu. There will be several options for modifying the code, depending on where your cursor is when you open the menu.

Experiment with different options to see how they affect the example function.

Solution 1: Quick Fix Refactoring

The Quick Fix menu will show different refactoring options depending on where your cursor is when you open it.

Inlining Variables

If you target randomPercentage, you can select an "Inline variable" option.

This would remove the variable and inline its value into the console.log:

const func = () => {
console.log(`${(Math.random() * 100).toFixed(2)}%`);

Extracting Constants

When selecting a smaller portion of code like Math.random() * 100, the option to "Extract constant in enclosing scope" will appear.

Selecting this option creates a new local variable that you are prompted to name, and assigns the selected value to it. After saving and running a code formatter, everything is cleaned up nicely:

const func = () => {
const randomTimes100 = Math.random() * 100;
const randomPercentage = `${randomTimes100.toFixed(2)}%`;

Similarly, the "Extract to Constant in Module Scope" option will create a new constant in the module scope:

const randomTimes100 = Math.random() * 100;
const func = () => {
const randomPercentage = `${randomTimes100.toFixed(2)}%`;

Inlining and Extracting Functions

Selecting the entire random percentage logic enables some other extraction options.

The "Extract to function in module scope" option will act similarly to the constant option, but create a function instead:

const func = () => {
const randomPercentage = getRandomPercentage();
function getRandomPercentage() {
return `${(Math.random() * 100).toFixed(2)}%`;

These are just some of the options provided by the Quick Fix menu. There's so much you can achieve with them, and we're only scratching the surface. Keep exploring and experimenting to discover their full potential!

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