by David Piepgrass

A quick yet in-depth tour of TypeScript and its types

Union types, generics, JSX, type system loopholes and more!

Photo by Luis Villasmil on Unsplash

This quick tour of TypeScript is mainly for people who have some experience with JavaScript.

I’ll explain a few surprising facts about JavaScript, too, in case you only studied something vaguely similar, like Java or C#. If you’d like to know how to set up a TypeScript project, see my previous article.

TypeScript is based on JavaScript. The TypeScript compiler (or other tools based on it, like ts-node or ts-jest) translates TypeScript into normal JavaScript simply by stripping out all the type information.

Alongside that process, type checking is performed in order to discover type errors — mistakes you’ve made that have something to do with types. Of course, occasionally, it also complains about things you did intentionally that nevertheless broke the rules of TypeScript.


Types can be attached to variables with a colon (:) in their definition, like so:

let z: number = 26;

However you often don’t have to write down the type. For example, if you write:

let z = 26;

TypeScript infers that z is a number. So if you write:

let z = 26;z = "Not a number";

You’ll get an error on the second line. TypeScript originally did adopt a loophole though: any variable can be null or undefined:

z = null;      // Allowed!z = undefined; // Allowed! 

If you’re new to JavaScript, you’re probably wondering what null and undefined are, or why they are two different things.

Well, I promised to tell you about TypeScript and null/undefined are JavaScript things. Ha!

Personally, I don’t use null very much. I find it convenient to use undefined consistently to avoid worrying about the distinction. undefined is the default value of new variables and function parameters that were not provided by the caller. It’s the value you get if you read a property that doesn’t exist on an object. By contrast, JavaScript itself only rarely uses null, so if you don’t use it yourself, you won’t encounter it very often. I’m sure some people do the opposite, and prefer null.

Anyway, some people — including me — are of the opinion that allowing every variable to be null/undefined was a bad idea. So TypeScript 2.0 allows you to take away that permission with the "strictNullChecks": true compiler option in “tsconfig.json”. You can use "strict": true for Maximum type checking. Instead, you would write:

let z: number | null = 26;

if you want z to be potentially null (| means “or”).

Union types

TypeScript has the ability to understand variables that can have multiple types. For example, here is some normal JavaScript code:

This is allowed in TypeScript by default, because var y (by itself) gives y a type of any, meaning anything. So we can assign anything, for example value or object, to y. We can certainly set it to a string, or a number, or an array of two things. any is a special type — it means “this value or variable should act like a JavaScript value or variable and, therefore, not give me any type errors.”

I recommend the "strict": true compiler option. But, in that mode, TypeScript doesn’t allow var y— it requires var y: any instead.

However, TypeScript allows us to be more specific by saying:

var y: string | number;

This means “variable y is a string or a number”. If y is created this way, using the example above, the if-else part is allowed. But the other part that says y = [y, y] is not allowed, because [y, y] is not a string and not a number either. y is an array of type number[] | string[]. This feature, in which a variable can have one of two (or more) types, is called union types and it’s often useful.

Tip: To help you learn TypeScript, it may help to do experiments in the playground.

To help you learn more about JavaScript, press F12 in Chrome, Firefox or Edge and look for the Console. In the console you can write JavaScript code, to find out what a small piece of JavaScript does and whether you are writing it correctly:


This console is fantastic because you can use it to run experiments in any browser tab — even this one! Since TypeScript is just JavaScript with static type checking, you can use the console to help you learn about the part of TypeScript that doesn’t have static types. In your TypeScript file you can call console.log(something) to print things in the browser’s console. In some browsers, log can display complex objects. For example, try writing console.log({name:"Steve", age:37, favoriteNumbers:[7, 666, -1]}):



As you know, classes are bundles of functions and variables that can be instantiated into multiple objects. Functions inside classes can refer to other functions and variables inside the class, but in JavaScript and TypeScript you must use the prefix this.A typical JavaScript class might look like this:

The console output is:

The big box is 10000 times larger than the small oneThe zero-size box has an area of 0.

JavaScript is a little picky. When you create a function outside a class, it has the word function in front of it. But, when you create a function inside a class, it is not allowed to have the word function in front of it.

Functions and methods are the same thing, except that methods in classes have access to this - a reference to the current object, except for static methods. static methods are called on the class , Box.ZeroSize in this example, so they do not have a “current object”. (Well, actually the current object of ZeroSize is the Box constructor function, which is not an instance of Box.)

Unlike JavaScript, TypeScript classes allow variable declarations, such as width and height in this example:

For convenience, TypeScript lets you define a constructor and the variables it initializes at the same time. So instead of

  width: number;  height: number;  constructor(width: number, height: number) {    this.width = width;    this.height = height;  }

you can simply write

constructor(public width: number, public height: number) {}

By the way, for any C# developers reading this, it works exactly like my LeMP system for C#.

Unlike JavaScript, TypeScript has private (and protected) variables and functions which are inaccessible outside the class:

private variables allow you to clearly mark parts of a class as “internal”. Users of the class cannot modify or read these.


Interfaces are a way of describing “shapes” of objects. Here’s an example:

IBox refers to any class that has a width and height property that are readable numbers. IArea refers to anything with a readable area property. The Box class satisfies both of these requirements. The get area() function counts as a property, because it is called without () parentheses. So I could write:

let a: IBox = new Box(10,100);  // OKlet b: IArea = new Box(10,100); // OK

Interfaces in TypeScript work like interfaces in the Go programming language, not like interfaces in Java and C#. That’s a good thing. It means that classes don’t have to explicitly say that they implement an interface. Box implements IBox and IArea without saying so.

This means we can define interfaces for types that originally were not designed for any particular interface. For example, my BTree package defines an IMap<Key,Val> interface that represents a dictionary of key-value pairs. The new Map class built into ES6 also conforms to this interface, so you can put a Map into an IMap variable. So, for example, you can write a function with an IMap parameter, and you can pass a Map or a BTree to the function, and the function doesn’t need to know or care which type it received.

readonly means we can read, but not change:

console.log(`The box is ${a.width} by ${a.height}.`); // OKa.width = 2; /* ERR: Cannot assign to 'width' because it is a                      constant or a read-only property. */

TypeScript does not require readonly for interface compatibility. For example, TypeScript accepts this code even though it doesn’t work:

interface IArea {  area: number; // area is not readonly, so it can be changed}
let ia: IArea = new Box(10,100);ia.area = 5; // Accepted by TypeScript, but causes a runtime error

I think of it as a bug in TypeScript.

TypeScript also has a concept of optional parts of an interface:

interface Person {  readonly name: string;  readonly age: number;  readonly spouse?: Person;}

For example we can write let p: Person = {name:'John Doe', age:37}. Since p is a Person, we can later refer to p.spouse. This is equal to undefined in this case, but could be a Person if a different object were assigned to it that has a spouse.

However, if you use p = {name:'Chad', age:19, spouse:'Jennifer'} with the wrong data type for spouse , TypeScript responds that Type string is not assignable to type Person | undefined.

Intersection types

Intersection types are the lesser-known cousin of union types. A union type like A | B means that a value can be either an A or a B, but not both. An intersection type like A & B means that a value is both A and B at the same time. For instance, this box is both IBox and IArea, so it has all the properties from both interfaces:

let box: IBox & IArea = new Box(5, 7);

If you mix union and intersection types, you can use parentheses to change the meaning:

// either a Date&IArea or IBox&IArealet box1: (Date | IBox) & IArea = new Box(5, 7);// either a Date or an IBox&IArealet box2: Date | (IBox & IArea) = new Box(5, 7);

& has higher precedence than |, so A & B | C means (A & B) | C.

Structural types

In some other programming languages, every type has a name, such as string or double or Component. In TypeScript, many types do have names but, more fundamentally, most types are defined by their structure. In other words, the type’s name, if it has one, is not important to the type system. Here’s an example where variables have a structural type:

var book1 = { title: "Adventures of Tom Sawyer",       year:1876 };var book2 = { title: "Adventures of Huckleberry Finn", year:1884 };

If you hover your mouse over book1 in VS Code, its type is described as { title: string; year: number; }. This is a structural type: a type defined entirely by the fact that it has a property called title which is a string, and another property called year which is a number. Thus book1 and book2 have the same type, and you can assign one to the other, or to a different book.

book1 = book2; // allowedbook2 = { year: 1995, title: "Vertical Run" }; // allowed

Generally speaking, you can assign a value with “more stuff” to a variable whose type includes “less stuff”, but not the other way around:

var book3 = { title: "The Duplicate",               author: "William Sleator", year:1988 };var book4 = { title: "The Boy Who Reversed Himself" };book1 = book3; // allowedbool1 = bool4; /* NOT allowed. Here is the error message:    Type '{ title: string; }' is not assignable to type     '{ title: string; year: number; }'. Property 'year'     is missing in type '{ title: string; }'.  */ 

In addition, if we have an interface like this:

interface Book {  title: string;  author?: string;  year: number;}

Then we can assign any Book value to either book1 or book2. But author is required in book3 and Book might not contain an author. We can assign any of the book variables to a new variable of type Book , except book4, of course.

Clearly, structural types are fantastic. This is obvious after you spend a few years using languages without them. For example, imagine if two people, Alfred and Barbara, write different modules A and B. They both deal with points using X-Y coordinates. So each module contains a Point interface:

interface Point {    x: number;    y: number;}

Many languages use nominal types instead of structural types. In these languages, A.Point is considered to be a completely different type than B.Point even though they are identical. So any points produced by A cannot be used by B and vice versa. This can be frustrating, so please take a moment to celebrate with me the wonder of TypeScript’s structural typing.

Structural types can be written either with semicolons or commas, e.g. { x: number, y: number } and { x: number; y: number; } are the same.

Flow-based typing and the exclamation mark

If s is a string, you could write s.match(/[0-9]+/) to find the first group of digits in that string. /[0-9]+/ is a RegExp - an object that can be used to search strings using Regular Expressions. Regular expressions are a string-matching system supported by many programming languages, including JavaScript.

match returns an array of strings, or null if the RegExp did not match the string. For example, if s = "I have 10 cats and 2 dogs" then s.match(/[0-9]+/) returns ["10"], but if s = "I have ten velociraptors and a weevil" then match returns null.

If you were looking for digits in a string, you’d want your code to behave differently depending on whether the string has digits or not, right? So you’d use an if statement:

var found: string[]|null = s.match(/[0-9]+/);if (found) {  console.log("The string has a number in it: " + found[0]);} else {  console.log("The string lacks digits.");}

As you probably know, if (found) means “if found is truthy”. It basically means if (found != null && found != 0 && found != false).

If you don’t check whether found !== null, TypeScript will give you an error:

var found = s.match(/[0-9]+/);console.log("The string has a number in it: " + found[0]);           // Error: Object is possibly 'null'  ^^^^^

So why don’t you get an error when you use the if statement? That’s the magic of TypeScript’s flow-based typing.

In the first branch of the if statement, TypeScript knows that found cannot be null, and so the type of found changes within that block to exclude null. Thus, its type becomes string[]. Similarly, inside the else {...} block, TypeScript knows that found cannot be string[], so string[] is excluded and the type of found becomes null in that region.

But TypeScript has a ! operator which is used to avoid certain error messages. It means “look, compiler, I know you think this variable could be null or undefined, but I promise you it isn’t. So if found has type string[]|null, thenfound! has type string[].”

If you’re sure that s has digits in it, you can use ! to avoid the error message:

var found = s.match(/[0-9]+/);console.log("The string has a number in it: " + found![0]);

TypeScript’s flow-based typing system supports the typeof and instanceof operators, as well as ordinary comparison operators. If you start with a variable that could have several types, you can use any of these operators to narrow down the type:

Note: JavaScript distinguishes between primitive and boxed primitive types, which are objects. For example, "yarn" is a primitive, and its type is string. However, there is also a boxed string type called String with a capital S, which is rarely used. You can create a String by writing new String("yarn"). The thing to keep in mind is that these are totally different types.

"yarn" instanceof String is false: "yarn" is a string, not a String!

"yarn" instanceof string is not false. Instead it’s a totally illegal expression — the right-hand side of instanceof must be a constructor function and string does not have a constructor.

JavaScript provides two different operators for testing the types of primitives and objects (non-primitives):

  • instanceof checks the prototype chain to find out if a value is a certain kind of object.
  • typeof checks whether something is a primitive and if so, what kind.

As you can see in the code above, instanceof is a binary operator that returns a boolean, while typeof is a unary operator that returns a string. For example, typeof "yarn" returns "string" and typeof 12345 returns "number". The primitive types are number, boolean, string, symbol, undefined, and null. Everything that is not a primitive is an Object, including functions.

But typeof treats functions specially. For example, typeof Math.sqrt === "function", and Math.sqrt instanceof Object === true. Symbols are new in ES6 and, although null is a primitive, typeof null === "object" is a mistake.

As you can see in the example above, TypeScript also understands Array.isArray as a way to detect an array. However, some other methods of detecting types in JavaScript are not supported:

  • if (thing.unshift) is sometimes used to distinguish strings from other things, because almost nothing except strings have an unshift method. This is not supported in TypeScript because it does not let you read a property that may not exist.
  • if (thing.hasOwnProperty("unshift")) isn’t recognized as a type test.
  • if (thing.constructor === String) isn’t recognized as a type test. In JavaScript, reading a property such as constructor promotes thing to Boxed status, so even if thing is a primitive string, its .constructor will be non-primitive.
  • if ("unshift" in thing) doesn’t work. “The right-hand side of an ‘in’ expression must be of type ‘any’, an object type or a type parameter.” (in should be avoided anyway because it is slow.)

Type aliases

The type statement creates a new name for a type. For example after writing:

type num = number;

You can use num as a synonym for number. type is similar to interface since you can write something like this…

type Point = {    x: number;    y: number;}

…instead of interface Point {...}. However, only interfaces support inheritance. For example I can create a new interface that is like Point but also has a new member z, like this:

interface Point3D extends Point {    z: number;}

You can’t do inheritance with type. However if Point was defined with type, you are still allowed to extend it with an interface.

Function types

In JavaScript you can pass functions to other functions, like this:

function doubler(x) { return x*2; }function squarer(x) { return x*x; }function experimenter(func){  console.log(`When I send 5 to my function, I get ${func(5)}.`);}experimenter(doubler);experimenter(squarer);


When I send 5 to my function, I get 10.When I send 5 to my function, I get 25.

In TypeScript you normally need to write down the types of function arguments — you need to know how to express the type of func. As you can see here, its type should be something like (param: number) => number:

function doubler(x: number) { return x*2; }function squarer(x: number) { return x*x; }function experimenter(func: (param: number) =&gt; number){  console.log(`When I send 5 to my function, I get ${func(5)}.`);}experimenter(doubler);experimenter(squarer);

TypeScript requires you to give a name to the parameter of func, but it doesn’t matter what that name is. I could have called it x, or Wednesday, or myFavoriteSwearWord and it would have made no difference whatsoever. But don’t even think of calling it asshat. The compiler won’t care, but what about your boss? Better safe than sorry, that’s all I can say.

In JavaScript, everything inside an object is a property — a kind of variable — and that includes functions. As a consequence, these two interfaces mean the same thing:

interface Thing1 {  func: (param: number) =&gt; number;}interface Thing2 {  func(param: number): number;}

And so this code is allowed:

class Thing {  func(x: number) { return x * x * x; }}let t1: Thing1 = new Thing();let t2: Thing2 = t1; 

Does it seem weird to you that TypeScript requires : before the return type of a “normal” function but it requires =&gt; before the return type of a function variable? Anyway, that’s the way it is.

Generics, and dates, and stuff


Let’s say I write a function that ensures a value is an array, like this:

function asArray(v: any): any[] {  // return v if it is an array, otherwise return [v]  return (Array.isArray(v) ? v : [v]);}

The asArray function works, but it loses type information. For example, what if this function calls it?

/** Prints one or more dates to the console */function printDates(dates: Date|Date[]) {  for (let date of asArray(dates)) {      // SUPER BUGGY!      var year = date.getYear();      var month = date.getMonth() + 1;      var day = date.getDay();      console.log(`${year}/${month}/${day}`);  }}

The TypeScript compiler accepts this code, but it has two bugs. The code correctly added 1 to the month, because getMonth() returns 0 for January and 11 for December. But the code for getting the year and day are both wrong. Since asArray returns any[], however, type checking and IntelliSense — which could have caught these bugs — is disabled on date. These bugs could have been avoided if asArray was generic:

function asArray<T>(v: T | T[]): T[] {  return Array.isArray(v) ? v : [v];}

This version of asArray does the same thing, but it has a type parameter, which I have decided to call T, to enable enhanced type checking. The type parameter can be any type, so it is similar to any. But it enables the function to describe the relationship between the parameter v and the return value.

Specifically, it says that v and the return value have, well, similar types. When you call asArray, the TypeScript compiler finds a value of T that allows the call to make sense. For example, if you call asArray(42) then the compiler chooses T=number because it is possible to use 42 as an argument to asArray(v: number|number[]): number[]. After choosing T=number, TypeScript realizes that asArray returns an array of numbers.

In printDates we called asArray(dates) and the compiler figures out that T=Date works best in that situation. After choosing T=Date, TypeScript realizes that asArray returns an array of Date. Therefore, the variable date has type Date, and then it finds the first bug: date.getYear does not exist! Well, actually it does exist, but it has been deprecated due to its behavior — it returns the number of years since 1900 — 118 in 2018. Instead, you should call getFullYear.

TypeScript itself doesn’t notice the second bug. But, when you type date.getDay, VS Code will inform you in a little popup box that this function “Gets the day of the week, using local time”. The day of the week? You have got to be kidding me!

Thanks to generics and VS Code, we fix our code to call date.getDate instead. This does not return the date without a time attached to it but, rather, the day of the current month. Unlike the month, the day does not start counting from zero.

/** Prints one or more dates to the console */function printDates(dates: Date|Date[]) {  for (let date of asArray(dates)) {      var year = date.getFullYear();      var month = date.getMonth() + 1;      var day = date.getDate();      console.log(`${year}/${month}/${day}`);  }}

One good thing about Dateis that they are normally stored in UTC — universal time zone, or GMT. This means that if the user changes the time zone on their computer, the Date objects in your program continue to represent the same point in time, but the string returned by .toString() changes. Usually this is what you want, especially in JavaScript where you might have client and server code running in different time zones.


An advanced example of generics appears in my simplertime module. In this case I had a timeToString function that accepted a list of formatting options like this:

export interface TimeFormatOptions {  /** If true, a 24-hour clock is used and AM/PM is hidden */  use24hourTime?: boolean;  /** Whether to include seconds in the output (null causes seconds   *  to be shown only if seconds or milliseconds are nonzero) */  showSeconds?: boolean|null;  ...}
export function timeToString(time: Date|number,                              opt?: TimeFormatOptions): string {  ...}

The export keyword is used for sharing code to other source files. For example you can import timeToString in your own code using import {timeToString} from 'simplertime' (after installing with npm i simplertime of course). If you want to import things from a different file in the same folder, add a ./ prefix on the filename, e.g. import * as stuff from './mystuff'.

Generics can also be used on classes and interfaces. For example, JavaScript has a Set type for holding an unordered collection of values. We might use it like this:

var primes = new Set([2, 3, 5, 7]);for (var i = 0; i < 10; i++)  console.log(`Is the number ${i} prime? ${primes.has(i)}`);

In TypeScript, though, Set has a type parameter, Set<;T>, meaning that all items in the set have type T. In this code TypeScript infers that T=number, so if you write primes.add("hello!") you’ll get a Type Error. If you actually want to create a set that can hold both strings and numbers, you can do it like this:

var primes = new Set&lt;string|number>([2, 3, 5, 7]); 

You can also create your own generic types. For example, I created a B+ Tree data structure called BTree<K, V>, which is a collection of key-value pairs, sorted by key, that supports fast cloning. It has two type parameters, K (a key) and V (a value) and its definition looks roughly like this. Note: function bodies have been omitted because I just want to show you how a generic class looks:

Literals as types

Remember how there is an error when you write this?

let z = 26;z = "Zed";

The error message sounds a bit strange:

Type '"Zed"' is not assignable to type 'number'

Why does it say that "Zed" is a “type”, instead of a “value” or a “string”? In order to understand this, it is necessary to understand that TypeScript has an ability to treat values as types. "Zed" is a string, of course, but it’s more than that — it has another type at the same time, a more specific type called "Zed" which represents the value "Zed" . We can even create a variable with this type:

let zed: "Zed" = "Zed";

Now we have created a completely useless variable called zed. We can set this variable to "Zed", but nothing else:

zed = "Zed"; // OKzed = "ZED"; // Error: Type '"ZED"' is not assignable to type '"Zed"'.

By default we can set zed to null and undefined. Luckily with the "strictNullChecks": true option, we can close that loophole so that this variable will never be anything except “Zed”. Thank God for that, is all I can say.

So what are these literal-types good for? Well, sometimes a function allows only certain particular strings. For example, imagine if you have a function that lets you turn("left") or turn("right") but nothing else. This function could be declared with a literal-type:

function turn(direction: "left"|"right") { … }

Fixed-length arrays

Here’s another puzzle for you: what’s the difference between the types number[] and [number]? The first is an array of numbers, the second is an array that contains only one element, which is a number.

Similarly [string,number] denotes an array of length 2 with the first element being a string and the second being a number. In addition, the array has a property length: 2, i.e. its type is 2, not just number. These fixed-length arrays are called tuple types.

Advanced generics

So, remember the simplertime module I was talking about? It also exports a defaultTimeFormat object which holds default values for the timeToString formatting options. I wanted to define a special function which would allow me to write things like get(options, 'use24hourTime') to retrieve the value of options.use24hourTime if it exists and defaultTimeFormat.use24hourTime if it does not exist.

In many languages it is impossible to write a function like that, but it is possible in “dynamic” languages such JavaScript. Here’s how the get function would look like in JavaScript:

function get(opt, name) {  if (opt === undefined || opt[name] === undefined)    return defaultTimeFormat[name]  return opt[name];}

In JavaScript and TypeScript, can be written as thing["property"] instead and, if the property does not exist, the result is undefined. But in the square-bracket version we can use a variable, so that the question “which property are we using?” can be answered by code located elsewhere.

Translating this to TypeScript is possible with a feature called keyof, but it’s very tricky. Here is the translation:

function get<;K extends keyof TimeFormatOptions>(         opt: TimeFormatOptions|undefined, name: K):          TimeFormatOptions[K]{  if (opt === undefined || opt[name] === undefined)    return defaultTimeFormat[name]  return opt[name];}

Here, the type variable K has a constraint attached to it, K extends keyof TimeFormatOptions. Here’s how it works:

  1. keyof X turns the properties of X into a union type of the names of the properties. For example, given the Book interface from earlier, keyof Book means "title" | "author" | "age". Likewise keyof TimeFormatOptions is any of the property names in TimeFormatOptions.
  2. The “extends” constraint, X extends Y, means that “X must be Y, or a subtype of Y”. For example X extends Object means that X must be some kind of Object, which means it can be an array or a Date or even a function, all of which are considered to be Objects, but it can’t be a string or a number or a boolean. Similarly X extends Point means that X is Point or a more specific type than Point, such as Point3D.
  3. What would B extends keyof Book mean? It would mean that B is a subtype of "title" | "author" | "age". And, remember, that we are talking about types here, not values. The string literal "title" has the value "title" but it also has the type "title", which is a different concept. The type is handled by the TypeScript type system, and the value is handled by the JavaScript. The "title" type no longer exists when the program is running, but the "title" value still does. Now, B can be assigned to types like "title" or "title" | "age", because every value of type "title" | "age" (or "title") can be assigned to a variable of type keyof Book. However B cannot be string, because some strings are not “title”, “author”, or “age”.
  4. Similarly, K is constrained to have a subtype of keyof TimeFormatOptions, such as "use24hourTime".
  5. The type X[Y] means “the type of the Y property of X, where Y is a number or string literal”. For example, the type Book["author"] is string | undefined.

Putting this all together, when I write get(options, 'use24hourTime'), the compiler decides that K='use24hourTime'. Therefore, the name parameter has type "use24hourTime" and the return type is TimeFormatOptions["use24hourTime"], which means boolean | undefined.

Holes in the type system

Since TypeScript is built on top of JavaScript, it accepts some flaws in its type system for various reasons. Earlier we saw one of these flaws, the fact that this code is legal:

class Box {  constructor(public width: number, public height: number) {}  get area() { return this.width*this.height; }}
interface IArea {  area: number; // area is not readonly}
let ia: IArea = new Box(10,100);ia.area = 5; // Accepted by TypeScript, but causes a runtime error

Here are some other interesting loopholes:

You can assign a derived class to a base class

A Date is a kind of Object so naturally you can write:

var d: Object = new Date();

So it makes sense that we can also assign this D interface to this O interface, right?

interface D { date: Date }interface O { date: Object }var de: D = { date: new Date() };    // okay...var oh: O = de;                      // makes = { date: {wait:"what?"} }   // wait, what?

Well, no, not really, because TypeScript now believes is a Date when it is actually an Object.

You can assign [A,B] to (A|B)[]

It makes sense that an array of two items, an A followed by a B, is also a an array of A|B, right? Actually, no, not really:

var array1: [number,string] = [5,"five"];var array2: (number|string)[] = array1;   // makes sense...array2[0] = "string!";                    // wait, what?

TypeScript now believes array1[0] is a number when it is actually a string. This is an example of a more general problem, that arrays are treated as covariant but they aren’t really covariant because they are editable.

Arrays? There be dragons.

In the recommended strict mode, you can’t put null or undefined in arrays of numbers…

var a = [1,2,3];a[3] = undefined; // 'undefined' is not assignable to type 'number'

So that means when we get a value from an array of numbers, it’s a number, right? Actually, no, not really:

var array = [1,2,3];var n = array[4];

TypeScript now believes n is a number when it is actually undefined.

A more obvious hole is that you can allocate a sized array of numbers… with no numbers in it:

var array = new Array<number>(2); // array of two "numbers"var n:number = array[0];

Function parameters are bivariant when overriding

Unlike other languages with static typing, TypeScript allows overriding with covariant parameters. Covariant parameter means that, as the class gets more specific (A to B), the parameter also gets more specific (Object to Date):

class A {    method(value: Object) { }}
class B extends A {    method(value: Date) { console.log(value.getFullYear()); }}
var a:A = new B();a.method({}); // Calls B.method, which has a runtime error

This is unsafe, but oddly it is allowed. In contrast, it is (relatively) safe to override with contravariant parameters, like this:

class A {    method(value: Date) { }}class B extends A {    method(value: Object) { console.log(value instanceof Date); }}

Covariant return types are also safe:

class A {    method(): Object { return {} }}class B extends A {    method(): Date { return new Date(); }}

TypeScript rightly rejects contravariant return types:

class A {    method(): Date { return new Date(); }}class B extends A {    // Property 'method' in type 'B' is not assignable to     // the same property in base type 'A'.    //   Type '() => Object' is not assignable to type '() => Date'    //     Type 'Object' is not assignable to type 'Date'    method(): Object { return {} }}

Classes think they are interfaces (but they’re not)

TypeScript allows you to treat a class as though it were an interface. For example, this is legal:

class Class {  content: string = "";}
var stuff: Class = {content:"stuff"};

Stuff isn’t a real Class, but TypeScript thinks it is, which can cause a runtime TypeError if you use instanceof Class somewhere else in the program:

function typeTest(x: Class|Date) {  if (x instanceof Class)    console.log("The class's content is " + x.content);  else    console.log("It's a Date in the year " + x.getFullYear());}

this isn’t necessarily what you think

this is a loophole of JavaScript, not TypeScript. Any time a function uses this, it might be accessing some completely unexpected object, with a different type than you think:

class Time {  constructor(public hours: number, public minutes: number) { }  toDate(day: Date) {    var clone = new Date(day);    clone.setHours(this.hours, this.minutes);    return clone;  }}
// Call toDate() with, new Date());

TypeScript’s only sin is that it won’t try to stop you from doing this.

Speaking of this, one thing JavaScript developers should know is that arrow functions like x => x+1 work slightly differently than anonymous functions like function(x) {return x+1}.

Arrow functions inherit the value of this from the outer function in which they are located. Normal functions receive a new value of this from the caller. So, if f is an arrow function,, x) doesn’t change this, so it’s like calling f(x). That’s usually a good thing, but if you write:

var obj = { x: 5, f: () => this.x }

You should realize that obj.f() does not return obj.x.


To avoid these holes, you need to:

  • Not treat an object as a “baser” type (e.g. don’t treat D as an O) unless you are sure that the baser type won’t be modified in a way that could violate the type system.
  • Not treat an array as a “baser” type (e.g. don’t treat D[] as O[], or [A,B] as (A|B)[]) unless you are sure that the baser type won’t be modified in a way that could violate the type system.
  • Be careful not to leave any “holes” with undefined values in your arrays.
  • Be careful not to use out-of-bounds array indexes.
  • Not override a base-class method with covariant parameters.
  • Avoid treating a class K as though it were an interface, unless you are sure that no code will ever check the type with instanceof.
  • Avoid using .call(...) , and be careful how you deal with references to functions.

TypeScript actually had more holes in the past, which are now plugged.


React introduced the concept of JSX code. Or, maybe, Hyperscript introduced it and React copied the idea soon afterward. In any case, JSX looks like HTML/XML code. But you are not making DOM elements, you’re making plain-old JavaScript objects, which we call a “virtual DOM”. For example, <img src={imageUrl}/> actually means React.createElement("img", { src: imageUrl }) in a .jsx or .tsx file.

If JSX is a React thing, why am I talking about it in the TypeScript section? Because support for JSX is built into the TypeScript compiler. To get JSX support in any TypeScript file, you just have to change the file’s extension from .ts to .tsx.

JSX can be used in the same places as normal expressions: you can pass JSX code to a function…

ReactDOM.render(<h1>I'm JSX code!</h1>, document.body);

you can store it in a variable…

let variable = <h1>I'm JSX code!</h1>;

and you can return it from a function…

return <h1>I'm JSX code!</h1>;

Because <h1>I'm JSX code!</h1> really just means React.createElement("h1", null, "I'm JSX code!").

It is important whether a JSX tag starts with a capital letters — it is translated to TypeScript (or JavaScript) differently if it does. For example:

  • <div class="foo"/> means React.createElement('div', {"class":"foo"}), but
  • <Div class="foo"/> means React.createElement(Div, {"class":"foo"}) (without quotes around Div).

Tips for using JSX:

  • JSX is XML-like, so all tags must be closed: write <br/>, not <br>.
  • JSX only supports string attributes and JavaScript expressions. When writing numeric attributes in TypeScript, use <input type="number" min={0} max={100}/>, because max=100 is a syntax error and max="100" is a type error.
  • In React/Preact, you can use an array of elements in any location where a list of children are expected. For example, instead of return <p>Ann<br/>Bob<br/>Cam&lt;/p>, you can write let x = [<br/>, 'Bob', &lt;br/>]; return <p>Ann{x}Cam</p>. This has the same effect because React/Preact “flattens” arrays in the child list.
  • In React, the class attribute is not supported for some reason. Use className instead.
  • JSX itself does not support optional property or children. For example, suppose you write <Foo prop={x}> but you want to omit the prop when x is undefined. Well, JSX itself doesn’t support anything like that. However, most components treat an undefined property the same as a missing property, so it usually works anyway. JSX doesn’t support optional children either, but you can get the same effect with an empty array: because arrays are “collapsed” by React/Preact, <Foo>{ [] }</Foo> has the same effect as <Foo></Foo>. <Foo>{undefined}</Foo> does not have this effect (you end up with a single child equal to undefined.)
  • If you have an object like obj = {a:1, b:2} and you would like to use all the properties of the object as properties of a Component, you can write <Component {...obj}/>. The dots are always required; <Component {obj}/> is not allowed.

At the top of the file, the @jsx pragma can control the “factory” function that is called to translate JSX elements. For example if you use /** @jsx h */ then <b>this</b> translates to h('b', null, "this") instead of React.createElement('b', null, "this"). Some Preact apps use this pragma (h is the preact function to create elements), but you won’t need to use it in this tutorial (createElement is a synonym for h). Also, in “tsconfig.json” you can get the same effect with "jsxFactory": "h" in the compilerOptions.

See also

TypeScript evolution explains the newest TypeScript features in more detail. You might also like to see Advanced Types in TypeScript’s manual.

Before you go…

If you liked this article, don’t forget to clap or tweet! And if you’d like to learn React, continue on to my next article.