Intersection Types

An intersection type combines multiple types into one. This allows you to add together existing types to get a single type that has all the features you need. For example, Person & Serializable & Loggable is a Person and Serializable and Loggable. That means an object of this type will have all members of all three types.

You will mostly see intersection types used for mixins and other concepts that don’t fit in the classic object-oriented mold. (There are a lot of these in JavaScript!) Here’s a simple example that shows how to create a mixin:

function extend<T, U>(first: T, second: U): T & U {
  let result = <T & U>{};
  for (let id in first) {
    (<any>result)[id] = (<any>first)[id];
  }
  for (let id in second) {
    if (!result.hasOwnProperty(id)) {
      (<any>result)[id] = (<any>second)[id];
    }
  }
  return result;
}

class Person {
  constructor(public name: string) { }
}
interface Loggable {
  log(): void;
}
class ConsoleLogger implements Loggable {
  log() {
    // ...
  }
}
var jim = extend(new Person("Jim"), new ConsoleLogger());
var n = jim.name;
jim.log();

Union Types

Union types are closely related to intersection types, but they are used very differently. Occasionally, you’ll run into a library that expects a parameter to be either a number or a string. For instance, take the following function:

/**
 * Takes a string and adds "padding" to the left.
 * If 'padding' is a string, then 'padding' is appended to the left side.
 * If 'padding' is a number, then that number of spaces is added to the left side.
 */
function padLeft(value: string, padding: any) {
  if (typeof padding === "number") {
    return Array(padding + 1).join(" ") + value;
  }
  if (typeof padding === "string") {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}

padLeft("Hello world", 4); // returns "  Hello world"

The problem with padLeft is that its padding parameter is typed as any. That means that we can call it with an argument that’s neither a number nor a string, but TypeScript will be okay with it.

let indentedString = padLeft("Hello world", true); // passes at compile time, fails at runtime.

In traditional object-oriented code, we might abstract over the two types by creating a hierarchy of types. While this is much more explicit, it’s also a little bit overkill. One of the nice things about the original version of padLeft was that we were able to just pass in primitives. That meant that usage was simple and concise. This new approach also wouldn’t help if we were just trying to use a function that already exists elsewhere.

Instead of any, we can use a union type for the padding parameter:

/**
 * Takes a string and adds "padding" to the left.
 * If 'padding' is a string, then 'padding' is appended to the left side.
 * If 'padding' is a number, then that number of spaces is added to the left side.
 */
function padLeft(value: string, padding: string | number) {
  // ...
}

let indentedString = padLeft("Hello world", true); // errors during compilation

A union type describes a value that can be one of several types. We use the vertical bar (|) to separate each type, so number | string | boolean is the type of a value that can be a number, a string, or a boolean.

If we have a value that has a union type, we can only access members that are common to all types in the union.

interface Bird {
  fly();
  layEggs();
}

interface Fish {
  swim();
  layEggs();
}

function getSmallPet(): Fish | Bird {
  // ...
}

let pet = getSmallPet();
pet.layEggs(); // okay
pet.swim();  // errors

Union types can be a bit tricky here, but it just takes a bit of intuition to get used to. If a value has the type A | B, we only know for certain that it has members that both A and B have. In this example, Bird has a member named fly. We can’t be sure whether a variable typed as Bird | Fish has a fly method. If the variable is really a Fish at runtime, then calling pet.fly() will fail.

Type Guards and Differentiating Types

Union types are useful for modeling situations when values can overlap in the types they can take on. What happens when we need to know specifically whether we have a Fish? A common idiom in JavaScript to differentiate between two possible values is to check for the presence of a member. As we mentioned, you can only access members that are guaranteed to be in all the constituents of a union type.

let pet = getSmallPet();

// Each of these property accesses will cause an error
if (pet.swim) {
  pet.swim();
}
else if (pet.fly) {
  pet.fly();
}

To get the same code working, we’ll need to use a type assertion:

let pet = getSmallPet();

if ((<Fish>pet).swim) {
  (<Fish>pet).swim();
}
else {
  (<Bird>pet).fly();
}

User-Defined Type Guards

Notice that we had to use type assertions several times. It would be much better if once we performed the check, we could know the type of pet within each branch.

It just so happens that TypeScript has something called a type guard. A type guard is some expression that performs a runtime check that guarantees the type in some scope. To define a type guard, we simply need to define a function whose return type is a type predicate:

function isFish(pet: Fish | Bird): pet is Fish {
  return (<Fish>pet).swim !== undefined;
}

pet is Fish is our type predicate in this example. A predicate takes the form parameterName is Type, where parameterName must be the name of a parameter from the current function signature.

Any time isFish is called with some variable, TypeScript will narrow that variable to that specific type if the original type is compatible.

// Both calls to 'swim' and 'fly' are now okay.

if (isFish(pet)) {
  pet.swim();
}
else {
  pet.fly();
}

Notice that TypeScript not only knows that pet is a Fish in the if branch; it also knows that in the else branch, you don’t have a Fish, so you must have a Bird.

typeof type guards

Let’s go back and write the code for the version of padLeft that uses union types. We could write it with type predicates as follows:

function isNumber(x: any): x is number {
  return typeof x === "number";
}

function isString(x: any): x is string {
  return typeof x === "string";
}

function padLeft(value: string, padding: string | number) {
  if (isNumber(padding)) {
    return Array(padding + 1).join(" ") + value;
  }
  if (isString(padding)) {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}

However, having to define a function to figure out if a type is a primitive is kind of a pain. Luckily, you don’t need to abstract typeof x === "number" into its own function because TypeScript will recognize it as a type guard on its own. That means we could just write these checks inline.

function padLeft(value: string, padding: string | number) {
  if (typeof padding === "number") {
    return Array(padding + 1).join(" ") + value;
  }
  if (typeof padding === "string") {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}

These typeof type guards are recognized in two different forms: typeof v === "typename" and typeof v !== "typename", where "typename" must be "number", "string", "boolean", or "symbol". While TypeScript won’t stop you from comparing to other strings, the language won’t recognize those expressions as type guards.

instanceof type guards

If you’ve read about typeof type guards and are familiar with the instanceof operator in JavaScript, you probably have some idea of what this section is about.

instanceof type guards are a way of narrowing types using their constructor function. For instance, let’s borrow our industrial string-padder example from earlier:

interface Padder {
  getPaddingString(): string
}

class SpaceRepeatingPadder implements Padder {
  constructor(private numSpaces: number) { }
  getPaddingString() {
    return Array(this.numSpaces + 1).join(" ");
  }
}

class StringPadder implements Padder {
  constructor(private value: string) { }
  getPaddingString() {
    return this.value;
  }
}

function getRandomPadder() {
  return Math.random() < 0.5 ?
    new SpaceRepeatingPadder(4) :
    new StringPadder("  ");
}

// Type is 'SpaceRepeatingPadder | StringPadder'
let padder: Padder = getRandomPadder();

if (padder instanceof SpaceRepeatingPadder) {
  padder; // type narrowed to 'SpaceRepeatingPadder'
}
if (padder instanceof StringPadder) {
  padder; // type narrowed to 'StringPadder'
}

The right side of the instanceof needs to be a constructor function, and TypeScript will narrow down to:

1. the type of the function’s prototype property if its type is not any
2. the union of types returned by that type’s construct signatures

in that order.

Type Aliases

Type aliases create a new name for a type. Type aliases are sometimes similar to interfaces, but can name primitives, unions, tuples, and any other types that you’d otherwise have to write by hand.

type Name = string;
type NameResolver = () => string;
type NameOrResolver = Name | NameResolver;
function getName(n: NameOrResolver): Name {
  if (typeof n === "string") {
    return n;
  }
  else {
    return n();
  }
}

Aliasing doesn’t actually create a new type - it creates a new name to refer to that type. Aliasing a primitive is not terribly useful, though it can be used as a form of documentation.

Just like interfaces, type aliases can also be generic - we can just add type parameters and use them on the right side of the alias declaration:

type Container<T> = { value: T };

We can also have a type alias refer to itself in a property:

type Tree<T> = {
  value: T;
  left: Tree<T>;
  right: Tree<T>;
}

Together with intersection types, we can make some pretty mind-bending types:

type LinkedList<T> = T & { next: LinkedList<T> };

interface Person {
  name: string;
}

var people: LinkedList<Person>;
var s = people.name;
var s = people.next.name;
var s = people.next.next.name;
var s = people.next.next.next.name;

However, it’s not possible for a type alias to appear anywhere else on the right side of the declaration:

type Yikes = Array<Yikes>; // error

Interfaces vs. Type Aliases

As we mentioned, type aliases can act sort of like interfaces; however, there are some subtle differences.

One difference is that interfaces create a new name that is used everywhere. Type aliases don’t create a new name — for instance, error messages won’t use the alias name. In the code below, hovering over interfaced in an editor will show that it returns an Interface, but will show that aliased returns object literal type.

type Alias = { num: number }
interface Interface {
  num: number;
}
declare function aliased(arg: Alias): Alias;
declare function interfaced(arg: Interface): Interface;

A second more important difference is that type aliases cannot be extended or implemented from (nor can they extend/implement other types). Because an ideal property of software is being open to extension, you should always use an interface over a type alias if possible.

On the other hand, if you can’t express some shape with an interface and you need to use a union or tuple type, type aliases are usually the way to go.

String Literal Types

String literal types allow you to specify the exact value a string must have. In practice string literal types combine nicely with union types, type guards, and type aliases. You can use these features together to get enum-like behavior with strings.

type Easing = "ease-in" | "ease-out" | "ease-in-out";
class UIElement {
  animate(dx: number, dy: number, easing: Easing) {
    if (easing === "ease-in") {
      // ...
    }
    else if (easing === "ease-out") {
    }
    else if (easing === "ease-in-out") {
    }
    else {
      // error! should not pass null or undefined.
    }
  }
}

let button = new UIElement();
button.animate(0, 0, "ease-in");
button.animate(0, 0, "uneasy"); // error: "uneasy" is not allowed here

You can pass any of the three allowed strings, but any other string will give the error

Argument of type '"uneasy"' is not assignable to parameter of type '"ease-in" | "ease-out" | "ease-in-out"'

String literal types can be used in the same way to distinguish overloads:

function createElement(tagName: "img"): HTMLImageElement;
function createElement(tagName: "input"): HTMLInputElement;
// ... more overloads ...
function createElement(tagName: string): Element {
  // ... code goes here ...
}

Discriminated Unions

You can combine string literal types, union types, type guards, and type aliases to build an advanced pattern called discriminated unions, also known as tagged unions or algebraic data types. Discriminated unions are useful in functional programming. Some languages automatically discriminate unions for you; TypeScript instead builds on JavaScript patterns as they exist today. There are four ingredients:

1. Types that have a common, string literal property — the discriminant.
2. A type alias that takes the union of those types — the union.
3. Type guards on the common property.

interface Square {
  kind: "square";
  size: number;
}
interface Rectangle {
  kind: "rectangle";
  width: number;
  height: number;
}
interface Circle {
  kind: "circle";
  radius: number;
}

First we declare the interfaces we will union. Each interface has a kind property with a different string literal type. The kind property is called the discriminant or tag. The other properties are specific to each interface. Notice that the interfaces are currently unrelated. Let’s put them into a union:

type Shape = Square | Rectangle | Circle;

Now let’s use the discriminated union:

function area(s: Shape) {
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
  }
}

Exhaustiveness checking

We would like the compiler to tell us when we don’t cover all variants of the discriminated union. For example, if we add Triangle to Shape, we need to update area as well:

type Shape = Square | Rectangle | Circle | Triangle;
function area(s: Shape) {
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
  }
  // should error here - we didn't handle case "triangle"
}

There are two ways to do this. The first is to turn on --strictNullChecks and specify a return type:

function area(s: Shape): number { // error: returns number | undefined
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
  }
}

Because the switch is no longer exhaustive, TypeScript is aware that the function could sometimes return undefined. If you have an explicit return type number, then you will get an error that the return type is actually number | undefined. However, this method is quite subtle and, besides, --strictNullChecks does not always work with old code.

The second method uses the never type that the compiler uses to check for exhaustiveness:

function assertNever(x: never): never {
  throw new Error("Unexpected object: " + x);
}
function area(s: Shape) {
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
    default: return assertNever(s); // error here if there are missing cases
  }
}

Here, assertNever checks that s is of type never — the type that’s left after all other cases have been removed. If you forget a case, then s will have a real type and you will get a type error. This method requires you to define an extra function, but it’s much more obvious when you forget it.

Polymorphic this types

A polymorphic this type represents a type that is the subtype of the containing class or interface. This is called F-bounded polymorphism. This makes hierarchical fluent interfaces much easier to express, for example. Take a simple calculator that returns this after each operation:

class BasicCalculator {
  public constructor(protected value: number = 0) { }
  public currentValue(): number {
    return this.value;
  }
  public add(operand: number): this {
    this.value += operand;
    return this;
  }
  public multiply(operand: number): this {
    this.value *= operand;
    return this;
  }
  // ... other operations go here ...
}

let v = new BasicCalculator(2)
      .multiply(5)
      .add(1)
      .currentValue();

Since the class uses this types, you can extend it and the new class can use the old methods with no changes.

class ScientificCalculator extends BasicCalculator {
  public constructor(value = 0) {
    super(value);
  }
  public sin() {
    this.value = Math.sin(this.value);
    return this;
  }
  // ... other operations go here ...
}

let v = new ScientificCalculator(2)
    .multiply(5)
    .sin()
    .add(1)
    .currentValue();

Without this types, ScientificCalculator would not have been able to extend BasicCalculator and keep the fluent interface. multiply would have returned BasicCalculator, which doesn’t have the sin method. However, with this types, multiply returns this, which is ScientificCalculator here.