Zoom Webinar: Learn and enhance your TypeScript skills | 28th March 2023 | 10 AM PST [10:30 PM IST] Register here
Education

Advanced TypeScript Cheat Sheet for Types, Interfaces, Control Flow, and More…

logo

DhiWise

February 28, 2023
image
Author
logo

DhiWise

{
February 28, 2023
}

Debugging and troubleshooting is the most annoying part of software development but TypeScript handles the inadequacies with grace. That’s why almost 60% of JavaScript developers use it and 22% of them wish to try it.

TypeScript which is often referred to as the superset of JavaScript has slowly gained the attention of many software organizations. After witnessing its benefits, software developers are incorporating the language into their tech stack.

This article will walk you through the Advanced TypeScript concepts and capabilities. So let’s dive in,

  • What is TypeScript?
  • Advantages of TypeScript
  • Advanced TypeScript cheat sheet
    — Types
    — Interfaces
    — Classes
    — Control Flow analysis
TypeScript

What is TypeScript?

TypeScript is the strongly typed programming language developed and maintained by Microsoft. In Oct 2012, TypeScript was first introduced by Microsoft after two years of internal development.

It is a strict syntactical superset of JavaScript and adds optional static typing to the language. It allows developers to specify the variable type, function parameter, returned values, and object properties. The language compiles into JavaScript and the compiled output is pretty neat and readable that can be used wherever you want. The syntax has few similarities with the C#.

Advantages of TypeScript

If you are a TypeScript user, you might have noticed that “once you start using it, you will stay with it.” Find out why it’s worth using TypeScript for programming:

  1. Higher code stability due to strongly typed variables.
  2. Strong tooling and community support.
  3. Easy to understand and thus best for the growing team of developers.
  4. It improves agility while refactoring and it’s better for the compiler to catch errors than wait until things fail during runtime.
  5. By leveraging the power of TypeScript and Dependency injection we can efficiently avoid bugs through efficient testing and debugging.
  6. With its auto-injection libraries, it makes the source code extremely maintainable and predictable.
  7. TypeScript is compiled down to JavaScript code that runs on all the browsers.
  8. With TypeScript, developers can write more clean, type-safe, and testable code with ease.

Advanced TypeScript cheat sheet

TypeScript is a simple language and allows developers to express types in terms of other types. However, while performing the basic tasks, having a profound understanding of how TypeScript works are critical for unlocking its advanced functionality.

As we know more about TypeScript, we can utilize this knowledge to write cleaner and testable code. In this section, we are combining all the basic concepts of TypeScript and its advanced features in a single cheatsheet, so here we go.

TypeScript Primitive+Advanced Types

Key points:
  • Think of Type as variables: the type can be created in the same way we create variables.
  • TypeScript has lots of global types that can be used to perform common tasks.
Common data types:
  • Built-in type primitives: boolean, string, number, undefined, null, any, unknown, never, void, bigint, symbol.
  • String: Represents string values like “Hello, world”.
  • Number: Represents all numbers including integers and floats. For example, 42 and 42.3 are both numbers.
  • Boolean: Represents boolean values true and false.
  • Array: Represents an array of numbers, strings, and so on. It can be represented as number[], string[].
  • any: The type is used when you don’t want any type checking.

Object Literal Type

In JavaScript, the fundamental way that we group and pass around data is through objects. In TypeScript, we represent those through object types.

Syntax:

Image
Object Literal Syntax

For Example:

Image

Tuple Type

A tuple is a special-cased array with known types at specific indexes.


Image

Union Type

A union type describes a value that can be one of several types. It allows us to use more than one data type for a variable or a functional parameter.


Image

Intersection Type

The type combines multiple types into one(A way to merge or extend types).

Syntax:


Image

For Example:


Image

Type Indexing

A way to extract and name from a subset of a type.


Image

Type From Value

Reuse the type from an existing JavaScript runtime value via the typeof operator.


Image

Type From Function Return

Reuse the return value from a function as a type.


Image

Mapped Types

Acts like a map statement for the type system, allowing an input type to change the structure of the new type.


Image

Conditional Types

It works as an “if statement” inside the type system. Created via generics, and then commonly used to reduce the number of options in a type union.


Image

Template Union Type

A template string can be used to combine and manipulate text inside the type system.


Image

Interfaces

Key Points:
  • Used for type checking whether the object has a specific structure(shape) or not.
  • Only contain the declaration of methods and fields but not implementations.
  • Almost everything in JavaScript is an object and the Interface is built to match its runtime behaviour.
  • Common built-in JS objects: Date, Error, Array, Map, Set, Regexp, Promise

Common syntax


Image
Interface Syntax

Generics

Declaring a type which can change in your interface.


Image
Generics in Interface

One can constrain what types are accepted into the generic parameter via the extends keyword.


Image
Set constraints on the type

Overloads

A callable interface can have multiple definitions for different sets of parameters.


Image
Interface Overloads

Extensions via merging interfaces

Interfaces are merged, so multiple declarations will add new fields to the type definition.


Image
Merging Interfaces

Type vs Interface

  • Interfaces can only describe object shapes.
  • Interfaces can be extended by declaring it multiple times
  • In performance-critical types interface comparison checks can be faster.

TypeScript Classes

Key Points:
  • TypeScript offers full support for the class keyword introduced in ES2015.
  • As with other JavaScript language features, TypeScript adds type annotations and other syntax to allow you to express relationships between classes and other types.
Creating a Class Instance
Image
Class Instance

Private keyword vs Private fields

Private keyword:

The prefix private is a type-only addition and has no effect at runtime. So, when a transpiler converts TypeScript code to JavaScript, the private keyword is removed.

Image
Private keyword

Private fields:

#private which is runtime private and has enforcement inside the JavaScript engine that is only accessible inside the class. The private fields remain private at runtime even after the TypeScript code gets converted to JavaScript by the transpiler.

Image
Private fields

this parameter in the classes

The value of this parameter inside the function depends on how the function is called. This parameter can be used to bind function or arrow function.

We can add ‘this’ parameter to the method definition to statically enforce that the method is called correctly.

Type and Value

A class can be used as both a type and a value.

Image
Class as a type and value

Here the first Bag is a Type and the second Bag is a Value, so be careful while using class.

The common syntax used in Classes


Image
Common Class Syntax

Generics

Used to declare the type that can be changed in the class method.


Image
Generics in Class

Parameter properties

A specific extension to classes automatically set an instance field to the input parameter.

Image
Parameter Properties in Class

Abstract classes

A class can be declared as not implementable, but as existing to be subclassed in the type system. As can members of the class. The class which extends the abstract class must define all the abstract methods. We cannot create an instance of an abstract class.


Image
Abstract Class

Static members

A static property and method are shared among all instances of a class. To declare a static property, you use the static keyword. To access a static property, you use the className.propertyName syntax.


Image
Static property and method

Static and non-static fields with the same name can exist without any error.


Image
Static and non-static fields with the same name

Control Flow Analysis

Control Flow Analysis is the core TypeScript feature that analyses your code to get the best type interface depending on the variable usage. CFA always takes a union and optimizes your code by reducing the number of types inside the union based on the logic in your code.

Though there are multiple ways to define functions that affect how TypeScript narrows types, most of the time it has been seen that CFA works inside the natural JavaScript boolean logic, but there are ways to define your own functions which affect how TypeScript narrows types.

If Statements:

Most narrowing comes from expression inside the if statements where different type operators narrow inside the new scope.

typeof operator (for primitives)


Image
typeof operator

instanceof operator (for classes)


Image
instanceof operator

“property” in the object (for objects)

Image
Property in the object

type-guard functions (for anything)

Image
type-guard functions

Expressions:

Narrowing also occurs on the same line as code, while performing the boolean operations.

Image
narrowing on the same line
Discriminated unions/algebraic data types:

Combining string literal types, union types, type guards, and type aliases to build an advanced pattern is called discriminated unions, also known as tagged unions or algebraic data types.

Discriminated Unions are the combination of three things:

  • The Discriminant - Types that have a common literal (or enum) property
  • The union - A type alias that takes the union of those types
  • Type guard

In the following example, all the members of the union have the same property name, however, CFA can discriminate on that.


Image
Discriminated unions

Assignment:

Narrowing types using ‘as const’

const assertion to narrow an object literal type to its element. The prefix ‘as const’ locks all types to their literal versions.

Image
Narrowing types using ‘as const’

Tracking through the related variables

Image
Tracking through the related variables

Reassigning updated types

Image
Reassigning updated types

Type Guards to perform runtime checks

A function with a return type describing the CFA change for a new scope when it is true.

Image
Type Guards

Assertion Function:

A set of functions that throws an error when something unexpected happens

Following is a function describing CFA changes affecting the current scope, because it throws instead of returning false.

Image
Assertion Function

If you find yourself having trouble with some of the concepts discussed above, try reading through the TypeScript Documentation first to make sure you’ve got a solid understanding of all the basic and advanced concepts.


Wrapping Up

Most JavaScript developers are already in the affair with TypeScript as it is more reliable and easy to refactor. The above cheat sheet gives you quick access to all the TypeScript concepts.

But why are we sharing all this with you?

At DhiWise we believe in empowering developers, no matter which technology or platform they are using.  The platform already supports JavaScript and now it goes versatile in app development with the support for TypeScript. 

Want to explore more about the amazing platform and its features?

Visit DhiWise today and sign up to experience the perks of cleaner, customizable, and faster app development.