Typescript: Beginner to Advanced

Beginner to Advanced
TYPESCRIPT
Swamiprasad Amin
Prabhu Yadav
Shashank Korthiwada
Sachin Sahu

What is Typescript?
 A strict syntactical superset of JavaScript, which adds type-safety to the JS
code.
 Free and open-source programming language developed and maintained by
Microsoft.
 It doesn’t change the runtime behaviour of JS.
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Why Typescript?
 It has the potential to move some kinds of errors from runtime to compile time.
Examples include
- Potentially absent values
- Incomplete Refactoring
 It serves as the foundation for a great code authoring experience.
Note:
• Only activate during development
• Doesn't provide any performance optimization
• In some other languages the type system can be used to optimize some code
that we write using the compiler.
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Typescript Installation
 Node.js should be installed
 npm install Typescript
 To compile a Typescript file
 npx tsc <filename>
 It catches type errors in the Typescript file and creates a JavaScript file
with the same name.
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Type Systems
Static vs dynamic
• whether type-checking is performed at compile time or runtime.
Nominal vs structural
• Nominal type systems are all about NAMES, two types are deemed to be the
same if they have the same name
• Structural type systems are all about STRUCTURE or SHAPE
TypeScript’s type system is static.
TypeScript's type system is structural
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Type Annotations
• Typescript uses type annotation to specify the data type of the variable, function,
object, or function return value.
• It uses the syntax :[type], where type is the Typescript type.
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Primitive types
The Primitives: String, Number, Boolean
• string represents string values like "Hello, world"
• number is for numbers like 42. JavaScript does not have a special runtime value
for integers, so there’s no equivalent to int or float - everything is simply number
• boolean is for the two values true and false
• It has some special types too unknown, any, void & never.
Literal Types
In addition to the general types string and number, we can refer to specific strings and
numbers in type positions
Enums
Allows for describing a value which could be one of a set of possible named constants.
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Union Types
 TypeScript’s type system allows us to build new types out of existing ones using a
large variety of operators.
 We can achieve different use cases by combining types in interesting ways.
 Syntax:
We define union types by using a pipe (|) to separate each of the possible types
(type1 | type2 | type3 | .. | typeN)
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Type Aliases and Interfaces
 TypeScript provides two mechanisms for centrally defining types and giving them
useful and meaningful names
• Interfaces
• type aliases
 Type aliases allows us to
• define a more meaningful name for this type
• declare the particulars of the type in a single place
• import and export this type from modules, the same as if it were an exported
value
 An interface is a way of defining an object type.
• Like type aliases, interfaces can be imported/exported between modules
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Objects
 Anything that is not a primitive data type is a Typescript object.
 It is a collection of key-value pairs.
 Each key-value pair is known as a property, where the key is the name of the
property and value its value.
 These provide a way to group several values into a single value.
 Few ways on how types are associated for Objects
• Object Literal Syntax
• Named Types using Type Aliases and Interfaces
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Arrays
 Syntax for typing an array is Type[] or Array<Type>.
 Here, Type can be combination of types.
e.g. const list: (string | number)[] = [10, "twenty"]
Tuple:
 Sometimes we may want to work with a multi-element, ordered data structure, where
position of each item has some special meaning or convention.
e.g. type StringNumberPair = [string, number]
 It can have optional properties and they come at the end.
 A tuple with a rest element has no set “length” - it only has a set of well-known elements in
different positions.
 Tuples tend to be created and left un-modified in most code.
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Functions
 The syntax (param: string) => void means “a function with one parameter,
named param, of type string, that doesn’t have a return value”.
 Optional parameters can be typed with ?.
 void is not the same as undefined.
 When writing a function type for a callback, marking a parameter optional should be
avoided unless it is intended to call the function without passing that argument.
 A function, that can be called in different ways, can be typed by writing overload
signatures.
 If this is not inferred from code-flow analysis, it can be typed.
 unknown is the type-safe counterpart of any.
 The type annotation on rest parameters is implicitly any[].
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Classes
 Fields of class can be typed and/or initialized.
e.g. class Point { x: number, y = 0 }
 Class constructors are very similar to functions. But, it doesn't have a return type
annotation.
 super must be called before accessing this in the constructor of a derived
class.
 implements clause is to check that a class satisfies a particular interface.
 Derived classes need to follow their base class contracts. But it may choose to
expose a subtype of base class with more capabilities.
 Soft-private allows access using bracket notation.
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Generics
A kind of tool that enables you to create reusable code components that
work with a number of types instead of a single type.
<Type> parameters are for relating the types of multiple values.
Type parameters can be constrained with extends keyword.
Benefits:
 Defining a relationship between input and output parameters types.
e.g. Utility Functions
 Stronger type checks at compile time will be available.
 You can remove some unnecessary type casts.
 You may leave type variable unassigned.
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<T>

Utility Types
Globally available types to facilitate common type
transformations.
Benefits:
 These types are themselves strongly typed and has specific
use cases.
e.g. Parameters<T>, here T extends (...args: any) => any
 Saves time of creating custom types for day-to-day type
manipulations.
 Saves you from having to think about naming your utilities.
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Pick x Omit
Readonly
ReturnType
NonNullable
Required x Partial
Extract x Extract
Parameters
Record

Type Manipulation
 Conditional types let us deterministically define type transformations depending on a
condition.
 A ternary conditional operator applied at the type level rather than at the value level.
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 Mapped types are useful when there is a need for a type to be derived from (and
remain in sync with) another type.
 Helps us keep our types definitions that are dependent on each other code in DRY
state.
Conditional Types
Mapped Types

Decorators
 A Decorator is a special kind of declaration that can be attached to a class
declaration, method, accessor, property or parameter.
 Decorators provide a way to add both annotations and a meta-programming
syntax for class declarations and members.
 Decorators are a stage 2 proposal for JavaScript and are available as an
experimental feature of TypeScript.
 To enable experimental support for decorators, you must enable the
experimentalDecorators compiler option either on the command line or in
your tsconfig.json
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Typescript: Beginner to Advanced

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