TypeScript - ReadOnly. TypeScript includes the readonly keyword that makes a property as read-only in the class, type or interface. Prefix readonly is used to make a property as read-only. Read-only members can be accessed outside the class, but their value cannot be changed. In TypeScript, abstraction can be achieved by using the abstract keyword - which can be applied to both classes and methods specified in classes. Let's go ahead and create an abstract class with an abstract method as well. Define an abstract class in Typescript using the abstract keyword. Abstract classes are mainly for inheritance where other classes may derive from them. We cannot create an instance of an abstract class. An abstract class typically includes one or more abstract methods or property declarations.
- Typescript Abstract Property Default Value
- Typescript Class Example
- Typescript Optional Abstract Property
- Typescript Abstract Getter
TypeScript is the one of the tools people want to learn most, according to a Stack Overflow Survey of 90,000 developers.
TypeScript has exploded in popularity, community size, and adoption over the past few years. Today, even Facebook's Jest project from Facebook is moving to TypeScript.
TypeScript is a statically-typed superset of JavaScript which aims to ease the development of large javascript applications. It is also knows as JavaScript that scales.
Why use TypeScript?
JavaScript has evolved a lot over the past few years. It is the most versatile cross-platform language used for both client and server side.
But JavaScript was never meant for such large-scale application development. It is a dynamic language with no type system, meaning that a variable can have any type of value, such as a string or boolean.
Type systems increase code quality, readability and make it easier to maintain and refactor codebase. More importantly, errors can be caught at compile time rather than at run time.
Without a type system, it's difficult to scale JavaScript to build complex applications with large teams working on the same code.
TypeScript provides guarantees between different parts of the code on compile time. A compiler error typically tells you exactly where something went wrong and what exactly went wrong whereas a runtime error is accompanied by a stack trace that may be misleading and results on significant amount of time spent on debug work.
TypeScript pros
- Catch potential errors earlier in the development cycle.
- Manage large codebases .
- Easier refactoring.
- Make it easier to work in teams — When contracts in the code are stronger it is easier for different developers to move in and out of the codebase without unintentionally breaking things.
- Documentation — Types inform some sort of documentation that your future self and that other developers can follow.
TypeScript cons
- It’s something additional to learn — It’s a tradeoff between short-term slowdown and long-term improvement in efficiency and maintenance.
- Type errors can be inconsistent.
- Configuration drastically changes its behaviour.
Boolean
Number
String
Array
Array types can be written in one of the two following ways:
Tuple
Tuple types allow you to express an organised array where the type of a fixed number of elements is known. This means that you will get an error
Any
any
is compatible with any and all types in the type system, meaning that anything can be assigned to it and it can be assigned to anything. It gives you the power to opt-out of type checking.Void
void
is the absence of having any type at all. It is commonly used as the return type of a function that do not return a value.Never
The
never
type represents the type of values that never occur. For instance, never
is the return type of a function which will always throw an exception or not reach its end point.Null and Undefined
Both
undefined
and null
actually have their own types named undefined
and null
, respectively. Much like void
, they’re not extremely useful on their own but they become useful when used within union types (more on that in a bit)Unknown
TypeScript 3.0 introduces the unknown type which is the type-safe counterpart of
any
. Anything is assignable to unknown
, but unknown
isn’t assignable to anything but itself and any.
No operations are permitted on an unknown
without first asserting or narrowing to a more specific type.Type Alias
Type alias provides names for type annotations allowing you to use it in several places. They are created using the following syntax:
Union Type
TypeScript allows us to use more than one data type for a property. This is called union type.
Intersection Type
Intersection types are types that combine properties of all of the member types.
Interfaces are like a contract between you and the compiler in which you specify in a single named annotation exactly what properties to expect with its respective type annotations.
Side-note: Interfaces have zero runtime JS impact, it is used solely for type checking.
Side-note: Interfaces have zero runtime JS impact, it is used solely for type checking.
- You may declare optionalproperties marking those with an
?
, meaning that objects of the interface may or may not define these properties. - You may declare read onlyproperties, meaning that once a property is assigned a value, it cannot be changed.
Extending Interfaces
Interfaces can extend one or more interfaces. This makes writing interfaces flexible and reusable.
Implementing an Interface
A class implementing an interface needs to strictly conform to the structure of the interface.
An
enum
(or enumeration) is a way to organise a collection of related values that can be numeric or string values.Under the hood, enums are number-based by default.
enum
values start from zero and increment by 1 for each member.The JavaScript code generated by our previous example:
Alternatively enums can be initialised with string values which is a more readable approach.
Reverse Mapping
Waves plugins mac crack.
Going back to our CardSuit example:
enum
supports reverse mapping which means we can access the value of a member and also a member name from its value.Going back to our CardSuit example:
You can add types to each of the parameters and then to the function itself to add a return type.
Function Overloads
TypeScript allows you to declare function overloads. Basically, you can have multiple functions with the same name but different parameter types and return type. Consider the following example:
The meaning of each parameter changes based on how many parameters are passed into the function. Moreover, this function only expects one, two or four parameters. To create a function overload, you just declare the function header multiple times. The last function header is the one that is actually active within the function body but is not available to the outside world.
You can add types to properties and method’s arguments
![Property Property](https://image.slidesharecdn.com/introductiontypescript-181110092010/95/introduction-to-typescript-40-638.jpg?cb=1541841756)
Access Modifiers
Typescript supports
public
,private
,protected
modifiers, which determine the accessibility of a class member.- A
public
member works the same as plain JavaScript members and is the default modifier. - A
private
member cannot be accessed from outside of its containing class. - A
protected
member differ from a private as it can also be accessed within deriving classes.
Readonly modifier
A
readonly
property must be initialised at their declaration or in the constructor.Parameter properties
Parameter properties lets you create and initialise a member in one place. They are declared by prefixing a constructor parameter with a modifier.
Abstract
The abstract keyword can be used both for classes and for abstract class methods.
- Abstract classes cannot be directly instantiated. They are mainly for inheritance where the class which extends the abstract class must define all the abstract methods.
- Abstract members do not contain an implementation, thus cannot be directly accessed. These members must be implemented in child classes (kinda like an interface)
TypeScript allows you to override its inferred types in any way you want to. This is used when you have a better understanding of a variable type than the compiler on its own.
Originally the syntax for type assertion was <type> Final fantasy x ost download.
But this created an ambiguity when used in JSX. Therefore it is recommended to use
as
instead.Type assertion are usually used when migrating code from JavaScript and you may know a more accurate type of the variable than what is currently assigned. But assertion can be considered harmful.
Let’s take a look at our Person interface from the previous example. Did you notice something wrong? If you noticed the missing property age, congratulations! The compiler might help you providing autocomplete for properties of Person but it will not complain if you miss any properties.
TypeScript infers types of variables when there is no explicit information available in the form of type annotations.
Type compatibility is based on structural typing, which relates types based solely on their members.
The basic rule for structural type is that
x
is compatible with y
if y
has at least the same members as x
.As
y
has a member name: string
, it matched the required properties for the Person interface, meaning that x
is a subtype of y
. Thus, the assignment is allowed.Functions
Number of arguments
In a function call you need to pass in at least enough arguments, meaning that extra arguments will not cause any errors.
In a function call you need to pass in at least enough arguments, meaning that extra arguments will not cause any errors.
Return type
The return type must contain at least enough data.
The return type must contain at least enough data.
Type Guards allow you to narrow down the type of an object within a conditional block.
Typescript Abstract Property Default Value
typeof
Using typeof in a conditional block, the compiler will know the type of a variable to be different. In the following example TypeScript understand that outside the conditional block,
x
might be a boolean and the function toFixed
cannot be called on it.Typescript Class Example
instanceof
in
The
in
operator checks for the existence of a property on an object.Literals are exact values that are JavaScript primitives. They can be combined in a type union to create useful abstractions.
Conditional Types
A conditional type describes a type relationship test and selects one of two possible types, depending on the outcome of that test.
This means that if type
A
is assignable to type B
, then X
is the same type as C
. Otherwise X
is the same as type D;
Generic type is a type that must include or reference another type in order to be complete. It enforce meaningful constraints between various variables.
In the following example a function returns an array of whatever type you pass in.
In the following example a function returns an array of whatever type you pass in.
keyof
The
keyof
operator queries the set of keys for a given type.Mapped Types
Mapped Types allow you to create new types from existing ones by mapping over property types. Each property of the existing type is transformed according to a rule that you specify.
Partial
- The generic Partial type is defined with a single type parameter
T
. keyof T
represents the union of all property names ofT
as string literal types.[P in keyof T]?: T[P]
denotes that the type of each propertyP
of typeT
should be optional and transformed toT[P]
.T[P]
represents the type of the propertyP
of the typeT
.
Readonly
As we have covered in the Interface section, TypeScript allows you to create readonly properties. There is a
Readonly
type that takes a type T
and sets all of its properties as readonly.Exclude
Exclude
allows you to remove certain types from another type. Exclude
from T
anything that is assignable to T
.Pick
Pick
allows you to pick certain types from another type. Pick
from T
anything that is assignable to T
.infer
You can use the
infer
keyword to infer a type variable within the extends
clause of a conditional type. Such inferred type variable can only be used in the true branch of the conditional type.ReturnType
Gets the return type of a function.
Typescript Optional Abstract Property
Let’s break down
MyReturnType
:- The return type of
T
is … - First of all, is
T
a function? - If so, then the type resolves to the inferred return type
R
. - Otherwise the type resolves to
any
.
Typescript Abstract Getter
In order to study and give TypeScript a try I’ve build a simple CurrencyConverter app using TS and React-Native with hooks. You can check this project here.
Thanks and congratulations for reading up to this point! If you have any thoughts on this, feel free to leave a comment.
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