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Learn TypeScript fundamentals in 60-ish minutes - part(2)

This post is in continuation with the previous post. If you have directly landed here, I would suggest you read the previous post first.

Classes

TypeScript uses the object-oriented approach to handle the creation of objects and inheritance. Typescript classes are essentially compiled to the given JavaScript version which allows greater browser compatibility. ES6(ECMAScript-2015) also includes a similar approach.

    /**
        A Person class is declared.
        name:string is a memeber variable
        introduce() is a method
    **/
    class Person {
      name:string;
      constructor(name:string){
        this.name = name;
      }
      introduce(){
        console.log(`HI my name is ${name}`);
      }

    }

    /**
        create sam as an Object of type Person
    **/

    var sam:Person = new Person("sam");
    sam.introduce();  // Hi my name is sam

Inheritance and Access Modifiers

A class can extend to other classes and thus Inherit all the protected and public members of the class. private members are not inherited. Each member is public by default. Methods also follow a similar rule as the members.

Constructor can be marked as protected which means the class can only be instantiated and is accessible within the class declaration. However, it can be extended by other classes.

If a constructor is declared private, the corresponding class cannot be extended to and is only accessible within the class declaration.

The below example is self-explanatory.


    class Person {
      private name:string;
      protected age:number;
      constructor(name:string, age:number){
        this.name = name;
        this.age = age;
      }
      introduce(){
        console.log(`HI my name is ${this.name}`);
      }

    }
    
    /** Student inherits all public and protected members 
        from class Person 
    **/
    class Student extends Person {
         public grade:string;
         constructor(name:string , age:number, grade:string){
           /** super has to be the first statement in the 
               constructor of the deriving class 
            **/
           super(name,age); 
         }
         introduce(){
           /** call the method of the parent class **/  
           super.introduce();  

          /**ERROR: name is a private memeber of parent class **/   
          console.log(`HI my name is ${this.name}`); 

          /** age can be accessed here as it is a protected member */
          console.log(`My age is ${this.age}`); 

          console.log(`My grade is ${this.grade}`);
         }

    }
        /** declare x as person **/
        var x = new Person("x",12);
        x.introduce();

        /** declare a variable jack of the type Person 
            representing a Student  
        **/
        var jack:Person =  new Student("jack", 24, "A");
        student.introduce();

ReadOnly modifier and Parameter properties

Declaring a member as readonly cannot be modified except in constructor or at the place of declaration using parameter properties.


     /**declare using paramter properties */
    class ReadonlyType{
      readonly name:string = 'james';
    }

    /** use a constructor to initailize */
    class ReadonlyTypeWithConstructor{
      readonly name:string;
      constructor(name:string){
        this.name = name;
      }
    }

Accessors: Getters and Setters

TypeScript supports getters/setters as a way of intercepting access to a member of an object. Get and Set keywords are used to provide the same. TypeScript compilers lower the ECMAScript 5 does not support this feature.

   /**
    Declare a class Employee with a private member username.
    set getters and setters for the member.
    **/
    class Employee{
        private username:string;
        constructor(theUsername:string){
          this.username = theUsername;
        }
        get userName(){
          return this.username;
        }
        set userName(name:string){
          this.username = name;
        }
    }

    var jack = new Employee('Jack');
    /**setter called**/
    jack.userName = "emanuel";

    /** getter called **/
    var uName = jack.userName;
Static Properties

Static properties are available to class itself rather than on the instances. static keyword is used to declare a property as static.


class Printer{
  /** static method **/  
  static printDocument(path:string){
    console.log("document is printed");
  }
}

/** call the static method using the class name **/
Printer.printDocument("path://to/the/document");
Abstract class

Abstract keyword is used to declare a class as abstract. An abstract class can not be instantiated. It can be inherited by other classes. Methods declared as abstract only have the method declaration. However, non-abstract methods can also be present.


    abstract class Person{

       private name:string;
       constructor(name:string){
         this.name = name;
       }
      printName(){
        console.log(`hey yo.. I m ${this.name}`);
      }
      abstract printAge(age:number);
    }

    class realPerson extends Person{
      constructor(name:string){super(name);}
      printAge(age:number){
        console.log(`I am ${age} years old`);
      }
    }

    /** Cannot create an instance of an abstract class. */
    var p = new Person();

    var rp:Person = new realPerson("sparrow");
    rp.printAge(12);

 

 

Generics

Generics is a facility of generic programming to produce reusable software components.

Generic programming is a style of computer programming in which algorithms are written in terms of types to-be-specified-later that are then instantiated when needed for specific types provided as parameters– wikipedia

A generic type could be represented by any character, commonly represented by any Capital letter such as T.

Generic Functions

When used in a function, TypeScript compiler will enforce correct use of the generic types inside the function. A typical Generic function in TypeScript is written as follows:

    /** generic function to declare a variable of various types */
    function declareVariable<T>(parameter: T):T{
        return parameter;
    }
    /** declare type as number as number **/
    let x =  declareVariable<number>(12);

    /**ERROR:  Type '"a dummy sring"' is not assignable to type 'number'. **/
    x = "a dummy string "

    /** declare type as number as string **/
    let yx = declareVariable<string>("iota");
    

A generic type typically represents an identity(type) of the Type. It does not control the structure of the parameters.

The below example clears a little bit air about it. We have declared a function with generic type T. If we try to access the length property, the TypeScript compiler is going to complain about it.

    /** Declare a function with generic type T
        and try to access the length property 
    **/
    function declareVariable<T>(parameter: T):T{
        /** ERROR": Property 'length' does not exist on type 'T' */
        console.log(parameter.length) 
        return parameter;
    }

Let’s take a step back and think– if our generic function is dealing the types having a length property, then our declaration of parameters is wrong. Typically, indexable types such as array have a length property. It does not necessarily have to be an array. It could be any type with a length property.

Let’s change the parameter type of the function.

    /** Parameter includes an array of T type **/
    function processIndexableGenericTypes<T>(params:Array<T>):T{
        console.log(`length = ${params.length}`);
        return params[0]
    }

    var firstElement = processIndexableGenericTypes<number>([1,2,3,4]);

we could use a user-defined type as below

    /** lengthPropType has a property length:number */
    interface lengthPropType{
        length: number;
    }

    /** Parameter includes an array of T type **/
        function processIndexableGenericTypes<T>(params:lengthPropType):T{
            console.log(`length = ${params.length}`);
            return params[0]
        }

        /**
         * Since the array has a length property.
         * the below statement will still work with array parameter 
         */
        var firstElement = processIndexableGenericTypes<number>([1,2,3,4]);

Generic Function Types

Generic function types are created as a typical function type except the explicit type name is replaced with a generic type.

    /** represents a function with parameter of any given type */
    var genType : <T>(param:T)=>T;

    /** assign a function of similar type */
    genType =  declareVariable;
    declareVariable(1212);

    /** declare a type of a function with array parameters of a type T */
    var indexgenType : <T>(params: T[]) =>T;
    indexgenType = processIndexableGenericTypes;
    indexgenType([1,2,3]);
Generic Interface

The below example describes a generic interface

    /** declare an interface which specifies
        a function type 
    **/
    interface genFunctionType<T,U>{
        <T,U>(param1:T,param2:U): T;
    }

    function logOuput<T,U>(a: T, b:U):T{
        console.log(a,b);
        return a;
    }

    var logNumStringOutPut : genFunctionType<number, string> =  logOuput;
    logNumStringOutPut(12, "dashing");
Generic constraints

Normally, Generic types can allow any predefined and user-defined types. To have limits on what a generic type can represent, constraints come in picture. The extends keyword is used to constrain a generic type.

The following example will provide a better picture of constraint usage.

  • Let’s say we want a function to accept only those types of parameters which have a name property of type string.
       /** declare a type which has a name:string property **/
      interface nameType{
          value:string;
      }
      /**getName function excepts parameters which extend to nameType **/
      function getName<T extends nameType>(nameobj:T){
        console.log(nameobj.value);
      }
    
      getName({value:"Batis"}); //OUTPUT: Batis
    
  • How about making things a little complex– Let’s say we want to create a class Person. It can take any types as parameters which have name and age property. Also, it wants to make sure the name is of the type which has at least a value property of type string and age is of a type which has at least a value of the property of type number
    /** ageType has a property value:number */
    interface ageType {
        value: number;
    }
    /** nameType has a prperty value:age*/
    interface nameType {
        value: string;
    }

    /** declare an interface with T which has a property T and U.
       Both T and U extend to nameType and ageType respectively.
     */
    interface PersonType<T extends nameType, U extends ageType> {
        name: T;
        age: U;
    }

    /** Person class takes type P which extends 
        to PersonType Interface 
     **/
    class Person<T extends PersonType<{ value: string }, { value: number } >> {
        name: string;
        age: number;
        constructor(param: T) {
            this.name = param.name.value;
            this.age = param.age.value;
        }
    }

    let personOne = new Person({ name: { value: 'batis' }, age: { value: 31 } });
Class Types In Generics

It is essentially useful when creating factories to generate classes which extends from a certain type. A slight twist here is– the factory function will take constructor signature as the parameter type.

/**function type of class based generic factory**/ 
var genericFactoryType : <T extends Entity>(param: new ()=>T) =>T;

new ()=>T represents a constructor signature with no arguments and returning a generic type.

The implementation looks as below–

    class Entity{
        name:string;
    }

    class Student extends Entity{
        GPA:number;
    }
    class PrimeryGrader extends Student{
        GRADE: string;
    }

    class Professor extends Entity{
        Band: number;
    }

    class UniversityPropessor extends Professor{
        Department:string;
    }

    /** factory function takes a constructor type parameters
        of type E which extends to Entity
    **/
    function createEntity<E extends Entity>(e : new ()=>E):E{
        return new e();
    }

    var student = createEntity<Student>(PrimeryGrader);
    var professor = createEntity<Professor>(UniversityPropessor);

Type Compatibility

TypeScript uses structural typing for comparison of the types unlike nominal typing using by object-oriented languages.

Type X is compatible with type Y if Y has at least same members as X
    interface X{
        a:number;
        b:number;
    }
    interface Y{
        a:number;
    }
    let x:X;
    let y:Y;

    y = x; //OK
  
    /** 
        ERROR: Type 'Y' is not assignable to type 'X'.
        Property 'b' is missing in type 'Y'.
    **/
    x = y;

Function type X is compatible with function type Y if the Y has at most or less number of parameters( of the same type) than X and the return type of Y is either same or a subtype of return of X
    let X : (a:number, b:number) => number;
    let Y :(a:number ) => number;
    X = Y  //OK
    /**
     ERROR: Type '(a: number, b: number) => number' is not assignable 
     to type '(a: number) => number'.
     **/
    Y = X;

Optional Parameters hold an optional place in comparison also. If the types are not compatible, optional parameters can pitch in and satisfy the compatibility– given if they are of same types. Let’s take the above example and add an optional parameter b to type Y

let X : (a:number, b:number) => number;
// b is an optional parameter
let Y :(a:number , b?:number) => number; 

x=y  //OK
y=x  //ok

If we remove the parameter b from type, X and Y being compatible would still hold true.

Rest Parameters are considered an infinite series of optional parameters.

Overloaded Function each overload in the source type must be matched by a compatible signature on the target type.

Class Compatibility. Since constructor and static side of the class are not considered, It is similar to comparing interface types. However private and protected affect the compatibility. Target type should have equal numbers of private or protected members as the source.

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