Scala Method Notations

Infix Notation/Operator Notation
Method with one parameter can be accessed with space (Natural Language)

class Person {
  def sayHello(name: String) = println(s"Hello $name")
}
val person = new Person
person sayHello "Niranjan"

In scala we can also use +/-/*/#/$ as method names which makes it much more comfortable to code with unlike other languages. This is exactly what is happening in mathematical operators as they are also methods

    println(1 + 3)
    // ALL OPERATORS ARE METHODS in SCALA
    println(1.+(3))

PREFIX NOTATION or Unary Operator
A Unary Operator is also a METHOD in SCALA
All UNARY Operators are methods with prefix “unary_”

val x = -1
// Its same as
val y = 1.unary_-

POSTFIX Notation
Method with no parameter can be accessed without paranthesis

class Person {
  def sayHello = println("Hello")
}
val person = new Person
person.sayHello
// OR
person sayHello

Classes, Instances, Objects, Companion Object, Scala Application

Class in scala is used to wrap instance level functionality.

class Foo //class

Instance

val foo = new Foo // instance

Note: In scala, we do not call instance of a class as an Object because Object has a special meaning.

Object Scala Object is a Singleton Instance by definition.
SCALA DOES NOT HAVE CLASS LEVEL FUNCTIONALITY (like “static” in java) – Equivalet in scala is Object
Everytime an object is defined, this object will be of its own type (“type person”) and it itself is the only instance.
Objects can be defined in a similar way to that of the classes with the only difference being the constructor parameters.

object Person {
  val N_HANDS = 2 // static in java
}
val person1 = Person // refers to the only instance on Object
val person2 = Person
println(person1 == person2) // returns true

Companion Object
We can define class with the same name as that of object to seperate instance level functionality (Class) from static/class level functionality (Object), and when class and object exists with same name in one file, they are called COMPANION OBJECTS or COMPANIONS

object Person {
  val N_HANDS = 2 // static in java
}
class Person(name: String, val age: Int) {
  // where as name and age are instance specific and hence they are referred in class members
}

Note:
In Java we access static member via class name and instance member via object reference
But in Scala, we access class level memebers via Object and instance level members via the instance of a Class, It turns out to be more Object Oriented than JAVA eventhough its created for functional programming

Scala Application
When a scala object inherts scala.App trait it becomes scala application. The App trait can be used to quickly turn objects into executable programs.

object Main extends App {
  Console.println("Hello World: " + (args mkString ", "))
}

Call By Value vs Call By Name

Call By Value is just like any other programming language where we use the static value of the argument directly

     def calledByValue(x: Long): Unit = {
         println("by value: " + x)
         println("by value: " + x)
     }
     calledByValue(System.nanoTime())

Call By Name
Here instead of the value, the expression is passed as is and it will be evaluated by the compiler everytime. Call by Name is lazily evaluated.

     def calledByName(x: => Long): Unit = {
         // the arrow above is going to tell the compiler to evaluate the parameter by NAME
         // x is evaluated for every use
         println("by name: " + x)
         println("by name: " + x)
     }
     calledByName(System.nanoTime())

Lazy Evaluation example

def infinite(): Int = 1 + infinite()
def printFirst(x: Int, y: => Int) = println(x)

// println(infinite(), 34) // errors with stackoverflow

println(34, infinite()) // runs fine as second parameter is lazily evaluated and is never executed

Scala Type Hierarchy and Expressions

Type Hierarchy

Expressions
In imperative languages like Java/Python we execute instructions. For example IF conditional statement is an Instruction. Whereas in Scala, we execute expressions.

IF conditional statements are expressions that can return some value.

val aConditionValue = if (aCondition) 5 else 3
Code Blocks are also expressions that can return something
val someOtherValue = {
        if (someValue) 233 else 422
        42
    }

Code Blocks are commonly used expression blocks which can contain one or more expressions and the return type is the return value from the last expression.

val aCodeBlock = {
   if(true) 54
   56
}

Throwing exception from a method returns Nothing “()”
Any methods (println) that have side effects returns Unit
so on..

In functional programming languages writing loops are not encouraged. Whenever there is a need to write a loop we should go for recursion. Optimized way to write recursion is to use TAIL RECURSION technique.
To ensure we are using tail recursion we can use “@tailrec” before the function definition

Closures in Javascript

Wherever the function is declared and defined, it remembers the complete snapshot of the function, i.e, the scope chains used within the function.

var a = 10;
function outer() {
  var b = 20;
  var inner = function() {
    console.log(a);
    console.log(b);
  }
  return inner;
}
var innerFn = outer();
innerFn();
var innerFn1 = outer();
innerFn1();

When innerFn() is executed, it prints both ‘a’ and ‘b’. Printing ‘a’ is fine because its in global scope but printing ‘b’ looks interesting because var ‘b’ is in outer scope which is run and complete and hence no longer exists and there scope for var ‘b’ should not exists. But its working because JS has feature of CLOSURES.

In above program, global variables are created only once, whereas function variables are created as many times as the function is invoked.
So when innerFn() is invoked first time, it remembers snapshot of global var ‘a’ (created only once) and function var ‘b’ (created with outer function execution first time)
So when innerFn() is invoked next time, it remembers snapshot of global var ‘a’ (as before) and function var ‘b’ (created again with outer function execution second time)
To verify this make a small modification

var a = 10;
function outer() {
  var b = 20;
  var inner = function() {
    a++;
    b++;
    console.log(a);
    console.log(b);
  }
  return inner;
}
var innerFn = outer();
innerFn();
var innerFn1 = outer();
innerFn1();

Output:
11
21
12
21

Asynchronous callbacks as closures
As JS engine is single threaded, there is no concept of pause/wait in the program. To achieve this we use a global function called ‘setTimeout’ where it fires the given function asynchronously after the specified time.

var a = 10;
var fn = function() {
  console.log(a);
}
setTimeout(fn, 1000);
console.log("done");

Important thing to notice here is that, this is the best example of CLOSUREs, because the function we declared is trying to access global variable ‘a’ which is remembered by the function eventhough its executed inside another function ‘setTimeout’

The Module Pattern in JS

var person = {
  "firstName": "Niranjan",
  "lastName": "Tallapalli",
  "getFirstName": function() {
    return this.firstName;
  },
  "getLastName": function() {
    return this.lastName;
  }
}
person.getFirstName(); // would give me first name BUT
person.firstName; // would also let me access the first name because there is no concept of private in JS

There is no concept of private variables in JS, everything is by default public. To make variable private, we can do one of the following

  • We can make use of closures
  • We can make use of Module Pattern

The idea here is to make use of the concept of new function SCOPE so that the variables are not visible outside the scope.

function createPerson() {
  var firstName = "Niranjan",
  var lastName = "Tallapalli",
  var person = {
    "getFirstName": function() {
      return this.firstName;
    },
    "getLastName": function() {
      return this.lastName;
    }
  }
}
person.getFirstName(); // would give me first name as usual
person.firstName; // will not work

Closures in Async Callbacks

var i;
var print = function() {
  console.log(i);
}

for(i = 0; i < 10; i++) {
  //print();
  setTimeout(print, 1000);
}

‘print()’ will work fine and prints 0-9
setTimeout will print 10 ten times, its because by the time setTimeout runs 10 different instances of the function, the value of ‘i’ is changed to 10 and then exited the loop.

Now HOW do we solve this problem???

var i;
for(i = 0; i < 10; i++) {
  (function() { // IIFE
    var currentValueOfI = i;
    setTimeout(function() { // make an inline function
      console.log(currentValueOfI);
    }, 1000);
  })();
}

// OR
var i;
for(i = 0; i < 10; i++) {
  (function(currentValueOfI) { // IIFE
    setTimeout(function() { // make an inline function
      console.log(currentValueOfI);
    }, 1000);
  })(i);
}

Compilation and Interpretation

To most of us it looks like JavaScript is an interpretor based language because we do see the direct source code in most of the web pages but it is actually not. JavaScript runtime engine compiles the source code into intermediate binary code on the fly and then interprets it.

Compilation Phase
The minute we create a function it actually creates an object behind the scenes and a variable by name function name binding to this object.

 
var a = 10;
function myFn(x) {
  var b = 20;
  var c = x;
  console.log(a+b+c);
}
myFn(30);

** If you notice carefully there are 5 variables (3 explicitly declared and 1 implicit myFn, 1 implicit param)
During compilation
line #1 // ‘a’ is put in global scope
line #2 // implicit var ‘myFn’ is put in global scope
line #2 // ‘x’ is put in myFn scope
line #3 // ‘b’ is put in myFn scope
line #4 // ‘c’ is put in myFn scope
Note that compilation phase does not look at RHS, it only looks at the LHS, i.e, declarations “var a”. Scope chains are created at compilation step.

Execution Phase
This phase ignores the declaration part and only looks at “a = 10” instead of “var a = 10”. It also refers to the scope chains created in compilation scope to find which variable to use where.
line #1 // Interpretor assigns var the value 10, similarly other variables at line 3 and 4
line #5 // It tries to search for ‘console’ var declaration in myFn scope, since its not declared, interpretor goes one level up and searches again (.. so on till it finds var console) and then invokes the log on it.

Global Scope Problem: Lets look at how interpretor interprets this code

var a = 10; // looks at 'a' from global scope from compilation step and assigns value 10
function myFn() {
  var b = a; // search for 'a' in myFn scope, its not declared, goes one level up to Global Scope and find it and uses it
  console.log(d); // search for 'd' in myFn scope, its not declared, goes one level up to Global Scope and still does not finds it and errors out. Note that it errors out because its a READ operation
  c = 100; // search for 'c' in myFn scope, its not declared, goes one level up to Global Scope and still does not finds it, instead of erroring out it creates variable 'c' because we are writing to it (NOT READING). Interesting thing is that, as the interpretor went up the scope levels all the way to Global Scope, it creates the variable in Global Scope itself eventhough this is the line responsible for the creation of this variable. Compiler skipped to identify the scope if the variable because it does not have the declaration with 'var'
}

Some surprising JS behaviors

var a = 10;
function outer() {
  var b = a;
  console.log(b);
  function inner() {
    var b = 20;
    var c = b;
    console.log(c);
  }
  inner();
}
outer();

This will have 3 scopes at compilation step and interpretor goes and resolves all variables from the compilation at the time of interpretation and runs the program. Its all good, lets see the same program with some minor modifications.

var a = 10;
function outer() {
  var b = a;
  console.log(b);
  function inner() {
    var c = b;
    console.log(c);
    var b = 20;
  }
  inner();
}
outer();

This will print ‘c’ as ‘undefined’ but NOT ’10’ as a typical programmer thinks. WHY???
#1 – The order of ‘var’ really does not matter in JS because compilation happens prior to interpretation.
#2 – There are two steps involved in every JS program, first step is compilation and the nest is interpretation. The problem here is that, compilation only looks at vars and interpretation does not look at vars at all.

At Interpretation phase
line #: When it looks at var ‘b’, it asks the ‘inner’ scope for this variable and its not initialized by the interpretor YET, its still available in ‘inner’ scope because its declared with var. And hence we are trying access a variable which is declared but not yet initialized, it gives us ‘undefined’. This is also referred to the problem of HOISTING

Problem of Hoisting
All the var declarations in JS code are moved/hoisted to the top in each of the execution scopes no matter where they are declared. Because of this reason we get ‘undefined’ as seen above. This is also the problem with function expressions (NOT WITH FUNCTION DECLARATION)

fnA();
function fnA() {
  console.log('hello')
}
Would run fine, BUT
fnA();
var fnA = function() {
  console.log('hello')
}

would give an error because the function expression is defined after execution command is invoked by the interpretor

Using strict mode
Its introduced in ECMA5 and this will not allow JS to initialize variables without declaring them.

"use strict"; // this should be the first line
var myName = "tekmarathon";
myname = "test"; // this will throw runtime error

Note: Strict mode can be applied to a function as well.

Reference
https://www.youtube.com/watch?v=9T3AIM3JMss&list=PLqq-6Pq4lTTZ_LyvzfrndUOkIvOF4y-_c&index=16
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

Scopes and Closures

Like in other programming languages, variable scope in JavaScript is not created by blocks, instead its created from functions.
Ex:
var x = 1;
if(x === 1) {
var y = 'one';
}
console.log(x);
console.log(y); // prints 'one' as the scope is not block based

For the most part, scopes in JS are created via functions
var x = 1;
function assignValue() {
if(x === 1) {
var y = 'one';
}
}
console.log(x);
console.log(y); // will throw runtime error

Note *** variables ‘x’ from outer scope can be used inside function but not vice versa as in case of ‘y’

Global variables in JS are evil especially from browser perspective because we load everything at once and we dont know if our global variable names are conflicting with others from different libraries. For this reason we put our logic in a function expression and invoke that function but again the problem could be same if some 3rd party library has same global function. Hence we go for IIFE – Immediately Invoked Function Expression
(function () {
var a = 10;
var b = 20;
console.log(a+b);
})();
With IIFEs global namespace is not polluted.

Without declaring and defining a variable, if we try to use it we get and error.
By declaring and without defining a variable we get ‘undefined’

If we declare a variable, we can do both read and write operations fine, but if dont declare a variable then we can do write operation alone (and not read operation)
var x;
x = 10; // write operation
console.log(x); // read operation

console.log(y); // leads to runtime error as we cannot read before declaring
z = 10; // is fine as we can write without declaring a variable

The Window Object
Whenever we declare a variable outside a function then that variable becomes global. All the global variables and functions are wrapped inside a GLOBAL/ROOT object. This global object name depends on which runtime we are using for running JS.
If its the browser then the Global Object name is ‘window’.
If its node, then its called ‘global’

Scopes can be hierarchical: Variables declared inside a function (INNER VARS) can access variables declared outside the function (OUTER VARS)

Introduction to Functions

Functions cannot be overloaded in JS: Function defined with n arguments can be passed with n args where extra args will be ignored.

function foo() {
return "Hello World";
}
foo();

Functions can be created as

  1. Function Expression
  2. Anonymous Function Expression
  3. Functions as arguments
  4. Functions on objects

1. Function Expression
Functions as values (FIRST CLASS CITIZENS)
var myFunc = function foo() {
return "Hello World";
}
myFunc();

2. Anonymous Function Expression
In the below expression function name is not required as we call with assigned variable anyways which is aka AFE
var myFunc = function() {
return "Hello World";
}
myFunc();

3. Functions as Arguments
var myFunc = function() {
return "Hello World";
}
var executorFunc = function(fn) {
fn();
}
executorFunc(myFunc);

4. Functions on Objects
var myObj = {
"prop1": true
}
myObj.myFunc = function() {
console.log("Hello World");
}
console.log(myObj);
myObj.myFunc();

Default Function Arguments
‘arguments’: is an indexed object with list of arguments passed to method
‘this’: Understanding ‘this’ keyword in JS
var person = {
"firstName": "Niranjan",
"lastName": "Tallapalli",
"getFullName": function() {
console.log(person.firstName + ' ' + person.lastName);
}
}

person.getFullName(); // would work fine BUT
// imagine this scenario
var person2 = person; // object address is copied
person = {};
person2.getFullName(); // this would have person reference which is empty hence results in “undefined undefined”
// instead refer to the object on which getFullName is invoked which is done via “this” keyword

Note: *** Functions are objects behind the scenes in JS ***

Array functions
myArr.push(element); // will add element at the end of an array
myArr.pop(); // will remove last element from array
myArr.shift(); // removes element from beginning of array
myArr.unshift(element); // push element at the beginning of array
myArr.forEach(function(item, index, arr) {
console.log(item +' '+index);
})

Arrays and Wrapper Objects in JavaScript

Arrays
Every array in JS behind the scenes is an Object which means array element can be accessed with index as an integer and also as a string
myArr[0] is same as myArr["0"]

As arrays are objects, can we use DOT NOTATION here? No because the property name is a number which is invalid identifier

If array is an object then how does myArray[0] works? Its because JS does Type Coercion behind the scenes and converts number to string.

Since array indexes are just property names in an object, we can assign element to any index, say

myArr[100] = "xyz";
*** myArr.length -> gives 101 because its actually not the length of an array rather its equal to (max_index + 1)
var myArr = ["hello", "how", "are"];
console.log(myArr.length);
myArr[20] = "you";
console.log(myArr.length);

If you try to access a index which is not defined then you get value ‘undefined’ rather than ArrayIndexOutOfBoundsException like in Java
var myArr = [100, 200];
console.log(myArr[0] +';'+ myArr[1] +';'+ myArr[2]);

Note: Additional elements in array can be added dynamically

Wrapper Objects in JS
By default when we create a variables, they are created as primitives but when we use DOT NOTATION on primitive, JS converts primitive into its corresponding wrapper object and performs the DOT OPERATION and then discards the object.
var value = "Hello World";
value.length; // here value primitive is converted into wrapper string object and then performs length
typeof value; // shows "string" which is a primitive

Similar wrappers are available for Number, Boolean and also Symbol(in ES6)

Variables in Javascript

Javascript is type inferenced which mean based on the value it will identify the type and hence we dont have to explicitly specify the type.

There are 3 primitive data types and undefined, null

  • Number: These are double precision 64 bit format (there are no Integers, Short, Float so on)
  • String: Sequqnces of Unicode Characters (no char type)
  • Boolean: It is of type ‘Boolean’ with two possible values ‘true/false’
  • undefined: It is of type ‘undefined’ with two possible values ‘undefined’. Value assigned to every declared variable until its defined.
    • Ex var value; value = 10; //value between these two statements is value ‘undefined’ of type ‘undefined’
  • null: It is of type ‘null’ with two possible values ‘null’.
  • In ECMA6, new variable called Symbol is introduced just like ENUMs

Note: There is no scoping information attached to variable declarations and hence all declared variables are by default global.

Variables and Values can be interrogated using ‘typeof’

typeof <variable>
typeof <value>
var a;
console.log(typeof a);
a = 10;
console.log(typeof a);
a = "hello";
console.log(typeof a);
a = true;
console.log(typeof a);
a = null;
console.log(typeof a);

Null is a typeof Object
* a=null; typeof a – returns object instead of null, it was a bug in early versions of JS but its not fixed in newer versions bcos it breaks backward compatibility and many web applications will break.

Type Coercion: As JS was introduced to be friendly  language with developers and they did type conversion of variables used in the expression and this lead to many confusions and later they fixed it but because of backward compatibility we still live with them. One such issue is ‘==’

  • double equals == and triple equals ===
  • 12 + “4” -> results in “124” because it looks at expression and finds one is string and other is number and hence coerce number into string so that it can do string concatenation
    * JS does a lot of type coercion and hence the behavior is unpredictable lot of times, beware of it
var a = 10;
var b = "10";
a == b -> returns true, whereas
a === b -> returns false

Values of all types have associated boolean value

  • Non zero numbers can be passed to a if loop which returns true
  • Non empty strings can be passed to a if loop which returns true
  • undefined and null are always false
var a = 10;
if(a) {
console.log("a is true");
} else {
console.log("a is false");
}

a = 0;
if(a) {
console.log("a is true");
} else {
console.log("a is false");
}

a = "Hello";
if(a) {
console.log("a is true");
} else {
console.log("a is false");
}

a = "";
if(a) {
console.log("a is true");
} else {
console.log("a is false");
}

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