Functions - Parameters, Arguements, understanding return keyword.

Author: sanmazum

03_basics/01_functions.js

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+function sayMyName () {
+    console.log("R");
+    console.log("O");
+    console.log("N");
+    console.log("Y");
+}
+
+sayMyName()    // exceution of function
+sayMyName  // reference of function
+
+/* When you write the function definition and you take any inputs in that those
+are called Function Parameters
+
+Here in addTwoNumbers -> number1 and number 2 are the parameters of the function.
+
+*/
+
+function addTwoNumbers(number1, number2) {
+    console.log( number1 + number2 );
+}
+
+addTwoNumbers(3, 4)  // o/p-> 7. Notice that , 3 and 4 are the arguements of the function name addTwoNumbers
+
+const result = addTwoNumbers(3,4)
+console.log("Result :" , result ); // o/p -> Result : undefined
+
+/*
+ Notice that you are trying to hold the output of addTwoNum in variable - result. 
+ But when you run the code, you get that Result has stored undefined. 
+
+ Why? - Because addTwoNum() is just printing the addition and not returning the result. 
+ That's why undefined is printed.
+*/
+
+function addThreeNumbers(number1, number2, number3) {
+   const result2 =  number1 + number2 + number3 
+   return result2
+   console.log(" Print me if you can? ") // This will never print as there is a rule in function that after return statement, code is undetectable.
+}
+
+const result2 = addThreeNumbers(5,10,10)
+console.log("Result of addition of Three Numbers:" , result2);  // Result of addition of Three Numbers: 25
+/*
+Now you can get the value stored in the variable result 2 as the
+ function addThreeNumbers is now returning the value as result2
+*/
+
+
+
+ /*
+ If you are thinking that only returning from a variable will give the result, 
+ it's not, you can directly return and get the function result stored in the variable. */ 
+
+ function addFourNumbers(number1, number2, number3, number4) {
+    return number1 + number2 + number3 + number4
+ }
+
+ const res3 = addFourNumbers(10,10,20,20)
+ console.log("Result of four no. addition :", res3); // o/p-> Result of four no. addition : 60
+
+
+
+ function loginUserMessage(username){
+    return `${username} just logged in`
+ }
+
+ console.log(loginUserMessage("Sandeepan")) // o/p-> Sandeepan just logged in
+
+ // if you don't pass any arguement to the loginUserName function, the output is undefined
+
+ console.log(loginUserMessage())  // o/p-> undefined just logged in, that's why we need validation.
+
+
+// Consider the below example of usecase of validation :-
+
+ function loginUserMessage2(username){
+    //another way of putting validation :-
+    // if(!username){
+    //     console.log("provide a username");
+    //     return 
+    // }
+    if(username === undefined){
+        console.log("provide a username");
+        return     /* This return will execute when no username is provided and terminate the code there only
+         and will print the o/p-> provide a username
+        */ 
+    }
+    return `${username} just logged in`
+ }
+
+ console.log(loginUserMessage2()) // o/p-> undefined
+ 
+
+ /* One more thing, we can by default also assign a username to the function.
+
+ For understanding it better, consider the function loginUserMessage3 in the below.
+ 
+ */
+
+ function loginUserMessage3(username = "Sam"){
+    //another way of putting validation :-
+    // if(!username){
+    //     console.log("provide a username");
+    //     return 
+    // }
+    if(username === undefined){
+        console.log("provide a username");
+        return     /* This return will execute when no username is provided and terminate the code there only
+         and will print the o/p-> provide a username
+        */ 
+    }
+    return `${username} just logged in`
+ }
+
+ console.log(loginUserMessage3()) // o/p-> Sam just logged in ( because the if username will never be undefined as Sam is by default parameter)
+
+ console.log(loginUserMessage3("Virat kohli")) // o/p-> Virat kohli just logged in ( The parameters are easily overriden.)