JavaScript - Chapter 7 - Advanced Functions

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Function
JavaScript


Invocation patterns

Recursion

Generator function

Arrow function

Variadic functions
Table of Content

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Invocation patterns
(JavaScript)

Function Invocation Patterns
●
The code in a function is not executed when the function is defined
●
Function is executed when it is invoked
●
Functions can be invoked in 4 different ways
– Invoking a function as a “function”
– Invoking a function as a “method”
– Invoking a function with a “Constructor function”
– Invoking a function with a “apply and call”

Invocation pattern
(invoking as function)
<script>
var obj;
function square(a) {
return a * a;
}
document.write(“Square of 3 is ” + square(3));
</script>

Invocation pattern
(invoking as method)
●
When a function is part of an object, it is called a method
●
Method invocation is the pattern of invoking a function
that is part of an object
●
JavaScript will set the this parameter to the object where
the method was invoked on
●
JavaScript binds this at execution (also known as late
binding)

Invocation pattern
(invoking as method)
<script>
var obj = { firstName: "Smith", lastName: "Doe",
fullName: function() {
return this.firstName + " " + this.lastName;
}
};
// In the example above, this would be set to obj
document.write("Full name is " + obj.fullName());
</script>

Invocation pattern
(Constructor invocation)
●
The constructor invocation pattern involves putting the new
operator just before the function is invoked
●
If the function returns a primitive type (number, string, boolean, null
or undefined)
– The return will be ignored and instead this will be returned
(which is set to the new object)
●
If the function returns an instance of Object
– The object will be returned instead of returning this

Invocation pattern
(Constructor invocation - 1)
<script>
var Fullname = function(firstname, lastname) {
// create a property fullname
return this.fullname = firstname + ‘ ’ + lastname;
};
var obj = new Fullname(“Tenali”, “Raman”);
document.write("Full name is " + obj.fullname);
</script>

Invocation pattern
(Constructor invocation - 2)
<script>
var Fullname = function(firstname, lastname) {
// return object
return { fullname : firstname + ‘ ’ + lastname }
};
var obj = new Fullname(“Tenali”, “Raman”);
document.write("Full name is " + obj.fullname);
</script>

Invocation pattern
(with apply() and call() methods)
●
JavaScript functions are objects and have properties and
methods
●
The call() and apply() are predefined method, invoke the
function indirectly
●
The call() method uses its own argument list as arguments to
the function
●
The apply() method expects an array of values to be used as
arguments

Invocation pattern
(by call() method)
<script>
var obj;
function square(a) {
return a * a;
}
document.write("Square of 3 is " + square.call(obj, 3));
</script>

Invocation pattern
(by apply() method)
<script>
var obj, arrSum;
function sum(x, y) {
return x + y;
}
arrSum = [5, 4];
document.write("Sum = " + sum.apply(obj, arrSum));
</script>

Function Expression
Syntax:
var func = function (param-1, param-2, . . . , param-n) {
statement(s);
}
●
Variable can be used to invoke the function
●
Above function is an anonymous function (a function
without a name)

Function Hoisting
●
JavaScript moves variable and function declarations to top of
the current scope; this is called hoisting
●
Due to hoisting JavaScript functions can be called before they
are declared
var s = sum (x, y);
function sum (x, y) {
return x + y;
}
function sum (x, y) {
return x + y;
}
var s = sum (x, y);

Function Hoisting
●
Function expressions are not hoisted onto the beginning of the
scope, therefore they cannot be used before they appear in the
code

Self Invoking Function
●
Function expressions can be used to self-invoke function
(start automatically without being called)
●
This is done by using parenthesis () -- also known as
function invocation operator
Syntax:
( function_expression ) ( );

Self Invoking Function
Example:
( function () {
document.write(“I am self invoking function”);
} ) ( );
●
Such expressions also known as IIFE (Immediately
Invokable Function Expression

JavaScript Scopes
(what?)
●
Scope determines the accessibility (or visibility) of variables,
objects, and functions from different parts of the code at
runtime
●
JavaScript has two types of scope
– Local scope
– Global scope
●
Please note that, in JavaScript, objects and functions are
also variables

JavaScript Scopes
(why?)
●
Scope provides security to data that, in principle, shall be
accessed only by intended part of the code
●
If data is exposed to all parts of program then it can be
modified anytime without your notice which will lead to
unexpected behaviour and results
●
Scope also allow use to use same names in different
functions

JavaScript Scopes
(Local Scope)
●
Variables defined within a function are local to the function
●
Such variables have local scope and can’t be accessed
outside the function
●
Since local variables are only recognized inside their
functions, variables with the same name can be used in
different functions
●
Local variables are created when a function is invoked
(started), and deleted when the function exits (ended)

JavaScript Scopes
(Local Scope)
<script>
function square(num) {
var result = num * num; // variable with local scope
}
Square(3); // invoke the function
document.write(“Square of number = ” + result); // Exception
</script>

JavaScript Scopes
(Global Scope)
●
Variables defined outside a function have global scope
●
Such variables are visible to all the functions within a
document, hence, can be shared across functions

JavaScript Scopes
(Global Scope)
<script>
var result; // variable with global scope
result = num * num;
}
square(3); // invoke the function
document.write(“Square of number = ” + result);
</script>

JavaScript Scopes
(Function and block scopes)
●
Further, in JavaScript, there are two kinds of local scope
– Function scope
– Block scope
●
A block of code is created with curly braces { }
●
Conditional statements (if, switch) and loops (for, while,
do-while) do not create new scope

JavaScript Scopes
(How variables are created?)
●
JavaScript processes all variable declarations before
executing any code, whether the declaration is inside a
conditional block or other construct
●
JavaScript first looks for all variable declarations in given
scope and creates the variables with an initial value of
undefined
●
If a variable is declared with a value, then it still initially has
the value undefined and takes on the declared value only
when the line that contains the declaration is executed

JavaScript Scopes
(How variables are created?)
●
Once JavaScript has found all the variables, it executes
the code
●
If a variable is implicitly declared inside a function -
– Variable has not been declared with keyword “var”
– And, appears on the left side of an assignment
expression
is created as a global variable

JavaScript Scopes
(Global Scope)
<script>
result = num * num; // Automatically global variable
}
document.write(“Square of number = ” + result);
</script>

JavaScript Scopes
(Global Scope)
●
Global variables are not automatically created in "Strict Mode"
<script>
“use strict”;
result = num * num;
}
document.write(“Square of number = ” + result); // Exception
</script>

JavaScript Scopes
(Global Scope)
●
Global variables belong to window object
<script>
result = num * num;
}
document.write(“Square of number = ” + window.result);
</script>

JavaScript Scopes
(Function scope variable)
●
Variables declared using keyword “var” within a function
has function scope

JavaScript Scopes
(Function scope variable)
<script>
function func() {
var x = 10; // function scope variable
{
var x = 20; // function scope variable
document.write("<br>x = " + x); // shall print 20
}
}
</script>

JavaScript Scopes
(Block scope variable)
●
ECMA script 6 has introduced keywords “let” and “const”
●
Variables declared using these keywords will have block
level scope
●
For these variables, the braces {. . .} define a new scope

JavaScript Scopes
(Block scope variable - let)
<script>
function func() {
let x = 10;
{
let x = 20;
}
}
</script>

JavaScript Scopes
(Block scope variable - const)
<script>
function func() {
const name = “Webstack Academy”;
{
const name = “Hello world!”;
document.write("<br>name = " + name);
}
document.write("<br>name = " + name);
}
</script>

JavaScript Scopes
(Life time of variables)
Variable Keyword Scope Life time
Local Var Function ●
Created when function is invoked
●
Deleted when function exits
Let, const Function or
block
●
Created when function is invoked
●
Deleted when function exits
Global var, let,
const
Browser
Window
●
Created when web page is loaded in the
browser window (tab)
●
Deleted when browser window (tab) is closed
var, let,
const
Block ●
Created when block is entered
●
Deleted when block is exited
●
Life time of a variable is the time duration between it’s creation
and deletion

JavaScript Scopes
(important notes)
●
Do not create global variables unless needed
●
Your global variables or functions can overwrite window
variables or functions
●
Opposite is also possible; any function (including window
object) can overwrite your global variables and functions

Function Closures
●
In JavaScript, an inner (nested) function stores
references to the local variables that are present in the
same scope as the function itself, even after the function
returns
●
The inner function has access to the outer function's
variables; this behavior is called lexical scoping
●
However, the outer function does not have access to the
inner function's variables

Function Closures
●
A closure is an inner function that has access to the outer
function’s variables – scope chain
●
The closure has three scope chains:
– It has access to its own block scope (variables defined
between its curly brackets)
– It has access to the outer function’s variables
– It has access to the global variables

Function Closures
(Example)
<script>
function disp() { // Parent function
var name = "Webstack Academy"; // name is a local variable
displayName() { // displayName() is the inner function, a closure
alert (name); // The inner function uses variable of parent function
}
displayName(); // child function call
}
disp(); // parent function call
</script>

JavaScript Scopes
(Lexical Scope)

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Recursion
(JavaScript)

Recursion
●
Recursion is the process in which a function is called by
itself
●
Recursion is a technique for iterating over an operation
by having a function call itself repeatedly until it arrives at
a result

Recursion
(Example)
<script>
var factorial = function(n) {
if (n <= 0) {
return 1;
} else {
return (n * factorial(n - 1));
}
};
document.write("factorial value"+factorial(5));
</script>

Exercise
●
Write a JavaScript function to find sum of digits of a number
●
Write a JavaScript program to compute x raise to the power y
using recursion

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