Unit 2 - Scope and Functions.md

JS is the only language that supports 3 paradigms

• Functional
• object-oriented
• async

What is scope

• When and where variables, constants, and arguments are considered to be defined/visible/available
• Internally, when their memory is allocated, used, and released
  • Memory release is through a process called garbage collection

function f(x) {
  return x + 3;
}
f(5); // 8 
console.log(x); // x is not defined

Four types of scoping

• Global Scope – ES5

  • Objects declared (with var, let, and const) outside of any function are global
  • Visible everywhere in a JavaScript program (or an HTML document in Browser)
  • use it without declaring – not allowed under ‘strict’ mode

• Function Scope (ES5)

  • Objects declared inside a function (yes, anywhere!) are visible only to code that appears inside that function (and its embedded functions)
  • Keyword ‘var’

• Block Scope

  • Objects declared inside a block are visible only to code that appears inside that block (and its embedded block)
  • Keywords ‘let’ and ‘const’

• Module Scope

  • When module system is used, no global variables. They become module variables
  • Later in Module System

var is a function scope, and let is block scope

cost o = {name: 'john', age = 20} o = {name: 'john1', age = 22} can't change because the "house is kept"

strict mode

• use variable before declaring
• using an object without declaring
• refered to w3cschool

Deep and shallow copy

Deep copy: value is the same, but two memory blocks, simple type always deep copy

Shallow copy, only copy the refereence

It's just that when a primitive is passed you get a copy of the primitive and when an object is passed you get a copy of the memory location of the object.

Exercise

function f() {
    var a = 10;
    if (a > 5) {
        a = 7;
    }
    console.log(a);
}

// When executed, what value will be logged?

Ans: 7
Explain: Inside the if condition, 10 > 5, so a has been re-initialized to 7.

function f() {
    if(true) {
        var a = 5;
    }
    console.log(a);
}
f();

Ans: 5
Explain: the above program will always go inside the if condition, where a was initialized. a is in a function scope defined by using var, so it still can be accessed inside the function

function f() {
	a = 3;
}
f();
console.log(a);

4.
var a = 5;
function first() {
    a = 6;
}


function second() {
    console.log(a);
}
first();
second();