How to create scope?

We create scope by creating function.

Example 1:

var a = 42; //In global scope, could also just write as a = 42, omitting the var keyword, as we are in global scope
var b = 57; //In global scope

function example() {
  var a = 10; //Attached to function
  var b = 8; //Attached to function 
  console.log(a); -> 10 
}
example(); -> 10 
console.log(a); -> 42

Example 2:

function example() {
  var a = 20;  
  function inner() {
    a = 40; // Mutating the example() a, as we did't use var (Explanation below) 
    console.log("inner " + a); -> 40 
  }
  inner();
  console.log("outer " + a); -> 40 //Value is 40 as it was mutated by inner(); 
}
example();

If you're in a function thenvarwill create a local variable, "no var" will look up the scope chain until it finds the variable or hits the global scope (at which point it will create it):

Example 3:

var a = 10;
function example() {
  var a = 20;  
  function inner() {
    var a = 30; //Bound to inner()
    console.log("inner" + a); -> 30 
  }
  inner();
  console.log("outer" + a); -> 20 
}

example();
console.log(a); -> 10

Example 4:

var a = 30; 
function example(a) {
  a = 20; 
  console.log(a); -> 20 
}
example(a); 
console.log(a) -> 30

For parameters that are passed in function, you can imagine that they are declared with the 'var' keyword.

Example 5:

Normally a compiled error in other languages such as Java..etc, but not Javascript.

function example() {
  for (var i=1-; i< 10; i++) {
    if(i > 5) {
      var test = i * 10; 
    }
  }
  console.log(i + test); -> 10 and 90 
}

Block statement does not create scope. Loops, switch, if, empty bracket {}..etc. It does not attach variable to the closest block, but to the closest function.