The purpose of this class is to introduce to the student:

• The importance of scope (global, functional and block)
• What hoisting is and the difference between compile time and run time
• The use and purpose of closures

FIRST HALF (12.00 - 13.30)

• synchronous vs. asynchronous
• callbacks
• eventloops
• map, filter, reduce

Note: You can ask students to explain a concept or summerise the last lecture themselves

Scopes define the visiblity of declarations of variables and functions.

The top level outside all your functions is called the global scope. Values defined in the global scope are accessible from everywhere in the code. Whereas, variables defined in local scope can only be accessed and altered inside the scope they were created.

• var and function declarations are visible with function scope.
• let and const declarations are visible with block scope. A block can be seen as a set of statements enclosed in curly brackets({}).

Global scope:

• Can be a real useful tool or a nightmare.
• Useful in scenarios where we want to export JS modules, use third party libraries like jQuery etc.
• Big risk of causing namespace clashes with multiple variables with same name being created in different places.

Local Scope:

• Think of local scope as any new scope that is created within the global scope.
• Each function written in JavaScript creates a new local scope.
• Variables defined within a function aren't available outside it. They are created when a function starts and are in a way destroyed/hidden when a function ends.

https://github.com/HackYourFuture/fundamentals/blob/master/fundamentals/scope.md

let villan = "Joker";                        // | global scope
                                             // |
function myFunction() {                      // |  | function scope
    let hero = "Batman";                     // |  |
    if (hero === "Batman") {                 // |  |  | block scope
        let coHero = "Robin";                // |  |  |
        console.log(hero);                   // |  |  |
        console.log(coHero);                 // |  |  |
        console.log(villan);                 // |  |  |
    }                                        // |  |  |
    console.log("------")                    // |  |
    console.log(hero);                       // |  |
    console.log(coHero);                     // |  |
    console.log(villan);                     // |  |
}                                            // |  |
                                             // |
myFunction();                                // |

And the same link as Explanation

What happens if we use the same variable name in different scopes?

function myFunction() {
    let hero = "Batman";
    if (true) {
        let hero = "The Flash";
        console.log(hero);
    }
    console.log(hero);
}

myFunction();

Made by Yash: https://github.com/yash-kapila/HYF-JS2-Week3/blob/master/src/scope.js

Same link as Explanation

Made by Yash: https://github.com/yash-kapila/HYF-JS2-Week3/blob/master/src/hoisting.js

Notes:

• Compile time is when the JavaScript is prepared to be executed in the browser
• Run time is when the JavaScript is actually executed, line by line

SECOND HALF (14.00 - 16.00)

Credits to Yash: A closure is when inner function remembers the environment in which it was created even after the outer function has returned.

One powerful use of closures is to use the outer function as a factory for creating functions that are somehow related.

In the code snippet underneath we can see that the carColor function has still got access to the outer function's properties like wheels, seats and brand even after the function manufactureCar has returned. We can then use the carColor as a factory to create multiple cards of the same type but with a different color.

function manufactureCar() {
  const wheels = 4;
  const seats = 5;
  const brand = 'Some Brand';

  return function carColor(color) {
    return {
      wheels,
      seats,
      brand,
      color,
    }
  };
}

const basicCar = manufactureCar();

const redCar = basicCar('red');
const blueCar = basicCar('blue');
const greenCar = basicCar('green');

{
  'use strict';

  const printName = () => {
    const message = 'My name is ';

    const displayName = name => {
      console.log(`${message}${name}`);
    }

    return displayName;
  };
  
  const name = printName();
  name('Yash Kapila');
}