# Session Plan

## Session Materials

These are some examples of previously created materials by mentors that you can use yourself, or for inspiration:

* [Notion Page Handout](https://dandy-birth-1b2.notion.site/HYF-Aarhus-JS-2-Week-3-6bce73b3a0bf47a3ad32ed12ee4d0519?pvs=4) (by [Thomas](https://github.com/te-online))
* [Demo](https://github.com/HackYourFuture-CPH/program/blob/main/courses/frontend/advanced-javascript/week2/session-materials/demo/README.md) – In-session code: **index.js** (worksheet with Task/Next comments to implement this session plan) and **index-solution.js**.
* [Event loop demo](https://github.com/HackYourFuture-CPH/program/blob/main/courses/frontend/advanced-javascript/week2/session-materials/event-loop-demo/README.md) – Draggable code blocks (sync, setTimeout) into Call stack, Timer, Async queue. Use it to explain how the event loop works under the hood.

## Session Outline

1. **Functions – passing a function as an argument**
   * Repeat everything about functions. Functions as a variable: pass as argument, call from inside another function.
   * This is super good at explaining function logic: [Function graphic](https://github.com/HackYourFuture-CPH/program/blob/main/courses/frontend/advanced-javascript/week2/session-materials/function-graphic.jpg).
   * [Demo](https://github.com/HackYourFuture-CPH/program/blob/main/courses/frontend/advanced-javascript/week2/session-materials/demo/README.md): implement `functionRunner`.
   * [Code inspiration](#calling-a-function-within-a-function).
2. **Callbacks – setTimeout and the event loop**
   * Explain how `setTimeout(callback, delay)` works, why the timer is never exact and why a 0 delay doesn't make it a sync operation.
   * [Event loop demo](https://github.com/HackYourFuture-CPH/program/blob/main/courses/frontend/advanced-javascript/week2/session-materials/event-loop-demo/README.md) can help with explaining how async operations work under the hood.
   * Solve [console-order.md](https://github.com/HackYourFuture-CPH/program/blob/main/courses/frontend/advanced-javascript/week2/session-materials/console-order.md) together with the trainees.
   * [Code inspiration](#callback-functions).
3. **(Optional) Infinite loop – sync vs async**
   * Implementation of an infinite loop with both sync and async operations and demonstrating whether it freezes the page or not. Helps to realise how async operations work under the hood.
4. **Callbacks – events (addEventListener)**
   * Another example of callbacks. Explain `addEventListener`.
   * [Code inspiration](#event-listeners), [Anonymous vs named](#anonymous-vs-named-function).

Also hammer in the point of the difference between:

```js
document.querySelector("button").addEventListener("click", logOuttext);
document.querySelector("button").addEventListener("click", logOuttext());
```

Here's a good [practical example of callbacks](https://github.com/HackYourFuture-CPH/JavaScript/blob/class08/JavaScript2/Week5/classwork/extra_examples.md) (TODO: move this to this repo).

## Code inspiration

### Calling a function within a function

```js
// Here we create a function that as a parameter takes a function!! Super weird right!?
// Functions works just like any other type in js.
function functionRunner(functionToRun) {
  console.log(typeof functionToRun);
  // Here we are calling the function that is provided as an argument when calling functionRunner
  functionToRun();
}

functionRunner(function () {
  console.log("hello");
});

// We don't see anything, why??
functionRunner(Math.random);

// Let's rewrite functionRunner to log out the return of a function
function functionRunnerImproved(functionToRun) {
  console.log(typeof functionToRun);
  // Here we are calling the function that is provided as an argument when calling functionRunner
  const capturedReturnValue = functionToRun();
  console.log(capturedReturnValue);
}

functionRunnerImproved(Math.random);
```

### Callback functions

```js
/*
Events
Events in javascript are thing like:
A timer has just finished, a user clicked a button, our page has loaded,
someone types into an input element or we have just gotten some data from a server.
When these events happen, we usually want to add some functionality.
e.g. when a user clicks the like button (event), we want to increment the like counter and color the like button blue.
Or when someone clicks "Close cookies" (event) we want to remove the cookie div.
Let's first try to create some js that waits 2 seconds and the console.logs out "2 seconds has elapsed!"
In javascript we use the word eventlistener to listen
*/

// Tried to find actual webkit implementation, but failed. To show that the setTimeout implementation is just calling the provided function after a given time
setTimeout(function () {
  console.log("2 seconds has elapsed!");
}, 2000);

// Cool, now let's make a function as a variable:
const fourSecondLog = function () {
  console.log("4 seconds has elapsed!");
};

setTimeout(fourSecondLog, 4000);
```

### Event Listeners

```javascript
// Now let's keep track of how many times a button is clicked.
// To do something whenever a button gets clicked, we use what is called an event listener.
// Imagine that the button is noisy. There's someone listening out for the click sound,
// and every time they hear it, they add 1 to a counter.

const buttonElement = document.querySelector("button");
let counter = 0;

buttonElement.addEventListener("click", function () {
  counter = counter + 1;
  console.log(`Button clicked ${counter} times so far`);
});

// or the same thing but assigning the event listener to a const:

const buttonElement = document.querySelector("button");
let counter = 0;

const buttonClicked = function () {
  counter = counter + 1;
  console.log(`Button clicked ${counter} times so far`);
};

buttonElement.addEventListener("click", buttonClicked);

//Callbacks
// Now let's learn about callbacks!
// Well actually you have already made callbacks!
// When you give a function to an event listener or a timer or when fetching data you are using a callback function

// Let's create a callback function when someone writes in a input element
const callback = function () {
  console.log("Someone is writing!!");
};

document.querySelector("input").addEventListener("input", callback);
```

### Anonymous vs named function

```js
// Named function
function myFunction() {
  console.log("myFunction");
}

// Anonymous function, assigned to a variable
const myFunctionAsVar = function () {
  console.log("myFunctionAsVar");
};

document.body.addEventListener("click", myFunctionAsVar);
document.body.addEventListener("click", myFunction);
```

## Exercises

See [Exercises](/course-content/frontend/advanced-javascript/week2/exercises.md). Trainees show results on the page (update the DOM), not in the console.


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