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JavaScript FUNctions

Objectives:

  • Understand how JavaScript affects HTML and CSS
  • Understand how JavaScript variable, function, methods work together
  • Use JavaScript to add data to a map
  • Add user interactivity with JavaScript functions

You can get the latest assignment by running in your assignments repository: 

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git pull upstream main

Returning home to the HTML/CSS/JS analogy

Recall from last week’s reading that a webpage is like a house: - HTML is the scaffolding of the house - CSS is the paint, carpets, etc. that makes the house look nice - JavaScript is the appliances that adds function to the house

Today we will be focusing on the appliances.

Picking up from last week

We will start this lab off with this Leaflet template code:

index.html 

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<!DOCTYPE html>
<html>
    <head>
        <title>Basic Leaflet Map</title>
        <meta charset="utf-8" />
        <link rel="shortcut icon" href="#">

        <style> #map{height:90vh}</style>

        <!-- Leaflet's css-->
        <link rel="stylesheet" href="https://unpkg.com/leaflet@1.7.1/dist/leaflet.css" />

        <!-- Leaflet's JavaScript-->
        <script src="https://unpkg.com/leaflet@1.7.1/dist/leaflet.js"></script>
    </head>

    <body>
        <div id="map"></div>
    </body>
    <script src="js/init.js"></script>
</html>

js/init.js 

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const map = L.map('map').setView([34.0709, -118.444], 5);

L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

// adding markers
let work = L.marker([34.0709, -118.444]).addTo(map)
        .bindPopup('Where I work on campus')

let home = L.marker([37.7409, -122.484]).addTo(map)
        .bindPopup('Where I currently am')

let random = L.marker([39.7409, -122.484]).addTo(map)
        .bindPopup('Third Point')

What is L.map and L.tile?

L.map is Leaflet’s lingo for its own mapping Application Programming Interface (API). Every API has its own unique language to utilize it. To learn more about Leaflet’s API visit here: https://leafletjs.com/reference-1.7.1.html

Some Variable Definitions

Last week I talked about let and const and var, but what we really need to understand about variables is that they act like boxes where you can store or take information out of. - const acts like a locked safe that will not let you put anything into it after you define it - let is like a regular box. - var is VARy problematic because it can be both locked and unlocked

Here are some of the types in JavaScript: 

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//number
let box1 = 5;
let box2 = 5.0;

//string
let box3 = 'five';
let box4 = "five";

// string literal, uses backticks and ${variable} to bring in another variable
let box5 = `this is from box #4: ${box4}`;

// array
let box6 = [1,2,3,4,5]; 

// object, stores variables together, can be of different types!
let box7 = {"number": 'five', "value":5};

// boolean (true or false)
let box8 = true;

// null value
let emptyBox;

To declare a variable or give it a value you use the = symbol, like so: 

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let my_variable = "exist!";

Notes: - let is the type of variable - my_variable is the variable’s name - "exist!" is the value for this variable - ; defines the end of a line in JavaScript

Remember: - Use let to define variables you want to change, - Use const to define unchangable variables

Let’s practice using variables in our init.js file.

js/init.js 

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// original code
const map = L.map('map').setView([34.0709, -118.444], 5);

L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

// adding markers
let work = L.marker([34.0709, -118.444]).addTo(map)
        .bindPopup('Where I work on campus')

let home = L.marker([37.7409, -122.484]).addTo(map)
        .bindPopup('Where I currently am')

let random = L.marker([39.7409, -122.484]).addTo(map)
        .bindPopup('Third Point')

Class Exercise #1

Replace the hard coded values of const map = L.map('map').setView([34.0709, -118.444], 5); with variables.

Bonus: Try to use an object.

Answer 
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// method 1
let zoomLevel = 5;
const mapCenter = [34.0709,-118.444];

const map = L.map('map').setView(mapCenter, zoomLevel);

// Bonus:
// let myMap = {'center': [34.0709,-118.444],'zoom':5}
// const map = L.map('map').setView(myMap.center, myMap.zoom);

Note: You cannot use spaces in variable definitions like let my map;, so stick with camelCase.

Think about the benefits of having the variables sitting outside like that, is it easier to read for you? Harder?

Checking our Dev Console

In VS Code, start Live Server.

After Firefox runs, open the Console: - You can either right click anywhere on the page with the mouse and clicking on Inspect or press F12 on the keyboard.

Think of the Console as the Command Line/Terminal for your browser.

  • In the console, type zoomLevel then press Enter.
  • What gets outputted?

Knowing how to check the console will help us test our functions.

Time for FUNctions

Programmers are often programming because they have to get something done, but a true programmer likes to automate (as well as copy and paste).

Look at our init.js file after the line //adding markers:

js/init.js 

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// original code
const map = L.map('map').setView([34.0709, -118.444], 5);

L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

// adding markers
let work = L.marker([34.0709, -118.444]).addTo(map)
        .bindPopup('Where I work on campus')

let home = L.marker([37.7409, -122.484]).addTo(map)
        .bindPopup('Where I currently am')

let random = L.marker([39.7409, -122.484]).addTo(map)
        .bindPopup('Third Point')

We can automate the marker creation by creating a function like this:

js/init.js

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function addMarker(lat,lng,message){
    console.log(message)
    L.marker([lat,lng]).addTo(map).bindPopup(message)
    return message
}

Notes: - function is the declaration of our function - addMarker is the name. - lat,lng,message is the parameter, which are passed in to a function to be utilized. Parameters are optional, but parentheses () are not!! - { is the begining of the function. - Notice how the function accesses lat,lng in the L.marker and message in the bindPopUp. - return tells the function to return a variable, it is also optional - } is the end of our function.

Note: Multiple parameters are seperated by a comma, (lat,lng,message) is 3 parameters.

The console.log in the body will tell us if the function is working.

Go ahead and check the console!

WHAT?! Nothing has changed!

Using Functions

In order for a function to run, it needs to be “plugged-in”. This is called “invoking” or “calling” the function. When a function has no parameters, you can call it like so: 

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    function_name()

But since our function does have parameters (namely the lat,lng,and message), you must specify them.

Add this to the end of our init.js file:

js/init.js 

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    addMaker(37,-122,'you are awesome! you automated a marker function')
Important: The order of the parameters (lat,lng,message) is the SAME order that the function reads them!! Try switching 37 and -122 to see what I mean.

Now your console should return the “message” AND you should see a new marker on the map!

Inside function blocks you can create variables, change HTML, and do all sorts of things like play videos and even create games.

Class Exercise #2 - Using the marker function

Create your own marker function that does the following: - Utilizes at least four parameters - Declare a new variable inside the function - Returns a value

Use your function to create 3 markers with it.

Answer 
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    // create function
    function addMarker(lat,lng,title,message){
        console.log(message)
        L.marker([lat,lng]).addTo(map).bindPopup(`<h2>${title}</h2>`)
        return message
    }

    // use the function
    addMarker(37,-122,'home','home land!')
    addMarker(32,-118,'work','where i work land!')
    addMarker(39,-119,'location 1','random location')
    addMarker(36,-120,'location 2','another random location')

If you finished early, try these extra challenges: - Try to style your pop-up with 2 attributes!

Bonus Exercise - Create your own function Create your own function that does the following: - Utilizes at least `two parameters` - Declare a `new variable` inside the function - `Returns` a value
Bonus Answer 
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    // create function
    function addNumbers(value1,value2){
        let result = value1 + value2
        return result
    }

    // use the function
    addNumbers(1,10)   // result: 11

Sidenote: String Literals

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let popup = `${zoomLevel} + ${zoomLevel}`
Declaring a string with ` instead of ' ' or " ", allows you to convert variables to strings. For example, the zoom level normally would be treated as a number, but when we brought it in with the ${} combination it became a string so it could not be summed.

This technique will be helpful for our pop-ups.

Functions and the DOM

The HTM-Elements: Avatag the last Airbender

When you see tags in HTML, like <body></body>, they are referred to as elements, so for example: 

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<water>Katara</water> 
<air>Aang</air> 
<earth>Toph</earth> 
<fire>Zuko</fire>
Above we have four elements. Each element has a content, for example, the earth element’s content is Toph. Unfortunately, despite how exciting those elements are, the most common HTML element is the <div></div> element, which is a generic container.

The DOM is basically where HTML elements exists and it has an API that JavaScript can interact with with functions.

Objective: Make a button that we can click on to fly to a location for each of the markers you made.

Steps:

  1. Add a new function to our addMarker function
  2. Create the function to add buttons
  3. Add a function to toggle the zoom

To create HTML elements with JavaScript you need to use the createElement method.

First, we will get our buttons ready by going to the addMarker function and adding a new function call for the function we haven’t created yet.

js/init.js 

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// Step 1 adding to our addMarker function
function addMarker(lat,lng,title,message){
    console.log(message)
    L.marker([lat,lng]).addTo(map).bindPopup(`<h2>${title}</h2>`)
    createButtons(lat,lng,title); // new line!!!
    return message
}
Next we will add our new function. Notice how we are using the lat,lng,and title from the addMarker function? That’s why it was helpful to do step one first. js/init.js 
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// Step 2 adding our new function
function createButtons(lat,lng,title){
    const newButton = document.createElement("button"); // adds a new button
    newButton.id = "button"+title; // gives the button a unique id
    newButton.innerHTML = title; // gives the button a title
    newButton.setAttribute("lat",lat); // sets the latitude 
    newButton.setAttribute("lng",lng); // sets the longitude 
    newButton.addEventListener('click', function(){
        map.flyTo([lat,lng]); //this is the flyTo from Leaflet
    })
    document.body.appendChild(newButton); //this adds the button to our page.
}
Try clicking the button on the webpage and it should fly to the location of that marker!

Congratulations on finishing the JavaScript FUNctions Lab!

Final Code

js/init.js 

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// declare variables
let zoomLevel = 5;
const mapCenter = [34.0709,-118.444];

// use the variables
const map = L.map('map').setView(mapCenter, zoomLevel);

L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

// create a function to add markers
function addMarker(lat,lng,title,message){
    console.log(message)
    L.marker([lat,lng]).addTo(map).bindPopup(`<h2>${title}</h2>`)
    createButtons(lat,lng,title); // new line!!!
    return message
}

// create a function to add buttons with a fly to command
function createButtons(lat,lng,title){
    const newButton = document.createElement("button"); // adds a new button
    newButton.id = "button"+title; // gives the button a unique id
    newButton.innerHTML = title; // gives the button a title
    newButton.setAttribute("lat",lat); // sets the latitude 
    newButton.setAttribute("lng",lng); // sets the longitude 

    // attach an event listner to the button with Leaflet's map.flyTo
    newButton.addEventListener('click', function(){
        map.flyTo([lat,lng]); 
    })
    document.body.appendChild(newButton); //this adds the button to our page.
}

// use our marker functions
addMarker(37,-122,'home','home land!')
addMarker(32,-118,'work','where i work land!')
addMarker(39,-119,'location 1','random location')
addMarker(36,-120,'location 2','another random location')

index.html 

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<!DOCTYPE html>
<html>
    <head>
        <title>Basic Leaflet Map</title>
        <meta charset="utf-8" />
        <link rel="shortcut icon" href="#">

        <style> #map{height:90vh}</style>

        <!-- Leaflet's css-->
        <link rel="stylesheet" href="https://unpkg.com/leaflet@1.7.1/dist/leaflet.css" />

        <!-- Leaflet's JavaScript-->
        <script src="https://unpkg.com/leaflet@1.7.1/dist/leaflet.js"></script>
    </head>

    <body>
        <div id="map"></div>
    </body>
    <script src="js/init.js"></script>
</html>

Lab Assignment #3 - JavaScript FUNctions

Due 4/22

In this lab, we learned how functions are helpful for automating tasks. Functions also form the basis of the programming we will be doing. Your assignment this week is to create a map that will pan to certain makers when a button is clicked.

The requirements are:

Extra Credit:

  • Use something else like images or text to move the map.
  • Try something new with the Leaflet API

Submission

Last update: 2022-01-24
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