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JavaScript jQuery Basics (2014) Creating a Simple Drawing Application Using toggle()

why do you need "+"

when assigning the valu of the rgb whith the value of the var "r" "g" and "b", why do you need to write rgb("+r+","+g+","+b+") and not just ("r","g","b")?

js/app.js
//Problem: No user interaction causes no change to application
//Solution: When user interacts cause changes appropriately
var color = $(".selected").css("background-color");

//When clicking on control list items
$(".controls li").click(function(){
  //Deselect sibling elements
  $(this).siblings().removeClass("selected");
  //Select clicked element
  $(this).addClass("selected");
  //cache current color
  color = $(this).css("background-color");
});

//When new color is pressed
$("#revealColorSelect").click(function(){
  //Show color select or hide the color select
  changeColor();
  $("#colorSelect");
});

//update the new color span
function changeColor() {
  var r = $("#red").val();
  var g = $("#green").val();
  var b = $("#blue").val();
  $("#newColor").css("background-color", "rgb(" + r + "," + g +", " + b + ")");
}

//When color sliders change
$("input[type=range]").change(changeColor);

//When add color is pressed
  //Append the color to the controls ul
  //Select the new color

//On mouse events on the canvas
  //Draw lines
index.html
<!DOCTYPE html>
<html>
<head>
    <title>Simple Drawing Application</title>
    <link rel="stylesheet" href="css/style.css" type="text/css" media="screen" title="no title" charset="utf-8">
</head>
<body>
    <canvas width="600" height="400"></canvas>
    <div class="controls">
        <ul>
            <li class="red selected"></li>
            <li class="blue"></li>
            <li class="yellow"></li>
        </ul>
        <button id="revealColorSelect">New Color</button>
        <div id="colorSelect">
            <span id="newColor"></span>
            <div class="sliders">
                <p>
                    <label for="red">Red</label>
                    <input id="red" name="red" type="range" min=0 max=255 value=0>
                </p>
                <p>
                    <label for="green">Green</label>
                    <input id="green" name="green" type="range" min=0 max=255 value=0>
                </p>
                <p>
                    <label for="blue">Blue</label>
                    <input id="blue" name="blue" type="range" min=0 max=255 value=0>
                </p>
            </div>
            <div>
            <button id="addNewColor">Add Color</button>
            </div>
        </div>
    </div>
    <script src="//code.jquery.com/jquery-1.11.0.min.js" type="text/javascript" charset="utf-8"></script>
    <script src="js/app.js" type="text/javascript" charset="utf-8"></script>    
</body>
</html>

1 Answer

Samuel Ferree
Samuel Ferree
31,721 Points

the jQuery function .css() expects two parameters, both of which should be strings.

you need to convert your numerical r, g and b values, into a string of text, that meets the requirements of the rgb() css property.

The '+' operator, concatenates strings. consider the following code

let greeting = 'hello' + ' ' + 'world'; //=> 'hello world'
let r = 100;
let g = 200;
let b = 255;
let text = 'rgb(' + r + ',' + g + ',' + b + ');'; //=> 'rgb(100,200,255)'
Samuel Ferree
Samuel Ferree
31,721 Points

If you prefer something a bit more readable, ES5 gave us template literals. which can interpolate values by placing them inside of ${}

consider the following code:

let r = 100;
let g = 200;
let b = 255;
let text = `rgb(${r},${g},${b})`; // => 'rgb(100,200,255)'

Oh I see. I didn'r realize that it took a string. thanx