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Using CSS Variables

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Updating CSS Variables with JavaScript

6:41 with Guil Hernandez

CSS Variables are not only updated using the cascade and inheritance, you can also read and write (or get and set) CSS variable values from your JavaScript.

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    CSS variables are not only updated using the cascade and inheritance. 0:00
    You can also read and write or get and set CSS variable values from your JavaScript. 0:05
    For example, in this demo, 0:10
    the ball moves around the page using a CSS transform based on where you click. 0:12
    And its color changes to a random hex value. 0:17
    The background color and transform properties of the ball 0:19
    are being updated live with JavaScript and CSS variables. 0:22
    To have CSS variables interact with JavaScript at runtime, 0:27
    you use the getPropertyValue method to get the value of a custom property and 0:31
    the setProperty method to set the value. 0:36
    If you haven't written a lot of JavaScript yet that's okay, 0:39
    I'm going to teach you how to create this fun demo using the set property method. 0:41
    We also have lots of JavaScript workshops and courses here at Treehouse, so 0:45
    be sure to check the teachers note to learn more. 0:49
    First, in variables.css, I'll use var and 0:51
    custom properties to set the background color and 0:55
    transform values of the ball class. 0:59
    So first I'll add the background color property. 1:03
    And set its value to var, passing it the variable ball-bg. 1:06
    Below that, I'll add a transform property. 1:14
    And I'll use the translate function as the transform. 1:20
    And like I did in an early video, I'll use calc and 1:25
    variables to set the x and y translation values in pixels. 1:29
    So first, I'll pass translate the calc function. 1:33
    And inside this calc function, I'll pass it var. 1:37
    And I'll specify the variable pos-x, or position-x. 1:40
    And I'll multiply it by one pixel to set it in pixels. 1:47
    Then I'll set the y value by adding a comma and another calc function. 1:52
    Inside here I'll specify var. 1:58
    And this time pass it, the variable position y. 2:01
    And multiply it by one pixel. 2:06
    And I'll set the ball-bg variable's fallback value to tomato. 2:10
    So, tomato will be the ball's initial background color, when the page loads. 2:14
    So now, instead of declaring the background color and 2:18
    translation variables in my style sheet, I'll do it with JavaScript. 2:21
    Over in scripts.js, 2:25
    I'll update the values with JavaScript by first selecting the body element and 2:28
    the element with the class ball using document.querySelector. 2:33
    So, I'll first select the body with const 2:37
    body = document.querySelector. 2:42
    And here I'll specify the body selector. 2:46
    Below that I'll target the class ball by creating a constant named ball. 2:51
    And here, I'll specify the class ball. 3:03
    So, we want the valleys to change when we click in the body. 3:05
    So, I'll give body an Event.Listener with body.addEventListener 3:10
    And here I'll set a background color and position value for 3:19
    ball each time the body is clicked. 3:24
    So I'll pass it, click event, and the event object followed by Function. 3:27
    So inside the function, 3:34
    I'll use the set property method on the style property to set each value. 3:36
    So first, I'll set the X position with ball.style.setProperty. 3:41
    I wanna set the value of the position x CSS variable. 3:52
    So I'll pass set property, the position x variable, 3:57
    and set the value to e.clientX. 4:04
    So, I'm setting the value using the x and y coordinates of the page, e.clientx 4:08
    will return the horizontal coordinate at which the click occured inside the body. 4:13
    And e.clientY is going to provide the vertical coordinates. 4:19
    So I'll copy this. 4:22
    Paste it below and here I'll set the value of the position y variable to e.clientY. 4:26
    And for this demo, I'll offset the coordinates by 80, 4:35
    to make the translation values a little more precise. 4:38
    That's going to place the center of the ball exactly where the mouse clicked. 4:42
    And you'll see the cursor in the center, like here in the demo. 4:46
    So, now to set a new background color each time the body is clicked. 4:52
    We'll say, ball.style.setProperty. 4:57
    Set the ball BGCCSS variable to a random hex value. 5:02
    The randomHex function already provided here in the JavaScript. 5:09
    So when body is clicked, this function returns a random 5:15
    hex value that gets assigned to the wall bg css variable. 5:19
    All right, let's have a look in the browser. 5:23
    I get this to save. 5:25
    Refresh and now if I click somewhere in the body, 5:28
    we see the ball move to that exact spot and it's background color changes. 5:32
    Good. 5:37
    You'll find lots of amazing examples examples out there of CSS variables and 5:40
    Java scripted action. 5:43
    For example this Google Chrome example, updates the layouts spacing unites, 5:45
    column and margin values live in the browser. 5:51
    And when you click on one of the use this color links in the cards it, 5:55
    changes the header background colors. 5:59
    And if you scroll down, you'll see the CSS snippet, 6:05
    displaying how they're using CSS variables page wise. 6:08
    And below that, you can see how JavaScript is used to get and 6:12
    set the custom property values. 6:15
    And this demo by developer Wes Boss is also a great example of how CSS variables 6:20
    can update with JavaScript. 6:24
    The padding, spacing, blur, and 6:26
    background color of the image update in real time when you adjust the sliders. 6:29
    I've posted links to more demos in the teachers notes this video. 6:36

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