CSS Variables

a.k.a css custom properties

CSSTricks ⟩
- The Big Gotcha With Custom Properties ⭐️ - 改變 CSS 變數如何影響其他屬性❗️
- Updating a CSS Variable with JavaScript - update with "mousemove" events
--css variables
JS.info ⟩ Computed styles: getComputedStyle

Valid Names

⭐️ valid CSS variable names ( begin with two dashes "--" ❗️):

/* ✅ valid names */
--spacing: var(--line_height);
--spacing_1: calc(var(--spacing) * var(--scale));
--spacing_2: calc(var(--spacing_1) * var(--scale));
--spacing_-1: calc(var(--spacing) / var(--scale));

/* ❌ not valid names */
--spacing_+1:

Cheatcheet

:root { 
    /* ⭐️ CSS variable */
    --main-font: Helvetica, Arial, sans-serif; 
}

p {
    /* ⭐️ use var() to reference the variable */
    font-family: var(--main-font); 
}

// ⭐️ set
elem.style.setProperty('--panel-bg', '#cdf');

// ⭐️ get
var styles = getComputedStyle(elem);
var bg = styles.getPropertyValue('--panel-bg');

console.log(bg.trim());

html {
  --color-1: red;
  --color-2: blue;
}

div {

  height: 50px;
  border: 1px solid black;
  margin-bottom: 10px;
  
  /*  
    ⭐️ 注意：
       如果此行放在上面 html { ... } 中，下面 .variation 的設定就會無效❗️ 
  
       理論上，更新 `--color-1` 應該也會帶動更新 `--bg` 才對，但這只是
       「一廂情願」的想法，CSS 實際上並不會因為更新 `--color-1` 而主動
       更新 <div> 中的 var(--bg)，除非我們將 `--bg` 的定義直接放在使用 
       var(--bg) 的 <div> 身上❗️
  */
  --bg: linear-gradient(to right, var(--color-1), var(--color-2));
  
  background: var(--bg);
}

.variation {
  --color-1: green;
}

Window ⟩ .getComputedStyle() - returns an object (live CSSStyleDeclaration object) containing the values of all CSS properties of an element.
CSSStyleDeclaration ⟩
- .getPropertyValue() : DOMString | ''
- .setProperty()

window.getComputedStyle(elem);
window.getComputedStyle(elem, pseudoElem);

// style get/set property
var value = style.getPropertyValue(prop);
style.setProperty(prop, value, priority);

key points

The custom properties behave as a sort of scoped variable because the values are inherited by descendant elements.

redefine css vars

:root {
    /* ⭐️ define css vars */
    --panel-bg: #fff;
    --panel-color: #000;
}

.panel {
    padding: 0.5em 1em;
    border: 1px solid #999;
    border-radius: 0.5em;

    /* ⭐️ use css vars */
    color: var(--panel-color);
    background-color: var(--panel-bg);
}

.panel > h2 {
    margin-block-start: 0;
    font-size: 0.8em;
    font-weight: 900;
    text-transform: uppercase;
}

.dark {
    margin-block-start: 1em;

    /* ⭐️ redefine css vars */
    --panel-bg: #333;
    --panel-color: #fff;
}

var root = document.documentElement;    // <html> element
var styles = getComputedStyle(root);

// get property value
var bg = styles.getPropertyValue('--panel-bg'); 
console.log(bg.trim());                 // #fff

// set property value
root.style.setProperty('--panel-bg', '#cdf');

// ------------ helpers --------------
// $ ⭐️
function $(selector, parent = document){
  return parent.querySelector(selector);
}

// $all ⭐️
function $all(selector, parent = document){
  return parent.querySelectorAll(selector);
}

<!-- A regular panel on the page -->
<div class="panel">
  <h2>Light Version</h2>
  <div class="body">
    We have built partnerships with small farms around the world to hand-select beans at the peak of season. We then careful roast in small batches to maximize their potential.
  </div>
</div>

<aside class="dark">
  <!-- The second panel inside a dark container -->
  <div class="panel">
    <h2>Dark Version</h2>
    <div class="body">
      We have built partnerships with small farms around the world to hand-select beans at the peak of season. We then careful roast in small batches to maximize their potential.
    </div>
  </div>
</aside>

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Last updated 2 years ago