// First, get all your elements:
const a = document.getElementById('a');
const b = document.getElementById('b');
const c = document.getElementById('c');
// And the buttons to update them:
const decreaseA = document.getElementById('decrease-a');
const decreaseAB = document.getElementById('decrease-ab');
const decreaseABC = document.getElementById('decrease-abc');
const increaseA = document.getElementById('increase-a');
const increaseAB = document.getElementById('increase-ab');
const increaseABC = document.getElementById('increase-abc');
const decreaseAll = document.getElementById('decrease-all');
const increaseAll = document.getElementById('increase-all');
// Define your function:
function updateColor(elements, change) {
// Make sure elements is always an Array, so that you can call the
// function with either an Array of elements or a single one (without
// having to wrap it in an Array yourself.
elements = Array.isArray(elements) ? elements : [elements];
// Process all elements:
elements.forEach(element => {
// Get the current background-color value:
const value = getComputedStyle(element).getPropertyValue("background-color");
// Get all color components (alpha may not be there if = 1):
const parts = value.match(/[\d.]+/g);
// If alpha is not there, add it:
if (parts.length === 3) {
parts.push(1);
}
// Modify alpha:
parts[3] = Math.min(1, Math.max(0, parseFloat(parts[3]) + change));
// Set the element's text to be the current alpha value (just for the example):
element.innerText = parts[3].toFixed(2);
// Apply new value:
element.style.backgroundColor = `rgba(${ parts.join(',') })`;
});
}
// Add event handlers for the buttons that will call the function with the right params:
decreaseA.onclick = () => updateColor(a, -0.1);
decreaseAB.onclick = () => updateColor(b, -0.1);
decreaseABC.onclick = () => updateColor(c, -0.1);
increaseA.onclick = () => updateColor(a, 0.1);
increaseAB.onclick = () => updateColor(b, 0.1);
increaseABC.onclick = () => updateColor(c, 0.1);
decreaseAll.onclick = () => updateColor([a, b, c] , -0.1);
increaseAll.onclick = () => updateColor([a, b, c], 0.1);
// Set the initial text inside each sample without modifying the color (just for the example):
updateColor([a, b, c] , 0);
body {
margin: 0;
font-family: monospace;
}
.samples {
overflow: hidden;
}
.buttons {
overflow: hidden;
border: 2px solid #FFF;
}
span {
float: left;
width: 33.33333333%;
line-height: 100px;
height: 100px;
text-align: center;
}
button {
float: left;
width: 33.33333333%;
background: #000;
color: #FFF;
border: none;
line-height: 32px;
font-family: monospace;
outline: none;
border: 2px solid #FFF;
}
#decrease-all,
#increase-all {
width: 50%;
}
#a { background: rgba(255, 0, 0, 1); }
#b { background: rgba(0, 255, 0, 1); }
#c { background: rgba(0, 0, 255, 1); }
<div class="samples">
<span id="a"></span>
<span id="b"></span>
<span id="c"></span>
</div>
<div class="buttons">
<button id="increase-a">INCREASE A</button>
<button id="increase-ab">INCREASE B</button>
<button id="increase-abc">INCREASE C</button>
<button id="decrease-a">REDUCE A</button>
<button id="decrease-ab">REDUCE B</button>
<button id="decrease-abc">REDUCE C</button>
<button id="increase-all">INCREASE ALL</button>
<button id="decrease-all">REDUCE ALL</button>
</div>