JSFiddle - React, Tailwind, and code Playground

by m1erickson

HTML

<input id="myRange" type="range" min="0" max="299">
<br>
<canvas id="canvas" width=300 height=45></canvas>

CSS

body{ background-color: ivory; }
#canvas{border:1px solid red;}
#myRange{width:300px;}

JavaScript

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.font = "14px verdana";

var $canvas = $("#canvas");
var canvasOffset = $canvas.offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;

var x1 = 0;
var y1 = 10;
var x2 = canvas.width;
var y2 = 10;

// create a gradient
var gradient = ctx.createLinearGradient(x1, y1, x2, y2);
gradient.addColorStop(0.00, "indigo");
gradient.addColorStop(1.00, "steelblue");

// draw the gradient across the canvas
ctx.fillStyle = gradient;
ctx.fillRect(x1, y1 - 10, x2 - x1, 20);

// get the pixel color array for the gradient
var data = ctx.getImageData(x1, y1, canvas.width, 1).data;

// get the gradient using a range control
$("#myRange").on("input change", function () {
    var value = $(this).val();
    var i = value * 4;
    var r = data[i];
    var g = data[i + 1];
    var b = data[i + 2];
    var a = data[i + 3];

    // change the result rectangle to the mouseX color
    var fill = "rgba(" + r + "," + g + "," + b + "," + a + ")";
    ctx.fillStyle = fill;
    ctx.clearRect(0, 20, canvas.width, 30);
    ctx.fillRect(0, 25, 50, 20);
    ctx.fillText(fill, 75, 40);
});