Heuristic to check whether a site appears dark or bright to a user

check_darkmode.js

/* round num to c digits */
function round(num, c) {
    return +(Math.round(num + "e+" + c)  + "e-" + c);
}

/* Adapted from Python from manojpandey, thanks!
   https://gist.github.com/manojpandey/f5ece715132c572c80421febebaf66ae
 */
function rgbTolab(inputColor) {

    var num = 0;
    var RGB = [0, 0, 0];

    inputColor.forEach(function(value) {
        value = parseFloat(value) / 255;
        
        if (value > 0.04045) {
            value = ((value + 0.055) / 1.055) ** 2.4;
        }
        else {
            value = value / 12.92;
        }
        
        RGB[num] = value * 100;
        num = num + 1;
    });
    
    var XYZ = [0, 0, 0];

    var X = RGB[0] * 0.4124 + RGB[1] * 0.3576 + RGB[2] * 0.1805;
    var Y = RGB[0] * 0.2126 + RGB[1] * 0.7152 + RGB[2] * 0.0722;
    var Z = RGB[0] * 0.0193 + RGB[1] * 0.1192 + RGB[2] * 0.9505;
    
    XYZ[0] = round(X, 4);
    XYZ[1] = round(Y, 4);
    XYZ[2] = round(Z, 4);

    // Observer= 2°, Illuminant= D65
    XYZ[0] = parseFloat(XYZ[0]) / 95.047;         // ref_X =  95.047
    XYZ[1] = parseFloat(XYZ[1]) / 100.0;          // ref_Y = 100.000
    XYZ[2] = parseFloat(XYZ[2]) / 108.883;        // ref_Z = 108.883

    var num = 0;
    XYZ.forEach(function(value) {

        if (value > 0.008856) {
            value = value ** (0.3333333333333333);
        }
        else {
            value = (7.787 * value) + (16 / 116);
        }

        XYZ[num] = value;
        num = num + 1;
    });

    var Lab = [0, 0, 0];

    var L = (116 * XYZ[1]) - 16;
    var a = 500 * (XYZ[0] - XYZ[1]);
    var b = 200 * (XYZ[1] - XYZ[2]);

    Lab[0] = round(L, 4);
    Lab[1] = round(a, 4);
    Lab[2] = round(b, 4);

    return Lab;
}

var mainTextColor = window.getComputedStyle(document.body, null).color;

var brightness = rgbTolab(mainTextColor.match(/\d+/g))[0];

var threshold = 50;

if (brightness < threshold) {
    console.log("This page has some dark text. Better use dark mode.");
}
else {
    console.log("This page has some bright text. Better leave it like that.");
}