hexToRgba, using lots of bitwise operations

hexToRgba.js

function hexToRgba(st) {
    if(st.startsWith("#"))
        st = st.slice(1);

    const len = st.length;                           // I don't know how JS works, I suspect this is recalculated
    const has_alpha = !(len % 4);
    if(len % 3 && !has_alpha)
        throw new Error("invalid hex code");

    var val = Number.parseInt(st, 16);              // Only parsing the string once is faster than slicing constantly

    const depth = Math.ceil(len / 4) << 2;          // Depth will be multiple of 4 bits
    const mask = (1 << depth) - 1;                  // Will mask bottom color

    let A = (val | (mask * !has_alpha)) & mask;     // In case there is no alpha, alpha will be 100%
    let B = (val >>= (depth * has_alpha)) & mask;   // Only shift when there is alpha
    let G = (val >>= depth) & mask;                 // Always shift for green and blue (red is always first)
    let R = (val >> depth) & mask;                  // Val is no longer needed; no need to store new value

    // 4bit to 8bit depth
    const four_bit = (depth == 4) << 2;             // Will not shift if depth > 4
    B |= B << four_bit;
    G |= G << four_bit;
    R |= R << four_bit;
    A |= A << four_bit;

    return [R, G, B, A];
}

module.exports = hexToRgba;

hexToRgba.md

Note

This package is officially published under @voxelprismatic/hextorgba on NPM.

Don't use this gist, as it uses Number, which conveniently breaks down at 12bit wide color channels with alpha channel (#RRRGGGBBBaaa). The NPM package uses BigInt, which goes up and beyond 20bit wide color channels.

The NPM package also supports returning array (default), object, percentages as array, and CSS rgb(...)