Lighten or darken a color

color_intensity.js

/**
 * Lighten (+ve) or darken (-ve) color.
 *
 * @param {string} col Color
 * @param {number} amt Intensity of light
 */
export const adjustColorIntensity = (col = '#000000', amt = 0) => {
    let usePound = false;

    if (col[0] === '#') {
        // eslint-disable-next-line no-param-reassign
        col = col.slice(1);
        usePound = true;
    }

    const num = parseInt(col, 16);

    let r = (num >> 16) + amt;

    if (r > 255) r = 255;
    else if (r < 0) r = 0;

    let b = ((num >> 8) & 0x00ff) + amt;

    if (b > 255) b = 255;
    else if (b < 0) b = 0;

    let g = (num & 0x0000ff) + amt;

    if (g > 255) g = 255;
    else if (g < 0) g = 0;
    const hexVal = (g | (b << 8) | (r << 16)).toString(16);
    let newHexVal = hexVal;

    for (let i = 0; i < 6 - hexVal.length; i += 1) {
        newHexVal = `0${newHexVal}`;
    }

    return (usePound ? '#' : '') + newHexVal;
};