Lightweight function that will return the appropriate foreground color (white or black) when given a HEX/HEXA background color.

contrastingColor.js

/**
contrastingColor(hex, factorAlpha)
@Description   Accepts a HEX/HEXA color in any valid format (#HEX/#HEXA/#HHEEXX/#HHEEXXAA; 
               '#' is optional), and a boolean indicating if its opacity should be 
               considered and returns either '#000' or '#FFF' representing the preferred
               foreground color for text atop it.
@param   <STRING>  hex           any valid hex color code.
@param   <BOOLEAN> factorAlpha   whether or opacity should be considered (DEFAULT: false)
@returns <STRING>                returns hex color code for white or black (#FFF/#000)
*/

function contrastingColor(hex, factorAlpha=false) {
    let [r,g,b,a]=hex.replace(/^#?(?:(?:(..)(..)(..)(..)?)|(?:(.)(.)(.)(.)?))$/, '$1$5$5$2$6$6$3$7$7$4$8$8').match(/(..)/g).map(rgb=>parseInt('0x'+rgb));
    return ((~~(r*299) + ~~(g*587) + ~~(b*114))/1000) >= 128 || (!!(~(128/a) + 1) && factorAlpha) ? '#000' : '#FFF';   
}