hsluv-gradient.js

const {hexToHsluv, hsluvToHex} = require('hsluv')

const interpolate = (a1, a2) => x => a1.map((c, i) => c + (a2[i] - c) * x)
const range = steps => Array.from({length: steps + 1}, (_, i) => i / steps)

const stopsToHsluv = (start, end) => {
    const hStart = hexToHsluv(start)
    const hEnd = hexToHsluv(end)
    
    const throughZero = Math.abs(hStart[0] - hEnd[0]) > 180
    
    if(throughZero) {
        if(hStart[0] < hEnd[0]) {
            hStart[0] += 360
        } else {
            hEnd[0] += 360
        }
    }
    
    return [hStart, hEnd]
}

const gradient = (start, end, steps = 10) => range(steps)
    .map(interpolate(...stopsToHsluv(start, end)))
    .map(hsluvToHex)

module.exports = gradient