JSFiddle - React, Tailwind, and code Playground
JavaScript
const arr = [1, 4, 3, 6, 8, 5]
const myNumber = 3
const below = arr.reduce((p, c) => c < myNumber ? c : p)
console.log('below ', below)
const above = arr.reduce((p, c) => myNumber > Math.max(c,p) ? c : p)
console.log('above ', above)
console.log()