chooseUser.js

const GREEN = 'green'
const YELLOW = 'yellow'
const RED = 'red'

const GREEN_WEIGHT = .2
const YELLOW_WEIGHT = .3
const RED_WEIGHT = .5

const people = [
  { user: 'user1', grade: GREEN },
  { user: 'user2', grade: GREEN },
  { user: 'user3', grade: GREEN },
  { user: 'user4', grade: GREEN },
  { user: 'user5', grade: GREEN },
  { user: 'user6', grade: YELLOW },
  { user: 'user7', grade: YELLOW },
  { user: 'user8', grade: YELLOW },
  { user: 'user9', grade: YELLOW },
  { user: 'user10', grade: RED },
  { user: 'user11', grade: RED },
  { user: 'user12', grade: RED },
  { user: 'user13', grade: RED },
]

function chooseUser(people) {
  const greens = people.filter(p => p.grade === GREEN)
  const yellows = people.filter(p => p.grade === YELLOW)
  const reds = people.filter(p => p.grade === RED)

  const random = Math.random()
  if (random < GREEN_WEIGHT) {
    return greens[Math.floor(Math.random() * greens.length)]
  } else if (random < GREEN_WEIGHT + YELLOW_WEIGHT) {
    return yellows[Math.floor(Math.random() * yellows.length)]
  } else {
    return reds[Math.floor(Math.random() * reds.length)]
  }
}

const ITERATIONS = 500
let greenCounter = 0
let yellowCounter = 0
let redCounter = 0

for (let i = 0; i < ITERATIONS; i++) {
  switch (chooseUser(people).grade) {
    case GREEN:
      greenCounter++
      break
    case YELLOW:
      yellowCounter++
      break
    case RED:
      redCounter++
      break
  }
}

console.log(`After ${ITERATIONS} iterations, this is the result:
- GREEN: #times: ${greenCounter}, actual: ${100 * greenCounter / (greenCounter + yellowCounter + redCounter)}%, expected: ${Math.round(GREEN_WEIGHT * 100)}%
- YELLOW: #times: ${yellowCounter}, actual: ${100 * yellowCounter / (greenCounter + yellowCounter + redCounter)}%, expected: ${Math.round(YELLOW_WEIGHT * 100)}%
- RED: #times: ${redCounter}, actual: ${100 * redCounter / (greenCounter + yellowCounter + redCounter)}%, expected: ${Math.round(RED_WEIGHT * 100)}%
`)