.map vs rx

let data = [...]

for (const i = 0; i < data.length; i++) {
  data[i] = mapper(data[i])
}

let newData = []

for (const i = 0; i < data.length; i++) {
  if (filter(data[i])) {
    newData.push(data[i])
  }
}

vs

let data = [...]
let newData = []

for (const i = 0; i < data.length; i++) {
  const newVal = mapper(data)
  if (filter(newVal)) {
    newData.push(newVal)
  }
}