fibonacci.js

// Returns Given Length Fibonacci Sequence (Array)

// Loop
function fibonacci(length) {
  let n1 = 0, n2 = 1, next, sequence = []

  for (let i = 1; i <= length; i++) {
    sequence.push(n1)
    next = n1 + n2
    n1 = n2
    n2 = next
  }

  return sequence
}

// Functional
let fibonacci = length => Array(length).fill().reduce(
  (sequence, _, index) => sequence.concat(
    index < 2 ? index : sequence[index - 1] + sequence[index - 2]
  ), []
)