walk the coordinates of a grid in a spiral to the center

spiralGridWalk.js

// walk around the edges of a grid
// and work toward the center
// Begin going right.
// When you are at an edge or a filled square, turn right.
const spiral = (w, h, fn) => {
  // a map where keys are visited indexes
  const visited = {}

  // translations represent increments for: right -> down -> left -> up
  const translations = [1, w, -1, -w]
  let direction = 0
  let idx = 0

  let step = w * h
  let nextIdx
  let x
  let y
  while (step--) {
    nextIdx = idx + translations[direction]
    x = idx % w
    y = (idx / w) >> 0

    if (fn) fn(x, y, idx)

    if (
      visited[nextIdx] ||
      (direction === 2 && x === 0) ||
      (direction === 3 && y === 0) ||
      (direction === 0 && x === w - 1) ||
      (direction === 1 && y === h - 1)
    ) {
      direction = direction < 3 ? direction + 1 : 0
      nextIdx = idx + translations[direction]
    }

    visited[idx] = true
    idx = nextIdx
  }
}