Use javascript array reduce given n inputs to yield m outputs

Question

I have a series of points that I want to change into a series of lines.

this is an example of what I want to the code do

[p1, p2, p3] -> [line1, line2]

each loop:

(p1, p2) -> line
(p2, p3) -> line

The standard way to do this is:

const triangle = [[0,0], [0,1], [1,2]]

const lines = []
for (let i = 1; i < triangle.length; ++i) {
  const slope = findSlopeFromPoints(...triangle[i - 1], ...triangle[i])
  const yIntercept = findYIntercept(...triangle[i], slope)
  lines.push({
    slope,
    yIntercept
  })
}

This is the closes I can get using Array.prototype.reduce. But it feels much harder to reason about

const initial = {
  array: [], // actual returned array we care about
  lastPoint: null // "triangle[i - 1]"
}
const linesR = triangle.reduce( (lines, point) => {
  if (lines.lastPoint === null)
    return { 
      ...lines, 
      lastPoint: point 
    }
  else {
    const slope = findSlopeFromPoints(...lines.lastPoint, ...point)
    const yIntercept = findYIntercept(...point, slope)
    lines.array.push({
      slope,
      yIntercept
    })
    lines.lastPoint = point
    return lines

  }
}, initial )

In short, is there a better way to use reduce to combine N inputs into N - 1 outputs?

Answer 1

Sure, use the currentIndex argument to apply an offset. Your callback function receives several more arguments¹ than you're using:

[{x:0, y:0}, {x:0, y:1}, {x:1, y:2}].reduce((lines, point, currentIndex, source) => {
  currentIndex < source.length -1 && lines.push({
    from: point,
    to: source[currentIndex + 1]
  }); 
  return lines;     
}, []);

_{¹See Array.prototype.reduce() for more info.}