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stub.js

// newtype Point = {x :: Number, y :: Number}
// newtype EnhancedPoint = {x :: Number, y :: Number, visible :: Boolean, distance :: Number}

/* Library functions */

//    getCenter :: Point => Point => Point
const getCenter =
    ({x: x1, y: y1}) =>
        ({x: x2, y: y2}) =>
            ({x: 0.5*(x1+x2), y: 0.5*(y1+y2)})

// XXX box corners need to be in order
// FIX by ordering the corners before the conditional check
//    inBox :: Point => Point => Point => Boolean
const inBox =
    ({x: x1, y: y1}) =>
        ({x: x2, y: y2}) =>
            ({x: xt, y: yt}) =>
                x1 <= xt && xt <= x2 &&
                y1 <= yt && yt <= y2

//    getDistance :: Point => Point => Number
const getDistance =
    ({x: x1, y: y1}) =>
        ({x: x2, y: y2}) =>
            Math.sqrt (Math.pow (x1 - x2, 2) + Math.pow (y1 - y2, 2))

/* Input data */

//    topLeft :: Point
const topLeft =
    {x: ..., y: ...}

//    bottomRight :: Point
const bottomRight =
    {x: ..., y: ...}

//    results :: [Point]
const results =
    [
        {x: ..., y: ...},
        ...
    ]

/* Auxiliary data */

//    center :: Point
const center =
    getCenter (topLeft) (bottomRight)

//    inScreen :: Point => Boolean
const inScreen =
    inBox (topLeft) (bottomRight)

/* Specific helper functions */

//    addDistanceAndVisibility :: Point => EnhancedPoint
const addDistanceAndVisibility =
    p =>
        ({x: p.x, y: p.y, visible: inScreen (p), distance: getDistance (p) (center)})

//    byVisibleThenClosest :: (EnhancedPoint, EnhancedPoint) => Number
const byVisibleThenClosest =
    ({inScreen1, distance1}, {inScreen2, distance2}) =>
        const visible = visible1 - visible2 // true before false
        const distance = distance2 - distance1 // low before high

        return visible !== 0 ? visible : distance
    }

/* The result */

//    result :: EnhancedPoint | undefined
const result = results
    .map (addDistanceAndVisibility)
    .sort (byVisibleThenClosest)
    [0]