maxPitDepth.js

const testArr = [
  /* 0 */ 0,
  /* 1 */ 1,
  /* 2 */ 0,
  /* 3 */ -3,
  /* 4 */ 1,
  /* 5 */ 3,
  /* 6 */ 4,
  /* 7 */ 0,
  /* 8 */ 1,
  /* 9 */ 3,
  /* 10 */ -2,
  /* 11 */ 0
]

function calculatePits(arr) {
  const pits = []

  for (let p = 0; p < arr.length; p++) {
    const pVal = arr[p]

    let qComp = pVal

    for (let q = p + 1; q < arr.length; q++) {
      const qVal = arr[q]

      if (qVal < qComp) {
        let rComp = qVal

        for (let r = q + 1; r < arr.length; r++) {
          const rVal = arr[r]

          if (rVal > rComp) {
            pits.push([p, q, r])

            rComp = rVal
          } else {
            break
          }
        }

        qComp = qVal
      } else {
        break
      }
    }
  }

  checkPits(pits, arr)

  return pits
}

function checkPits(pits, arr) {
  pits.forEach(([p, q, r]) => {
    for (let i = p; i <= q - 1; i++) {
      if (arr[i] < arr[i + 1]) {
        throw [p, q, r]
      }
    }

    for (let i = q; i <= r - 1; i++) {
      if (arr[i] > arr[i + 1]) {
        throw [p, q, r]
      }
    }
  })
}

function calculateDepths(pits, arr) {
  return pits.map(([p, q, r]) => Math.min(arr[p] - arr[q], arr[r] - arr[q]))
}

function maxPitDepth(arr) {
  const pits = calculatePits(arr)

  const depths = calculateDepths(pits, arr)

  return Math.max(...depths)
}

console.log(maxPitDepth(testArr))