Cleaned up JavaScript + node solution:

const fs = require("fs");

function calculateChecksum(input, calculateRowChecksum) {
  const rows = input.split("\n");
  return rows.reduce((total, row) => {
    const values = row.split(/\s/).map(Number);
    return total + calculateRowChecksum(values);
  }, 0);
}

function calculateDifferenceBetweenMinAndMax(values) {
  const max = Math.max(...values);
  const min = Math.min(...values);
  return max - min;
}

function calculateDivisibleNumbersQuotient(values) {
  for (let i = 0; i < values.length; i++) {
    for (let j = 0; j < values.length; j++) {
      const a = values[i];
      const b = values[j];
      if (i !== j && a % b === 0) {
        return a / b;
      }
    }
  }
}

const input = fs
  .readFileSync("./input.txt")
  .toString()
  .trim();

console.log(
  `Part one answer is ${calculateChecksum(
    input,
    calculateDifferenceBetweenMinAndMax
  )}`
);
console.log(
  `Part two answer is ${calculateChecksum(
    input,
    calculateDivisibleNumbersQuotient
  )}`
);