fillBlanks takes an array with falsy/null values, and fills them with the previous value in the array. If null values appear without a previous value, you can use nullReplace to autofill it.

fillBlanks.js

/**
 * fillBlanks takes an array with falsy/null values, and fills them with the previous value in the array. 
 * If null values appear without a previous value, you can use nullReplace to autofill it.
 * @example
 * const arr = ["", "", 0, "", "", 1, "", 2, "", "", "", "Test", ""]
 * fillBlanks(arr) 
 * // => [ null, null, 0, 0, 0, 1, 1, 2, 2, 2, 2, 'Test', 'Test' ]
 * @example
 * const arr = ["", "", 0, "", "", 1, "", 2, "", "", "", "Test", ""]
 * fillBlanks(arr, "replaced") 
 * // => [ 'replaced', 'replaced', 0, 0, 0, 1, 1, 2, 2, 2, 2, 'Test', 'Test' ]
 * @function fillBlanks
 * @param {array} arr - the input array
 * @param {string} [nullReplace=null] - the replacement value when a null value is encountered
 * @returns {string[]}
 */

function fillBlanks(arr, nullReplace = null) {
  const hasValue = val => (val || val === 0 ? true : false)

  return arr.reduce((acc, val, idx) => {
    if (hasValue(val)) {
      acc.push(val)
    } else {
      const hasPrevValue = hasValue(acc[idx - 1])
      const prevVal = hasPrevValue ? acc[idx - 1] : nullReplace
      acc.push(prevVal)
    }
    return acc
  }, [])
}

// Example:
const arr = ["", "", 0, "", "", 1, "", 2, "", "", "", "Test", ""]
fillBlanks(arr) 
// => [ null, null, 0, 0, 0, 1, 1, 2, 2, 2, 2, 'Test', 'Test' ]
fillBlanks(arr, "replaced") 
// => [ 'replaced', 'replaced', 0, 0, 0, 1, 1, 2, 2, 2, 2, 'Test', 'Test' ]