function replaceHolders(array, placeholder) {
    var index = -1,
        length = array.length,
        resIndex = -1,
        result = [];

    while (++index < length) {
      if (array[index] === placeholder) {
        array[index] = PLACEHOLDER;
        result[++resIndex] = index;
      }
    }
    return result;
  }
Replaces all `placeholder` elements in `array` with an internal placeholder
and returns an array of their indexes.

@private
@param {Array} array The array to modify.
@param {*} placeholder The placeholder to replace.
@returns {Array} Returns the new array of placeholder indexes.