pack.js

var testCases = [
  [[1, 2, 3, 7, 8, 9], "1-3, 7-9"],
  [[1, 2, 3, 1, 2, 3], "1-3, 1-3"],
  [[0, 1, 1, 2],       "0-1, 1-2"],
  [[0, 7, 6, 5],       "0, 7, 6, 5"],
  [[0], '0'],
  [[], '']
]

for (let i = 0; i < testCases.length; i++) { 
  let [arr,expected] = testCases[i];
  console.info(`Testing case ${i+1}`)
  let result = compact(arr);
  if(result===expected) {
    console.info(`Test ${i+1} passed`)
  } else {
    console.error(`Expected: ${expected} but got ${result}`)
    break;
  }
}

function compact(arr){
  if(arr.length===0) return '';
  let prevVal = arr[0];
  let result = [];
  let initialValue = prevVal;
  if(arr.length===1) return String(initialValue);
  for(let i=1; i<arr.length; i++){
    let curVal = arr[i];
    if(curVal !== prevVal + 1){
      if(initialValue===prevVal){
        result.push(initialValue);
      } else {
      	result.push(initialValue + '-' + prevVal)
      }
      initialValue = curVal;
    }
    if(i===arr.length-1){
      if(initialValue===curVal){
        result.push(initialValue);
      } else {
      	result.push(initialValue + '-' + curVal)
      }
    }
    prevVal = curVal;
  }
  
  return result.join(', ');
}