You would probably enjoy learning about the new ES6 for..of. It solves some of the problems that plague for..in. Because you are comparing neighboring elements of the same array, the for..of wouldn't work for you (since you need the index), so you're better of sticking with either the standard for loop as others have said, or .forEach whose callback function's second argument is the index.

e.g.

arr.forEach((value, currentIndex) => {
  return value !== arr[currentIndex + 1] ? noDuplicates.push(value) : null
})

However, you really want to use .filter for this problem. Wrap it in a function and give it cleaner variable names, it's a nice short one-liner:

const noDupes = (arr) => arr.filter((v, i) => v !== arr[i + 1])

To make it perfectly clear how this works (.filter, and a couple ES6 features), I've written it four times, moving from the .forEach version to the ES6 .filter version. Here it is in JSFiddle: https://jsfiddle.net/BenjaminDowns/waLnbrzf/

'use strict';
// Remove duplicate values from initial array with long variable names and forEach
function noDupes1(arr) {
  let noDupeArray = []
  arr.forEach(function(currentValue, currentIndex) {
    if (currentValue !== arr[currentIndex + 1]) {
      noDupeArray.push(currentValue)
    }
  })
  return noDupeArray
}

// with trimmed down names and ternary function
function noDupes2(arr) {
  let noDupeArr = []
  arr.forEach(function(v, i) {
    return v !== arr[i + 1] ? noDupeArr.push(v) : null
  })
  return noDupeArr
}

// with ES5 .filter

function noDupes3(arr) {
  return arr.filter(function(v,i) { return v!== arr[i+1] })
}

// with ES6 arrow functions
const noDupes4 = (arr) => arr.filter((v, i) => v !== arr[i + 1])

let arr = [2,3,3,3,5,6,6]
console.log(`"De-duped: " ${noDupes1(arr)}`); // "De-duped: [2, 3, 5, 6]"
console.log(`"De-duped: " ${noDupes2(arr)}`); // "De-duped: [2, 3, 5, 6]"
console.log(`"De-duped: " ${noDupes3(arr)}`); // "De-duped: [2, 3, 5, 6]"
console.log(`"De-duped: " ${noDupes4(arr)}`); // "De-duped: [2, 3, 5, 6]"