guaranteed indexed object to array in order

Question

extension of this question

So I was playing around with a project and I found myself needing to create a string where I didn't know the exact order of the lettering right away, but I did know their position(and by extension, the length of this string).

Part of the reason for doing it this way is that I don't want to skip array elements at any point during the process(NO ["a", , "t"] during the process). Meaning that the order of insertion matters.

let string_length = 10;

let o = {
  0: "a",
  4: "q",
  7: "t",
  3: "p",
  6: "h",
  1: "z",
  2: "t",
  5: "a",
  9: "b",
  8: "z"
};

function obj_to_arr(o) {
  // this returns ["a","z","t","p","q","a","h","t","z","b"]
}

All the answers I've found until now don't necessarily guarantee that given an index object, it will give the corresponding ordered object, and this is mainly because of the nature of object, being unordered.

Is there a way to implement this?

Answer 1

Using Object.keys(), you can retrieve an array of keys, sort them numerically, then map that sorted array to one with the values.

The result will be an array of values in an order corresponding to the numeric values of the keys, even if they are not sequential.

For example

let o = {"0":"a","1":"z","2":"t","3":"p","4":"q","5":"a","6":"h","7":"t","8":"z","10":"b"}

let sortedValues = Object.keys(o)
    .sort((a, b) => a - b)  // numeric comparison
    .map(k => o[k])         // map to values

console.info(sortedValues)

Answer 2

You could fix this in two possible ways (and probably more).

Array

Either you add objects containing both the key and the value to an array.

array.push({c: "a"}) or array.push({ key: c, value: "a"}), then add the next and so on.

Push puts an element at the end of an array.

Sorted object

Or you insert into the object using alphabetic keys, in which case you can use the example below.

You could sort the keys (properties) of the object, and use them to access each value and put them into another array. Here's a verbose example of how you could do that.

const someObject = {
  c: "a",
  4: "q",
  7: "t",
  b: "p",
  6: "h",
  1: "z",
  2: "t",
  5: "a",
  a: "b",
  8: "z"
};


function obj_to_arr(obj) {
  let result = []
  
  // Get keys from object as an array
  let keys = Object.keys(obj)
  console.log('keys: ', keys)
  
  // Sort the array in alphabetical order
  let sorted = keys.sort()
  console.log('sorted: ', sorted)
  
  // Add each value to the resulting array
  sorted.forEach(function(key) {
    console.log('adding ' + obj[key] + ' to array')
    result.push(obj[key])
  })

  console.log('result: ', result)
  return result
}

// Call the function
obj_to_arr(someObject)

Answer 3

The short answer is "you can't" because of the Javascript spec.

Objects in Javascript are not guaranteed to have their keys ordered in any way due it being a primitive type that engines are allowed to optimize the hell out of, including reordering keys to speed up property access.

If ordering really matters, then you shouldn't be using an object primitive. Either an array of tuples (which has guaranteed ordering):

let fixedList = [
  { key: 0, value: 'a' },
  { key: 4, vlaue: 'q' },
  ...
];

or you can use a Map, which exists specifically as a way to have "an object, with the same ordering of key/value pairs as you declared".

Of the two, for data transport you probably want the former, and for running code you typically want the latter.