If you have a list of objects like so:

const readings = [
  {temp: 40.3, stage: 'preferment'}, 
  {temp: 10.1, stage: 'post fridge retardation'}, 
  {temp: 20.1, stage: 'pre oven'}
];

If you want to sort this list by temp and you try and run readings.sort() nothing will happen - it will return the array in same order as JS doesn't know what you're trying to do.  To help, you need to provide a custom compare function to sort.  A basic compare function to compare two numbers (a, and b) looks like this:

function myCompare(a,b){
  if(a > b) return 1;
  if(a < b) return -1;
  if(a === b) return 0;
 }

All we need to do is tell this function to use our desired key:

function tempCompare(a,b){
  const x = a.temp;
  const y = b.temp;

  if(x > y) return 1;
  if(x < y) return -1;
  if(x === y) return 0;
 }

Now we can run readings.sort(tempCompare) and you'll now have the following:

[
{temp: 10.1, stage: "post fridge retardation"},
{temp: 20.1, stage: "pre oven"},
{temp: 40.3, stage: "preferment"}
];

If you wanted this list in descending order then you alter the return values in the tempCompare(a,b) function like so:

function tempCompareDesc(a,b){
  const x = a.temp;
  const y = b.temp;

  if(x > y) return -1; //This is now -1 instead of 1
  if(x < y) return 1; //This is now 1 instead of -1
  if(x === y) return 0;
 }

Then you'll have:

[
  {temp: 40.3, stage: "preferment"},
  {temp: 20.1, stage: "pre oven"},
  {temp: 10.1, stage: "post fridge retardation"}
]