You cannot know the variable name because the variable name is not passed into javascript functions, only the values, but you can have a workaround

with arrays:

var numbers = [48,420,39];

const index = numbers.indexOf(Math.max(...numbers))

console.log(`The max value is the ${index+1}nth value in the array`)

with objects

var num1 = 48;
var num2 = 420;
var num3 = 39;

var numbers = {num1, num2, num3}

const maxVal = Math.max(...Object.values(numbers))
const key = Object.keys(numbers).find(key => numbers[key] === maxVal)

console.log(key, maxVal)

Since you are using var, it will get binded to the window object. Then you can iterate over window object, and get the name of variable. Also, if you follow same naming convention, like num1, num2 etc, you can use a if condition to check if num is present in key or not.

var num1 = 48;
var num2 = 420;
var num3 = 39;

var max = Math.max(num1, num2, num3);

let s = Object.keys(window).filter(key => {
      if (key.includes("num")) {
          return window[key] == max
        }
      });

console.log(s)

O(n) Solution

function getMaxIndex(nums) {
  let max = -Infinity, maxIndex = -1;
  
  for (let i = 0; i < nums.length; i++) {
    if (nums[i] > max) {
      max = nums[i]
      maxIndex = i
    }
  }
  return maxIndex
}

const nums = [48, 420, 39]
let i = getMaxIndex(nums)
console.log(i) // 1, because the index of 420 is 1

An alternative solution using reduce

function getHighestNumber(nums) {
  return nums.reduce((acc, num, index) => {
    if(num > acc.value) {
      acc.value = num;
      acc.index = index;
    }
    
    return acc;
  }, {value: null, index: null});
}

let nums = [48,420,39];
const max = getHighestNumber(nums);

console.log(max);

I would suggest putting the values into an array / list, so that you can compare easily in a for loop. Example:

let nums = [48,420,39]; 
let max = Math.max(nums[0], nums[1], nums[2]);
for(let i = 0; i < 3; i++){
    if(nums[i] == max){
        console.log("The largest is num" + i)
    }
}

this is the best i could think of.

create an array that will hold the variable name and its value like this:

let varAndVal = 
[
  {"varName":"num1",
   "value": 50},
   {"varName":"num2",
   "value": 51},
   {"varName":"num3",
   "value": 52},
]

then I would run over the array and find the max value and its name like this:

 function getMaxNumAndVarName(arr) {
    let i;
   
    // Initialize maximum element
    let maxVar = arr[0];

    // Traverse array elements 
    // from second and compare
    // every element with current max 
    for (i = 1; i < arr.length; i++) {
        if (arr[i].value > maxVar.value)
            maxVar = arr[i];
    }
     
  return maxVar;
}

let varAndVal = [{
    "varName": "num1",
    "value": 50
  },
  {
    "varName": "num2",
    "value": 51
  },
  {
    "varName": "num3",
    "value": 52
  },
]

function getMaxNumAndVarName(arr) {
  let i;

  // Initialize maximum element
  let maxVar = arr[0];

  // Traverse array elements 
  // from second and compare
  // every element with current max 
  for (i = 1; i < arr.length; i++) {
    if (arr[i].value > maxVar.value)
      maxVar = arr[i];
  }

  return maxVar;
}

console.log(getMaxNumAndVarName(varAndVal));

Your variables can be properties of an object, members of a function scope or reside in a module.

1. If they are properties of an object including the global/window object you can access them with Object.keys() and .reduce() to determine the variable with maximum value.

   var object = { var1: 1
                , var2: 2
                , var3: 3
                },
       maxVar = Object.keys(object).reduce((p,c) => object[p] > object[c] ? p : c);
   
   console.log(maxVar); // "var3";
   

   Or use a for in loop.

2. However if you need to define your variables in a function scope then you best make your function a constructor and claim your variable names like this.var1 ... this.varN etc in order to access the scope variables from the instantiated object and repeat 1.

3. Put your variables in a module and do like

   export let name1 = …, name2 = …, …, nameN; // also var, const`
   

   to be imported in your code like

   import * as object from "./myModule";
   

   and repeat 1.