Code Efficiency: Any Quick Way to Determine?

Hello,
I was wondering if anyone can tell me if there’s any way to calculate/determine how efficient/not efficient one’s code is. In the following code below, which is my noob-solution to the “Smallest Common Multiple” algorithm challenge, I assume that if I coded something like Euclidean’s algorithm into my code (I notated where I’m referring to below somewhere mid-way through the code) to determine the LCM of the numbers being checked, that the code would be more “efficient”. Actually, I assumed the code wouldn’t even work without figuring specific LCM’s but it did. The code passed but I feel a bit guilty about proceeding without something more “efficient”. Can someone explain how I, as a noob and not very naturally mathematically gifted, could determine the efficiency of one version of code vs. another version?

function smallestCommons(arr) {
  if (arr[0] > arr[1]){                     //sorts array into ascending order
    arr.push(arr.shift());
  }
  let aArr = arr[0];
  let bArr = arr[1];  
  let betweenArr = [];
  for (let i = aArr; i <= bArr;i++){ betweenArr.unshift(i)};//arr of numbers between a & b
  console.log(betweenArr);
  let a = betweenArr[0];
  let b = betweenArr[1];
  
  let lcm;
  lcm = ((a*b)/b); // .  ** PERHAPS THIS COULD BE MORE PRECISELY DEFINED USING THE EUCLIDEAN ALGORITHM? **
    
  let varPass;
  let lcmControl = lcm; //keeps a static lcm value in order to add to lcm for tests
  while (varPass = "not ok"){
    for (let i = 0; i < betweenArr.length; i++){
      if ((lcm % betweenArr[i]) != 0){
        varPass = "not ok";
        lcm += lcmControl;
      }
    }   
  }                                       

console.log(lcm);
return lcm;

}
smallestCommons([23,18]);

If by “efficient” you mean performant/fast, you can empirically find out how performant your code is using tools like jsPerf. You can also compare the performance of your solution vs. other solutions. No math knowledge required.

Exactly the sort of thing I was talking about! Thanks!